Sansui AU-20000: Restoration
This is one of the most complex amplifiers that I ever worked on. Not in terms of construction or engineering challenges, but in terms of the number of parts that have to be replaced. I took no chance and shotgunned all small signal transistors with corroded and non-corroded legs. I worked on other Sansui units and from my experience these old transistors might fail whenever you don't expect them to. So why assume this risk?
And since I spotted some cracked solder joints, I decided that it would be a good idea to reflow all of them on each and every printed circuit board. And then I just thought to myself that I'd better remove the old solder altogether and replace it with new eutectic formulation solder alloy. And while I did that, I decided to dismantle everything and do a good cleaning of the PCBs and the various electronic components. In some cases I even took the effort to retest them before reinstalling them.
Just a side note, the presentation order of the various printed circuit board restorations in this article, is not the order I chose to work on in real life. I attacked the cards without any particular order in mind, but for completion purposes, I presented the work in a natural amplification stage order, based on the electrical schematic diagram.
Since I already put a lot of effort in documenting the parts list at the beginning of this thread, I will spare you of the details of parts replacements and I will focus more on the reconstruction process.
To some readers, the work I carried on the AU-20000 might seem a bit overreaching and maybe also a bit on the extreme side. And these readers might be just right. I haven't lost my mind, yet, but I just wanted the best I could do in terms of amplifier restoration. I put all my electronics experience into this project so I hope you will find this article an interesting read. It is worth mentioning that I also learned a couple of new things along this reconstruction adventure.
By the way, in case you're asking, this whole restoration took me around three to four months to complete from start to end. I mostly worked a couple of hours during the evenings and a bit more during the weekends. As you can imagine, compiling all my handwritten notes and sorting through the pictures I took during my work, took me another two weeks or so to write these articles. Nevertheless I had my share of fun and I enjoyed almost every step of the work I carried.
I should begin by saying that working on this amplifier presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for more than 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.
Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also I only use parts that are suitable as replacements in various sections of the amplifier, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.
I have found out that working with a temperature of exactly 300 degrees Celsius is sane for these vintage printed circuit boards. I have never lifted a pad with this temperature. But it is also true that I never wait more than three seconds with the soldering tip on a pad. While working on the chassis, I am pumping up between 360 and 440 degrees Celsius in the soldering iron.
Flux fumes are extremely toxic and should be avoided at all costs. Especially toxic are the fumes released while working on these old Japanese electronic boards.
Mains Power Switch Capacitor Replacement
This high voltage ceramic capacitor is used for spark canceling purposes. Its job is to cancel the electric arc that appears between the internal electrodes of the mains power switch upon switching from OFF to ON and vice versa.
For some reason, the schematic lists this capacitor with a working voltage of 150 V. I think that is a mistake and they probably wanted to type 1.5 kV. Anyway, for safety reasons I used a 2 kV ceramic capacitor instead.
This is the initial situation.
Here's the new Vishay modern ceramic disc capacitor. It is slightly smaller than the old one. In addition, it is coated with a flame retardant epoxy resin that complies to the 94V-0 standard.
Mains voltage is potentially deadly so good insulation is mandatory. Thus I used some glass tubing on the capacitor terminals.
The operation went straightforward and posed no real issues.
Zobel Filters Restoration
Each of the four speaker outputs has a Zobel filter installed directly on the signal and ground terminals. These simple series RC networks are used to correct the speakers impedance. In my unit the Zobel filters appeared to be in good condition. But given the fact that they are installed directly on the speaker outputs, I am thinking that if for some reason, any of the four capacitors goes bad in short circuit, the affiliated resistor will burn in flames in a split second at high power levels.
Here they are before parts replacement.
These are the new Zobel filters. I used glass tubes on the component terminals and the old PVC tubes for insulation purposes. The new 250 V film capacitors will ensure protection at any output signal level as their rated voltage is about six times larger than what this amplifier can supply at its outputs.
And the speaker terminals with the new RC networks.
Details on the soldering job.
While here, I took some scientific impedance measurements of my new Zobel filters. Let's start with the upper row which corresponds to SYSTEM-A.
|100 Hz||31.63 kΩ||32.56 kΩ||2.89 %|
|120 Hz||26.36 kΩ||27.10 kΩ||2.76 %|
|1 kHz||3.16 kΩ||3.26 kΩ||3.11 %|
|10 kHz||319 Ω||328 Ω||2.78 %|
|20 kHz||160 Ω||165 Ω||3.07 %|
And the impedance measurements for the Zobel filters on the lower row, corresponding to SYSTEM-B.
|100 Hz||33.30 kΩ||32.88 kΩ||1.26 %|
|120 Hz||27.75 kΩ||27.41 kΩ||1.23 %|
|1 kHz||3.34 kΩ||3.29 kΩ||1.50 %|
|10 kHz||337 Ω||332 Ω||1.49 %|
|20 kHz||169 Ω||167 Ω||1.19 %|
As a side note, before taking these measurements, I let the Hewlett-Packard 4276A LCZ Meter warm up for around 30 minutes in order to ensure accurate readings.
I have used 10 % tolerance film capacitors which reflects in the impedance differences between the four Zobel filters. I individually measured the 1 % resistors and all of them were pretty tight around the 10 Ω value. However I don't see any issues with these readings. Once the speakers system is connected, the very low impedance of 8 Ω - 16 Ω of a very complex load (hopefully guaranteed throughout the entire audio frequency range) paralleled with the high impedance of the Zobel filters will help stabilize the low output impedance that the power sections have to drive over the entire audio frequency range.
I also added a percentage difference column showing the relationship between the left and right channel measurements at each of the five frequencies. The imbalance is under 5 % for both SYSTEM-A and SYSTEM-B. While I can say that the percentage difference values for SYSTEM-B are better than those of SYSTEM-A, I doubt I could ever hear any difference in the audio program. Sure, high quality laboratory equipment can easily reveal even the slightest deviation, but finally my ears will just act as a filter for any audio signal that my brain has to decode. And taking into consideration the variable (with frequency) human ear signal integration in the time domain, I'd say that these values are perfect.
All speaker terminals are now correctly wired and the whole assembly is ready to be reinstalled on the back panel.
These simple RC networks are easy to service, albeit not mandatory, but their physical position on the back panel makes the job a bit awkward as it involves a bit of mechanical disassembly.
F-2501 Equalizer Circuit Board Restoration
This is a half-crowded PCB that implements the phono cartridge preamplifier with RIAA equalization curve. The other half of the PCB is empty and is used just for signals pass-through.
Here is the initial state of the equalizer board.
Stripping the components off the PCB is a tedious operation.
And a view on the solder side, midway through.
Details on the components. New matched hFE transistors are installed.
I finished the work and the restored circuit board looks as follows.
And a macro detailed view.
Followed by a picture of the solder side.
Working on this circuit board gave me a typical Sansui restoration experience. There are a lot of parts, all crowded on the left side while the right side is just a plain pass-through. Classic old-school Japanese engineering.
F-2505 Flat Amplifier Circuit Board Restoration
This PCB implements an intermediary amplification stage that takes the signal from the audio program sources and further feeds it to the tone amplifier stage and the VU-meter driver circuit.
This is the electrical schematic diagram.
Here is the printed circuit board before restoration. I don't get it why that capacitor is mounted in such a way on the PCB. My initial though was that in its original mounting position, it would've captured some electrical noise. But that is a bit of a nonsense, given this design and the position of the PCB within the amplifier. I will correct that.
And I am making progress. As always, first I am stripping down these PCBs of all the components. I also thoroughly cleaned the corrosive glue.
More progress. I have cleaned the PCB and all parts, individually. Also I installed new wire bridges, new Kapton tape instead of the paper tape, new transistors, and new capacitors.
And here is the printed circuit board after the rebuild.
Some details on my reconstruction of this PCB. I have replaced the carbon composition resistors with precision metal film parts of the same rating. All transistors were replaced with modern equivalents in matched hFE pairs.
More details on the circuit section that drives the tape record RCA outputs. In addition, this stage also serves as input signal for the VU-meter driver circuit.
The solder side looks very good, as expected. Absolutely zero lifted pads.
It wouldn't be a Sansui without bodge wires, additional capacitors, or resistors on the solder side.
In conclusion, this PCB was easy to service. I would've preferred that Sansui engineers would've provided a bit more space so I could replace the 4.7 uF capacitors on the top-left side of the board with film counterparts. But bipolar parts are more than enough in this position. The audio program that would pass through these capacitors will go to the VU-meter driver circuit and to the tape record RCA sockets. Also the ugly resistors on the solder side give this board an amateurish feeling. I think they could've done a bit of a better job when they initially designed this card. But it's a Sansui after all. These kind of corrections (some not even mentioned in the service manual) were very normal for them.
F-2502 Main Volume Circuit Board Restoration
There are no parts that need replacement on this circuit board assembly. But in my unit, this PCB was a bit oily. And especially the volume potentiometer was soaked in some kind of greasy solution. The same oily compound was found under most of the connectors.
Stripped of parts and cleaned.
The solder side is in very good condition. I particularly like the attention to details for low level audio signal propagation: they added ground guard traces interspread with signal lines.
Next I completely disassembled the volume potentiometer. Since it was a fairly dirty operation, I didn't took pictures. But the inner assemblies of the potentiometer were easy to service. On the inside, it was completely soaked with some kind of oily substance and there was a lot of dirt as well. I cleaned the two independent printed circuit carbon deposit resistors and the sliding metallic cursors. Then I reassembled everything in place. There are only four metal clips per circuit section that are used to secure the metallic outer cage in place.
I installed new jumper wires, new Kapton tape and cleaned all the connectors. The restored circuit board looks as follows.
And the solder side looks like almost in pristine state.
This circuit board was very easy to service and despite that I don't like to work on mechanical assemblies, the volume potentiometer and the switches were pretty easy to clean.
F-2495 Tone Amplifier Circuit Board Restoration
This is a straightforward implementation of a triple tone control amplifier stage. It is easy to understand the principle of operation and to choose the best components to replace the aging ones.
Here is the electrical schematic diagram.
This is the initial situation of the tone amplifier printed circuit board.
Making progress: halfway through.
And this is how I proceed. I strip an entire section of the PCB completely of all the electronic components. I clean the PCB thoroughly and then I clean each component. In some cases, I test them individually before I install them back in their respective positions. I always keep track of the component and its original position on the PCB so that I can place it in the exact same spot.
And a view of the solder side. You can see the rebuilt section, the work in progress section, and the original solder joints. Just a side note, I found some very brittle solder joints and some cracked ones. Also in some cases, the old soldering alloy was of bad quality. Especially on the connector solder pins.
Instead of paper tape, I went with Kapton tape beneath the connectors. Since the PCB holes matching the connector terminals are large, the tape helps with the soldering process in such way that it doesn't allow the solder to flow down the pins on the other side of the PCB.
This is how the rebuilt card looks like after I finished my work on it.
Details. I used glass beads as spacers for some of the capacitors. All transistors received a tubular glass bead on the middle terminal.
And a view from another angle. The Kapton tape looks very unobtrusive and very much like it's been there since always.
Even more details.
Finally, here is a view of the solder side.
This is the first PCB that I serviced in this unit. While it was a tedious work, I enjoyed every small step of the process. In addition, the rebuilding process of this card set my overall rhythm of work on this Sansui amplifier.
F-2488 Tone Control Circuit Board Restoration
This printed circuit board assembly contains the triple tone control potentiometers and the support circuitry. The tone amplifier circuit board is interfaced to the tone control circuit board directly through the central pin header array. There is nothing to change here. But I will just disassemble everything, remove the old solder, and clean everything thoroughly, including the mechanical parts.
This is how this PCB looks prior to restoration. A particularity of this implementation is that the clicky individual position tone controls are not actually heavy-duty individual switches, as in case of the Sansui AU-9500, for example. At a first glance, these appear to be classic potentiometers with a ratchet mechanism that helps position the cursor over a dedicated section on the carbon deposit of the potentiometer PCB assembly. However that's not integrally true. The various contacts printed on the wafer actually point to individual sections of the carbon deposited resistors. So there are basically multiple resistors printed on the same PCB wafer. That's clever, but it is definitely not a conventional standalone switch variating device.
Nevertheless, the tone control stack is comprised of an interesting RC network, according to the schematic. The only variating parameter is the resistance while all capacitors are fixed. Again, clever and cheaper than actually switching individual capacitances.
As I started disassembling the circuit board, I found out this oily substance under the connectors.
The bare circuit board looks like this.
And the solder side.
There is one interesting detail. The interrupted track is not a mistake. That is a ground guard trace interrupted in order to avoid a ground loop. Sansui engineers of 1975 were very attentive to such details.
I continued with dismantling of all potentiometers and cleaned the individual wavers of each switch assembly. On the left of the image, the PCB is cleaned of oil and corrosion while on the right of the image, the waver is dirty and corroded. No wonder there were signal interruptions or audible noises while operating the tone controls before the restoration.
I didn't took more pictures since this was a dirty operation and I don't particularly enjoy breaking my work rhythm, washing the hands, and taking pictures in the process.
The restored PCB looks like this now.
And the solder side.
Together with the tone amplifier circuit board.
And a view from another angle.
I don't particularly enjoy servicing boards that involve mechanical controls. But I know that the amplifier will deliver its best performance, only if each and every component is working properly. And since initially this unit had some noisy potentiometers and switches, I decided to follow the long road.
F-2496 Meter Circuit Board Restoration
The schematic implements a dual mono VU-meter driver circuit. As a particularity, this is the only part of the schematic where integrated circuits are used. There are two operational amplifiers, one for each channel. I can imagine these must've cost a lot back in the day. Another particularity is that the supply voltage is dropped via two power resistors and then regulated to ± 15 V with Zener diodes. Otherwise, the schematic is straightforward.
First I removed all the wires soldered directly on this PCB. In my opinion, the circuit layout is not very professionally designed. At least comparing to the other PCBs in this unit. But it does it's the job, as intended.
Then I noted the position of every wire and proceeded to dismantle everything. Next I cleaned everything thoroughly. The resulting bare PCB is pictured below. There is some slight heat discoloration around the power resistors and Zener diodes.
And the solder side. This circuit board is in very good condition, both mechanically and optically. It is not warped like most of the printed circuit boards in this amplifier.
Making visible progress by soldering various passive components and the two operational amplifier integrated circuits. Kapton tape covers the surface under the connectors and the switches.
A view from a different angle. I have used all new connection bridges.
And a picture with better lighting conditions.
I soldered most of the parts already.
Here are some details on the MV13SA varistor diode replacements.
And a detailed view of the densely packed section. There are a lot of parts crowded in a small place.
In the meantime I dismantled all switches and cleaned the contacts both on the inside and on the outside. I didn't took pictures since I don't particularly like to operate the mobile phone or the digital still camera while having my hands dirty with various solutions that I use to clean the mechanical assemblies. This kind of Alps switches are very easy to clean. However I don't know what are their contacts are made of, since they gather some kind of corrosion that resembles the one developed on the terminals of some transistors of the era.
This is the reworked circuit board.
A view from a different angle shows some more constructive details.
And the solder side.
Detailed view on the trimmer resistors.
Finally, I soldered back all connection wires. The audio signal wires are connected on the solder side of this circuit board.
This was a fun PCB to rework and also pretty different than the rest of the circuit boards in this unit. In some ways, it brought back memories from when I was taking my first steps into the electronics realm. The simplicity of the schematic diagram cumulated with the implementation of the circuit layout makes me think about a VU-meter driver that I built back in the late '90s.
F-2507 Driver Circuit Boards Restoration
The power amplification section of this amplifier is constructed around two push-pull amplifier stages configured as independent dual mono driver cards. The schematic diagram for one driver card is available below.
The left driver printed circuit board assembly in its initial stage.
And everything went away. I cleaned the PCB thoroughly. The glue was pretty stubborn but I removed it after all.
And the solder side. Unfortunately, previously it was a bit scratched by the factory workers during the initial assembly phase. But fortunately, there were no lifted pads and no thermal or mechanical issues.
Next I started to plant the components. I also added new Kapton tape under the connectors. All small signal diodes received glass tubes on their terminals. Ceramic capacitors are seated on glass beads. The input stage coupling capacitor was replaced with a film part of the same rating. All wire bridges are new as well.
All resistors, non polarized capacitors, small signal diodes, and variable resistors are installed. I also replaced the VD1212 diode with two 1N4148 diodes in series.
Almost all electrolytic capacitors are now soldered in their respective positions.
The completed driver card.
And the solder side.
Details on the input section.
Details on how I replaced the VD1212 diode by two 1N4148 small signal diodes in series. I used some heat shrink tube to protect the solder joint.
The right channel driver printed circuit board follows next. This is the initial situation.
I removed every electronic component and cleaned both sided of the circuit board.
In terms of factory scratches, this board looks better than that of the left channel presented above.
Making visible progress by advancing with the population of the PCB.
Detailed view of the power section, showing new components alongside verified and cleaned old parts. All are ready for years of service.
Detailed view on the input section. We can see the same mixture of verified old and new components.
And here is the completed right channel driver circuit board assembly.
The solder side looks good.
Again, details on the VD1212 replacement.
And a detailed view on the trimmer resistors section showing the new Bourns single turn, cermet, industrial, sealed variable resistors. Even though I prefer multi turn variable resistors, the 3386 series are a more appropriate fit for these old Sansui amplifiers.
I enjoyed working on the driver circuit boards as I found them straightforward and easy to follow. However, I truly disliked cleaning the hardened glue.
F-2497 Power Section Circuit Boards Restoration
The power section is implemented using six power transistors on each channel. The supporting PCB contains the emitter resistors and a PTH thermistor. One particularity of my unit is that the bias transistor is not located on this power section PCB, but directly on the pre-driver transistors heat sink on each of the two driver cards.
Let's start with the left channel. This is the circuit board in initial state.
I removed all components and cleaned the dual layer PCB thoroughly.
On both sides.
Each and every via between the two layers of this circuit board is done via rivets, additionally filled with melted solder. I already cleaned most of the old solder from these rivets and will apply new eutectic solder alloy.
I replaced the power resistors and applied new solder. I measured each of those old Micron power transistor emitter resistors and I got readings in range 450 mΩ to 490 mΩ. The new MIL-SPEC precision power resistors are all measuring within a steady range of 469 mΩ to 470 mΩ.
There are three power resistors on the other side of the circuit board.
Details on the power resistors.
Next follows this small connector circuit board that is coded F-2510. Normally there is nothing to do here. But I reworked it anyway. I removed the wires and the connectors. Then I cleaned the board.
Including the solder side.
Then I added new tape and soldered back the connectors.
And the solder side.
Next I soldered the wires.
Positions 7, 8, and 9 are unused in my unit. These would've been used by the bias transistor on early revisions of this amplifier.
The complete left power section assembly is too large to fit on my usual photographing spot on my workbench due to space constraints and lighting conditions. So I photographed it outside.
I replaced the old mica insulators with new ones and I added just enough thermal paste to ensure a good heat transfer without clogging the transistors or their terminals. While I used a quality thermal paste that I ordered from Mouser, I don't really remember its name. I don't like transistors bathing in thermal paste. Besides the ugly looks, the performance is suboptimal. The paste should fill in all discontinuities in the heatsink and the transistor metallic capsule so that it ensures a good thermal contact between the capsule and the mica insulator, and finally the heat radiator itself.
And now let's continue with the right channel. This is the PCB in its initial state.
Then I removed everything off this board and did a good cleaning. I extracted the old solder from each of the through hole rivets.
This is the other circuit side.
As with the left channel, I measured each of the old emitter power resistors. The lowest value was 444 mΩ while the highest reading was a spot on 470 mΩ. The new DALE resistors are measuring between 469 mΩ and 470 mΩ on my LCZ Meter.
And the other side of the PCB with the remaining three power resistors.
Again, a detailed view on the precision power resistors.
Next up is the corresponding F-2510 connector circuit board for the right channel power section. This is the cleaned circuit board.
And this is the solder side.
I installed new tape, and soldered back all the connection wires in their respective positions.
Here is the restored solder side.
This is the complete right power section assembly. Each of the power transistors received new mica insulators and just about sufficient thermal paste to ensure an optimal heat transfer.
Detailed view on some of the connection wires. Hidden inside the heat sink assembly is the power resistors array.
Better focus on the resistors. That's an interesting design implementation.
I enjoyed working on these circuit boards but I absolutely hated cleaning the heat sinks of old thermal paste remnants. I also disliked cleaning the power transistors of the same solidified thermal compound. But in the end, I completed both power sections, albeit with significant effort.
F-2508 Regulated Power Supply Circuit Board Restoration
An implementation of a classic dual linear dual-rail regulated power supply implementation capable of supplying ± 65 V and ± 35 V.
Let's start by examining the electrical schematic diagram.
The initial situation is depicted in the following picture. Immediately we can see some overheated capacitors. Other visible defects include the nuts for the two 2SD382 transistors which make an intermittent contact with the solder on the appropriate PCB pad. And as a matter of course, the omnipresent Sansui corrosive glue. Eventually the VD1212 failure-prone diodes are clearly visible in the lower left corner. Their package reminds me of some old Soviet diodes that used to fail in my old Rostov 105 S-1 reel to reel tape deck.
I was very curious how the most thermally stressed capacitor on this board measures up on the LCZ Meter. It is a Nippon Chemi-Con 10 uF / 50 V capacitor that measures a perfect 11.85 uF @ 120 Hz and registers an ESR of 5.36 Ω. I am impressed and I decided to open it up. Internally it was in very good condition and the paper layer was still well impregnated with electrolyte.
But let's make some progress with rebuilding of this PCB. Stripping down most of the parts reveals a messy corrosive glue leftover.
First I applied some heat and mechanically scrapped off glue remnants. Next I went with my trusty method of scrubbing the glue with acetone on cotton swabs. It takes some time and it kind of stinks but it works in the end.In fact the PCB appears to be in an almost like new condition. Well, despite the heat-discolored areas beneath the 2SD382 power transistor radiators.
Whoever assembled this circuit board in the first place was a bit careless and managed to scratch the soldermask in multiple places. This is all factory damage as there are no traces that anybody did any rework on this PCB ever since.
The discoloration is superficial and although it is not pretty, the PCB is not structurally affected in any way. An interesting heat spot is indicated by the position of C18. The source of this discoloration is questionable though and I wouldn't necessarily link it to a malfunctioning capacitor. Most probably it's a side effect of the glue used to prevent the capacitors from flying around while wave-soldering the board at the factory. Then I ask myself why the other capacitors don't show this pattern?
I am slowly advancing with the planting of the various electronic components. I also added new wire bridges and new Kapton tape under the connectors. I already installed some of the transistors and added tubular glass beads on the collector terminal. A trained eye will spot the replacement of the VD1212 diode with two 1N4148 diodes in series.
I added a small piece of heat shrink tube on the serial diodes solder joint for extra protection and looks. The diodes are installed on glass beads for some spacing against the PCB.
Let's zoom in a bit to see the tiny diodes.
Here is an overview of my work so far.
And the finished circuit board.
A view from another angle.
Detailed view on the replaced power resistors and the header for the 7 W power resistors that are mounted elsewhere in the amplifier.
Here is the solder side.
And some more details on the protection diodes. Those are all factory scratches. I didn't do any of them.
There is another protection diode located here. I have my reasons to think that nobody previously serviced or even inspected this card. Thus I would say that these are some genuine Sansui factory scratches.
Let's finish the F-2508 regulated power supply PCB restoration with another macro picture of my work.
Besides cleaning the dreadful glue, the rest of the circuit board was manageable. As I said, the layout might have been better in terms of design. But it's also true that I've seen worse power supply designs.
F-1512 Connector Circuit Board Restoration
This is a simple albeit fairly large circuit board that contains four electrolytic capacitors and a lot of connectors for two the driver cards and the regulated power supply card. On the solder side there is a metal fixture that secures a small point to point PCB that holds two power resistors. All connection wires are soldered directly on their respective terminals on the solder side.
I didn't took pictures of the initial state. However it was very dirty with a lot of hardened dust. Some of the solder joints seemed brittle but they still ensured very good contact. I removed everything and cleaned the circuit board on both sides. I also cleaned all the connectors.
Attention to details: I cut some rectangular holes in the Kapton tape for the connector pins.
This is the restored circuit board.
Detailed view on the new capacitors. Unfortunately I couldn't find three terminal capacitors with the same raster like the old ones. But I found some snap in capacitors that fit well, even though they will not be exactly centered within their corresponding holes in the steel chassis.
I cleaned the solder side and now everything looks ready for years of service. There are some factory scratches that were already filled with solder from the wave flow soldering process.
Next I reinstalled the PCB on the metal tray.
And a view from a different angle. It is easy to see that tha snap in capacitors are not physically centered in the steel tray holes. But they fit with a remaining clearance of one or two millimeters -- which is perfect.
I dismantled everything bar the connection wires. I sensed trouble and I avoided disconnecting them. Otherwise the PCB came up pretty nice and it took me only a couple of hours to restore it.
F-2503 Tape Copy Circuit Board Restoration
This circuit board implements the tape copy switching circuitry. In addition, it introduces an audio buffer between the flat amplifier (or the tone amplifier, if switched) and the filter circuit section. The schematic is a bit larger than what is depicted in the following picture. But the missing section contains just a bunch of switches that only require a good cleaning. On the other hand, the amplification stage has four transistors and six capacitors that are due for replacement.
There is one issue, though. On the solder side, there are a lot of audio signal wires, directly soldered on various pads. This makes life difficult. First I cataloged all wires and carefully noted their connection points.
After I removed all wires, this is the initial situation.
The circuit board stripped of all components.
The solder side cleaned up well. There were some scratches from the factory. I took the chance to repair them by filling with solder. That's why they appear to be bright white. As a particularity, there is a factory cut on one of the ground traces that leads to one of the Molex connector pins. I also took the liberty to completely tin the exposed copper ground isle.
I prepared the circuit board for restoration by installing new Kapton tape where needed.
The completed buffer stage.
And a view from another angle. New tape, new transistors, new capacitors, and a new wire bridge. Old components have been cleaned and tested.
I cleaned the switches and I didn't took pictures. They were dirty but not as bad as I initially thought. Finally the restored circuit board is depicted in the following picture.
And this is the solder side.
In the end, I soldered back all audio signal wires on their corresponding pads.
Although I initially thought this would be one of the most annoying circuit boards to work on, in the end it proved to be the contrary. Once I labeled all audio signal wires and carefully desoldered them, I found out this PCB was easy to service. And I really like the end result. Reattaching the wires proved to be straightforward, given my annotations on paper and the various pictures that I took before removing those twelve audio signal wires.
The switches cleaned up very well but I skipped the balance potentiometer. I don't feel that confident to disassemble that kind of potentiometer. I'm sure that it can be done, but it is just not my call. I can live with that as I operate the balance knob as frequent as rarely or even never.
F-2504 Filter Circuit Board Restoration
This PCB acts like a mainboard for the two F-2498 filter amplifier circuit boards. Also it contains the switches that operate the high and low filters. There is also a small signal relay that briefly turns the preamplifier outputs off at unit power-on, or completely off while the protector detects a fault.
This is how the printed circuit board looks like before the restoration.
First I removed everything and then I cleaned the PCB on both sides.
The solder side is in very good condition, without any scratches or lifted pads.
Making preparations for the components soldering phase.
But in the meantime I have cleaned the small contacts inside the filter switches. I have used a chemical solution that eats away oxides and sulphides and then I washed the contacts with isopropyl alcohol.
In the end, the switch assembly came up pretty clean.
I cleaned the other switch as well. I also disassembled the headphones jack and cleaned it with the same method. I didn't took pictures as I had my hands dirty with various chemicals involved in the cleaning.
I finished working on this printed circuit board.
The solder side looks as depicted below.
Local filtering capacitors have been installed with glass beads on the terminals since one of the PCB rasters is a bit larger than the capacitor lead spacing. Otherwise there is absolutely no benefit of choosing such a method.
While I don't particularly like working on mechanical assemblies, these Alps switches proved to be serviceable. However I don't recommend opening them more than once. Thus I would expect that after the next 40 years, whomever might open them, will probably severe the small metal tabs that are holding the lower PCB assembly of the switch. However this kind of power hungry linear audio amplifiers might be as well part of a museum by then.
F-2498 Filter Amplifier Circuit Boards Restoration
The electrical schematic diagram is very simple and easy to understand.
Here is the first filter amplifier stage. The board implements a simple stereo amplification stage with two transistors per channel.
Bare PCB that was stripped of all electronic components.
And the solder side. There are some factory scratches. I cleaned the circuit board of old gunk and dirt.
Then I applied a stripe of new Kapton tape.
A view from a different perspective. These brownish circuit boards are made of some sort of epoxy impregnated cellulose or similar material. Once heated, they emit an interesting chemical scent that I have always associated with old audio-class Japanese electronics.
Finally, I have reinstalled the electronic components. All old resistors are in place, all transistors are brand new and the electrolytic capacitors have been replaced with film parts.
All good on the solder side. Eutectic solder allows for very high quality solder joints.
And now comes the second filter amplifier stage. The schematic diagram and actual implementation is identical to the first filter amplifier circuit board that was presented above.
Again, stripped of all components.
The solder side looks better than that of the other PCB.
I installed all parts.
And the solder side looks great, as always.
Both filter amplifier circuit boards are ready for another lifetime (or more) of service.
A detailed view shows that all transistors received tubular glass beads on their collector terminal while all ceramic capacitors are seated on miniature glass beads as well. The 4.7 uF film capacitors are of the same height as that of the former electrolytic capacitors.
I particularly enjoyed working on these small PCB assemblies. I think these are the smallest industrially built circuit boards that I ever serviced.
The filter circuit board with both filter amplifiers circuit boards installed looks like this.
And a view from a different angle.
The filter PCB assembly with individual vertically mounted modules, reminded me of a radio transceiver that I was building in the early 2000s. I designed the mainboard with individually installable audio preamplifier modules for the main audio chain and the VOX control. I even found a low quality picture that I took back then. I cropped and attached it to this article so that you can see those modules below my analog CRT monitor (with screen protector glass) that I had back then. There were three small PCB assemblies. Two of them were audio preamplifier boards and the third one controlled the input of the VOX circuit for automated switching between transmit or receive modes based on whether I was talking or not. So basically there was no PTT circuit involved thus the voice (or telegraphy) transmission was fully automated. I still have the hand drawn circuit layouts on paper, however I lost the schematic diagrams. Dusty memories...
But anyway, coming back to the Sansui. The next step was to solder the wires where they were originally connected and install this assembly back into the amplifier. That's it for this section. Let's move further.
F-2511 Protector Circuit Board Restoration
This is a fairly large printed circuit board with a lot of connection points and a lot of assorted parts and sub-optimal layout every here and there. The schematic diagram is fairly simple for such a large board. Most of the PCB space is used by fuses, relays, and various mechanical connection points.
Somebody was here before. He changed almost all factory-installed carbon composition resistors with an assortment of carbon film and metal film resistors. Also all capacitors were previously replaced with Panasonic series. All transistors were replaced with European models from the BC series with a different terminal disposition. Most small signal diodes were also replaced with 1N4148 diodes. The previous technician worked mostly in a clean fashion but he still managed to lift some pads! I'm glad the technician wasn't a butcher (like I've seen in some other cases) but I would've preferred he was more careful.
According to my style, I will do a correct restoration worthy of the Sansui name. Remember their slogan? Better than the best. Everything has to go out, in the first place. Removal of the mechanical sub-assemblies that are present on this PCB, cannot be done without a good thermally regulated soldering station and a very good desoldering pump. These are mandatory tools in order not to lift any pads. Well, experience also plays its part in this situation.
The solder side presents some minor scratches but otherwise is in very good shape.
However, this printed circuit board is very warped. Either it was like that from the start, or it became warped due to heat, which I find highly improbable.
Previously lifted tracks are visible here.
I cleaned everything and then I started to reassemble the protector circuit board. First I soldered the mechanical connectors. All rectifier and small signal diodes followed next. So far I soldered some other resistors and capacitors. Can you spot the Kapton tape under the connectors and on the relays terminals?
Everything is now correctly soldered in place. All new Dale precision resistors have replaced the old ones. All small signal transistors have been replaced with modern equivalents. New high temperature capacitors are installed. Some power resistors have been changed. Given the tremendous power per channel of this amplifier, I find the protector unit very important in all aspects. I wouldn't want it to fail at its job and fry something in the amplifier or worse, destroy the speakers. That is why I chose to replace everything that was prone to failures or appeared to be tired.
Details on the power resistors and small signal diodes.
More details on the high voltage power resistors used for the soft start circuit of this amplifier.
Some more details on the other new components.
Finally let's inspect the solder side. I repaired all pads lifted by the previous technician.
On the solder side I replaced the decoupling capacitor and the reverse-biased suppressor diode for the relay coil on the tape copy circuit board.
Finally I have installed the protector circuit board on the steel tray that contains the toroidal power transformer. Pictured below is the entire power conversion section. This is how a good 400 W linear power supply should look like. It is literally dwarfing my workbench. For references, the cutting mat is in A2 size. The toroidal mains power transformer is mounted on the other side of the steel tray and accounts for around 8 kg of the total weight of this unit.
Initially I thought this PCB would pose some big problems and I will end up damaging (lifting) some pads. Fortunately that proved wrong and I managed to work on it without bringing even the smallest damage to the solder side. And while it initially appeared to be complicated and challenging, it ended up being pretty easy to service. And rewarding too, as I was making visible progress with each step I took.
Just one more update before moving on. I decided to replace the two cement filled soft start power resistors with new parts. Due to thermal stress, the resistance could go up in time. In fact, in my case, both were measuring around 5.30 Ω. While I don't think this is an issue for now, it might become a problem in the future. As for replacements, I went with high quality Vishay / Huntington resistors, rated 4 Ω / 7 W and 1 % tolerance.
Also I made an improvement to the LED indicator on the front panel. I observed the circuit used to light up the protector LED indicator and there are provisions on the circuit board for an additional resistor (R25) that can be installed in order to allow for a bi-color LED indicator. This way, the same LED can signal both power-on (green) and protection (red) states. I went for a two terminal bi-color LED with a wavelength of 569 nm for the green color and 621 nm for a red-orange color.
I got this idea from the Sansui BA-3000 power amplifier. R25 is rated 560 Ω / 2 W in the BA-3000 service manual. I used a 3 W part instead. Got to love the efficiency of power resistors in these kind of circuits. But I assume that back in 1973, this method was cheaper than using dedicated small signal transistors for lighting an LED alone.
It would've been nice if I had done these modifications from the start. At least I did them before actually reassembling the entire amplifier steel chassis around the power supply core chassis.
F-2500 Input Circuit Board Restoration
This circuit board is located on the back of the unit, on the rear steel panel. While it should be easy to remove, in fact it is not. There is a plethora of audio signal wires soldered directly on the RCA terminals on the solder side. Also there is a high chance these RCA connectors are corroded, like in my case. Now comes the tricky part since in order to remove this circuit board, it is not only necessary to undo the screws, but you have to remove a plastic chuck that is installed on top of each of the RCA connectors, additionally securing the socket to the back panel. I had to use fine machine oil to ease the removal of these plastic parts. This was an operation that I totally disliked.
The schematic is pretty simple and there is no component to replace. However given the corrosion state of the RCA connectors, I decided to disassemble everything and thoroughly clean the PCB and every mechanical component. This means I will also replace the old solder with new eutectic formulation soldering alloy.
This is the PCB before restoration. First, I carefully documented the pad where each audio signal wire was previously connected, prior to dismantling.
There were some cracked solder joints like those in the following picture. However, given the fact that each of the RCA connectors are using three ground terminals, there is no immediate problem if one of them is making intermittent contact. However when two of them are intermittent, then the intended redundancy is beginning to look questionable.
And another cracked solder joint, in a different circuit section. There were a few others as well. There is some ground copper plane corrosion trapped beneath the soldermask in the upper left section of this circuit board. There is nothing I can do in this situation without dramatically altering the original looks of the PCB.
I stripped the circuit board of all mechanical and electrical components.
This is the bare PCB solder side. In some cases, the soldermask wasn't completely lifted for some of the RCA connectors ground terminal pads. These minor mistakes were quite common in some other vintage Japanese audio gear as well. But that is not a real issue as I can still do almost perfect solder joints even in this situation.
I installed new wire bridges, added new Kapton tape, and soldered all carbon resistors.
Next I cleaned the two switches.
And I individually cleaned every RCA connector. The finished PCB now looks as shown below.
On the solder side there is one wire bridge that connects two ground planes.
Finally I soldered all of the audio signal wires as per my documentation on where they initially were. There are also two ceramic disc capacitors mounted directly on the solder side. An additional ceramic disc capacitor is installed between the steel frame and a certain ground spot on this PCB once it's put back in its place on the rear panel.
And the complete RCA inputs assembly, viewed from the top. Between the circuit board and the steel cage there is a black plastic material that prevents short circuits between various exposed metal areas and the ground signal represented by the chassis.
This PCB took a lot of time to complete due to the individual polishing of the RCA connectors. Otherwise, from an electrical point of view, despite the audio signal connection wires, everything was pretty simple and straightforward. Chronologically, this was the last circuit board that I serviced.
Here are the old parts that I replaced in this unit.
A lot of solder-related waste resulted after this rebuilt. I'd guess it's around half a kilogram of metallic waste, including component terminal remnants and used desoldering wick. I used two desoldering pumps (a heavy-duty one and a precision pump) and around four meters of desoldering wick.
Many of us might question the effort involved into replacing all these parts and cleaning everything. Especially since the amplifier was in a (quasi-)working state. However I stand by my reasons regarding vintage audio gear and I think that I will better enjoy the amplifier in its new, restored state.
This is the end of the main electrical restoration (rebuild!?) work. If you read up until here then either you don't get bored so easily or you really like electronics. Either way, you get a glimpse of what this whole process was like.
The following articles will treat other operations that I carried on this unit, some data collection as well as a few personal thoughts.