Sansui AU-9500: Restoration
Normally when starting the restoration process I organize all the parts so that I can quickly replace the faulty ones in the shortest time possible. And this is what I did in this case.
Here is a batch of Panasonic stacked film non polarized capacitors. Very good for stage coupling. They are rated 1 uF / 50 V. As you can see, two of them are already missing because they were just mounted in the equalizer block.
And here are about a hundred KSA992 transistors. I am using them in hFE matched pairs that I discover with my transistor tester. I ordered a lot of them because they are very cheap and having more than necessary is perfect for sorting purposes. Careful with these transistors as their terminals are designated as ECB. The old 2SA726 are BCE. So the replacements must be mounted in reverse.
Next I scored about a hundred 2SC1845 transistors. I am using them in sorted by hFE pairs as well. Also care should be taken with these transistors as their terminals are designated as ECB. The old 2SC1313 are BCE. So the replacements must be mounted in reverse. A great but weird particularity of these transistors is that I have measured about 20 of them from the entire lot and all have the same hFE of 428. So no need to sort. I will however test them before soldering.
Sorting transistors, however, is a boring task that requires a lot of patience and time.
This thing is dwarfing my workplace. I had a hard time positioning the unit on my bench.
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 almost 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 323 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.
Power Supply / Chassis Restoration
On the chassis there are four really large filtering capacitors. I have tested these against the specifications and they are doing fine. So no replacement for these yet. But I am already searching at my favorite electronic parts store for future-proof replacements. While ordering spare parts I need to be really careful so that they will be identical in diameter and hopefully better than the originals on ripple current filtering. I am still searching and once I will find what I am looking for, I will revisit this page.
However there are some capacitors that I have decided to change. I am talking about C010, C011, C012, C013, rated 4.7 uF / 100 V in the schematic. In my unit they are 3.3 uF / 100 V. I have used Nichicon MUSE KZ series replacements rated 10 uF / 100 V. These capacitors provide local filtering and decoupling of the supply rails for the two driver boards. So bigger is better in this case. I was thinking about bypassing these with 100nF / 100 V non-polarized polypropylene capacitors to cut up on high-frequency oscillations, should they appear. But I have had a hard time observing such oscillations with my 100 MHz oscilloscope. Thus I quickly abandoned this idea.
Left chanel before.
Right chanel before.
It is fairly easy to work on the chassis. But I have used a high power 80 W soldering iron because of the steel chassis. It works like a massive heatsink and the smaller irons will be quickly drained of caloric energy. A large tip helps as well.
F-2030 Power Supply / Protector Board Restoration
This board is really easy to remove but you need to note the order of the wires that are attached to it. Besides four soldered wires, all the other wires are using connectors. I have used a 1 mm permanent marker to mark the order of the cables. Before mounting the board back, I will use isopropyl alcohol to wipe clean the cables so that my markings will be erased. The four heavy duty power wires which are soldered are very easy to desolder using a vacuum pump and a powerful iron. I managed to easily desolder them without damaging anything, in about two to three seconds of heat application.
Inspecting the board reveals that there is a number of twelve electrolytic capacitors. I have decided to let alone most of the transistors on this board. Beware that this board has very large ground planes so soldering and desoldering capacitors is difficult to impossible for a low power soldering tool. What I use is a Weller 80 W electronically controlled temperature soldering station with appropriate tips. You do not want to damage tracks of these unobtainium printed circuit boards. So good tools and a lot of precaution is necessary for a clean job.
The board prior to the restoration.
Detailed view on the relay and the coils. In my case the OMRON relay is in very good working order. And it appears it is the original part. Now either this amplifier was not used that much or I don't know. I decided to let it as is. Should you want to replace it, OMRON MY2-02-DC24 is the modern counterpart. It is expensive but it worths every cent.
I used Nichicon MUSE KZ series capacitor replacements for the old ELNA ones. These capacitors are fairly large in diameter and also in height. I dare to say that in most cases they are even the same size as the original parts. So I changed the two 220 uF / 75 V capacitors with 220 uF / 100 V capacitors. Then I changed the two 100 uF / 75 V capacitors with 100 uF / 100 V parts. C057 was rated at 470 uF / 16 V. The replacement is 470 uF / 25 V. Regarding the replacements for C060 and C062, I have used the same 220 uF / 50 V rating for both of them. C907 is used as filter for the protector circuit. It was originally rated at 100 uF / 50 V. I replaced it with the same rating modern counterpart. In the protection circuit, C901 and C902 were bipolar capacitors, originally rated at 47 uF / 10 V. I replaced both of them with Nichicon ES series 47 uF / 16 V bipolar parts. C908 is a 1 uF / 50 V bipolar capacitor. I have replaced it with the same type modern part. C906 is part of the relay delay circuit and was originally rated at 1000 uF / 6.3 V. My replacement is a Sprague part rated at 1000 uF / 16 V and 105 ℃.
The board after the job.
Then I have proceeded to renewing of all solder joints on the back side of the board. I am using quality eutectic solder.
F-2029 Driver Board Restoration
These two modules are the easiest boards to work on of them all. Only four capacitors and two transistors to change per board. There is also a thermal fuse glued onto a small signal transistor. If you want you can take care of that too. I did as there is no apparent need for a thermal fuse there. Here is the board prior to refurbishing.
The filtering capacitors are designated as C815 and C817. Originally they were rated at 47 uF / 80 V. I have used Nichicon MUSE KZ series rated at 47 uF / 100 V. They fit like a glove. C801 is used as an input capacitor and is rated at 2.2 uF / 50 V. I used a Nichicon FG series capacitor of the same characteristics. C807 is originally rated at 220 uF / 6.3 V. I have replaced it with a Nichicon MUSE KZ series capacitor rated at 220 uF / 25 V.
Transistors TR801 and TR803 are the input differential pair and are of type 2SA726. They are known to develop annoying hiss and crackling sound in time. As a matter of fact, this Sansui amplifier cracks after a few minutes of use. Also it has a disturbing hiss when idling. I don't want to take any chances and I have decided to replace these transistors as well. I have used hFE matched KSA992 transistors.
As said above, I also took care of the fuse. The glue is very corrosive so I thoroughly removed it with isopropyl alcohol. The fuse is mounted in series with a resistor. I have removed the thermal fuse and soldered the resistor directly in its place, as factory-marked on the board.
And here is the board after the restoration.
Here you can see a detailed view of one of the 47 uF / 100 V replacement capacitors.
On the soldering side I carefully looked for faulty joints. There were none. But I am preparing for the long future so I renewed the solder joints using the eutectic soldering alloy.
The other channel is following as well. This is before restoration.
Detailed view on the thermal fuse. I experimented a bit after dismantling this fuse. So I triggered it with the soldering iron. Once popped there is no means to reset. Or so I think.
And after the job.
Detailed view of the hFE matched KSA992 transistors.
The other channel enjoys the same soldering renewal treatment.
F-2006 Equalizer Board Restoration
Removal of the board is extremely simple. The procedure is to first remove the two side screws holding the steel Faraday cage. The the cage itself. After that you gently pull the board upright out of its socket.
Working on this board is easy as well. There are a twelve capacitors and at least two transistors to change. Why at least? Because there are two 2SA726 transistors which are mandatory to change. But there are another two 2SC1313. While these are not yet causing issues, as a prevention, you can change these with 2SC1845. I decided to change them as well.
This is the board before restoration.
C601 and C602 are the input stage coupling capacitors. Originally they were 1 uF / 50 V, electrolytic. I have changed them with 1 uF / 50 V Panasonic stacked film capacitors. C603 and C604 which were originally 10 uF / 10 V were replaced with 10 uF / 16 V Nichicon ES series bipolar capacitors. C608 and C609, originally 47 uF / 6.3 V were replaced with 47 uF / 16 V Nichicon ES series bipolar counterparts. C613 and C614 are used to stabilize the static functioning point of two transistors and were originally rated at 47 uF / 6.3 V. I have used 47 uF / 25 V Nichicon FG series replacements. C615 and C616 are part of the RIAA correction network and originally were 4.7 uF / 50 V. I have replaced them with 4.7 uF / 50 V Nichicon ES series bipolar parts. The output coupling stage capacitors, originally rated at 3.3 uF / 50 V were replaced with 3.3 uF / 50 V Nichicon ES series bipolar capacitors.
On the back side I have renewed the solder joints.
F-2019-1 Tone Control Board Restoration
Removal of the board is extremely difficult. The procedure involves removing of the power supply board. Then the metallic shielding cage. After you have access to the solder side of the board which is bolted to the chassis with six screws. There is another fixing point in the left side, near the local filtering capacitors. This one requires a powerful soldering iron to detach. But this is not all. There are also a whole lot of wires attached to the solder side of this board. So I decided to let this board alone and adopt a different approach. I have positioned the amplifier so that I can easily reach the components from beneath, while having enough space to work on the solder side. It is an awkward job but the wires were untouched.
This is the board before restoration.
C701 and C702 are the input stage coupling capacitors. They are rated 470 nF / 25 V and are of tantalum type. I really cannot imagine why they have chosen this type of capacitor when it is clear that had the oversized mylar film green drops in production. Better dielectric for audio purposes and less problems caused should these capacitors fail. I have measured these old tantalum ELNA capacitors with my tester. Capacity is ranging from 460 nF (left) to 570 nF (right) while ESR is ranging from 29 R (left) to 120 R (right). Clearly out of specifications. Because I forgot to order replacements for these along with all the other parts, I have sorted through my capacitors box and found two Philips MKT orange drops rated 470 nF / 63 V. They fit like a glove on the PCB. On the tester, ESR is 0.9 R and leakage is 0 for both. Measured capacity is 467 nF. Even though they are not new, I believe they do qualify. Regarding their sound, it is fine. I like it. I have some experience with these capacitors in various tube amplifiers that I have built in the past. No disappointments yet.
The local emitter decoupling capacitors C703, C704, C705, C706, C719, C720 were originally rated 47 uF / 6.3 V. I have used Nichicon MUSE KZ series replacements rated 47 uF / 25 V. Stage coupling capacitors C709, C710, C723, C724, C731, C732 were rated 4.7 uF / 50 V. Nichicon FG series 4.7 uF / 50 V were used as replacement. I have used the same counterparts for C717, C718 2.2 uF / 50 V stage coupling capacitors. The local filtering capacitors C733 and C734 were 220 uF / 35 V. I have used Nichicon MUSE KZ series capacitors rated 220 uF / 50 V. The same for C799, originally 220 uF / 25 V, now replaced with 220 uF / 35 V.
There are a also six 2SA726 and six 2SC1313 transistors to change on this board. I have used hFE matched pairs as before.
Overall this board is very boring to work on. It took me at least four hours to exchange all the parts, due to the difficult clearance and maintenance access. Progress is very slow and barely visible. But I finished it after all. Here it is.
On the back side I have renewed the solder joints.
Beautiful track layout.
This board concludes the replacement of parts in this audio amplifier.
While having this amplifier on my bench I also redid the soldering of various other joints on all the other boards scattered around. I am not going to step into details as there is nothing interesting. But here are some pictures.
Also these packs of RCA connectors were suffering from a transportation hit.
The repair was done with spare parts from disassembled units. I got the parts from eBay. In the end it looks like this.
All good on the other side as well.
This is everything that I did on this amplifier in the electrical field.
Cleaning the Front Panel
If you ask me, besides the sonic qualities, this is one of the most important things. I cannot stand dirty units. I believe keeping them dirty is equivalent to disrespecting them and the work that was put on them. So I proceeded to disassemble everything on the front panel and clean up all the parts.
Now it is time to put back the cover of this amplifier and off I go for another 40 years.