Sansui AU-7500: Restoration
As always before I start working on a vintage unit restoration, I organize all the parts that will be changed. Further more I am sorting transistors based on the hFE factor. This is a boring job but it pays for itself. Here I am sorting Fairchild KSA992 transistors. Careful with these transistors as their terminals are designated as ECB. The old 2SA726 are BCE. So the replacements must be mounted in reverse.
An array of Panasonic stacked film capacitors. Good quality.
This is one of the most easy to repair of them all amplifiers. I have seen quite a few units during my restorations but none like this. Nearly all boards can be serviced directly in place. Not a screw to remove. My way of working on this is to put it upside-down on the workbench. I can reach with my hand under the chassis all the components that I need to service. On the soldering side there is enough clearance to work in a clean manner. If you are careful enough (!) then you don't need to even desolder any wire. With a lot of patience, I managed to do a clean job without heat-damaging the various soldered wires. Overall it is a joy to work on.
Working on this unit exposes you to electrical hazards. There are lethal voltages inside.
Severe accidents and possibly death by electrocution might occur. I am qualified and skilled with electronics and I have been doing audio gear repairs for over 20 years. If you lack experience, please take these articles as just a knowledge base. Do not attempt to repair something that you cannot handle as there is a high chance of doing further damage while also possibly suffering accidents.
Good tools are a must for a quality restoration. I use eutectic soldering alloy and a temperature-controlled soldering station equipped with various tip shapes. I a standard and a precision desoldering pumps and desoldering wick in various widths. To clean the flux, I use isopropyl alcohol and high purity acetone.
Empirically, I found that working with a temperature of exactly 300 °C is safe for these vintage printed circuit boards. I have never lifted any pads and I never wait more than a couple of seconds with the hot tip on any pad. While working on the chassis, I use between 360 and 440 °C. Flux fumes are extremely toxic and should be avoided at all costs.
Every replacement part is brand new, from a reputable manufacturer, ordered from the U.S.A., Japan, or Germany. In addition, I only use parts that are suitable in specific circuit sections, after inspecting and comprehending the original schematic diagrams. Last but not least, I have years of experience backing up my choices and actions.
The two decoupling capacitors, subject to change on the chassis are not figured in the original schematics. Or I could not find them. They are decoupling the power rails to the power transistors array. There is one on the leftmost side of the unit and another one on the rightmost side, directly under the fuse board. You cannot replace the one under the fuse board without removing this first. Disassembly is simple because only two screws are holding the board in its place. Changing these capacitors is very difficult due to the direct soldering to the steel chassis. This acts like a very large heatsink which gave a hard time to my 80 W soldering station. Even though the temperature was set to 450 degrees and I was using the widest tip I had available. But in the end I managed to solder the terminals. Here I show the original parts.
I have used Nichicon FG series replacements rated 3.3 uF / 50 V. Unfortunately I have not took before pictures. But this is after the replacement.
Very hard to spot the small electrolytic capacitor under the fuse board.
F-2013-1 Power Supply Board Restoration
Easy to work on; no need for disassembly.
Here it is in a before state.
A particularity is that C009 is rated 4.7 uF / 63 V on the schematic but in my unit it was 10 uF / 50 V. For C009, C010, C012 I have used Nichicon KZ MUSE series rated 10 uF / 100 V. C014 was replaced with a Nichicon KZ MUSE modern counterpart rated the same as the original.
This is after the restoration.
And on the track side I have re-soldered the terminals.
F-2041 / F-1215-A Protector Board Restoration
F-2041 is a joy to restore. A few capacitors and the coupling relay are targeted here.
This is before.
C901 was a 47 uF / 10 V bipolar capacitor and it was replaced with a Nichicon ES bipolar series modern part rated 47 uF / 16 V. C902 acts as the relay turn-on delay and it was rated 220 uF / 6.3 V. I replaced it with a modern part rated 220 uF / 25 V. C903 is a supply rail filter capacitor for this circuit. I replaced it with the same 100 uF / 50 V rating capacitor from the Nichicon KZ MUSE series. For C906 rated 1 uF / 50 V I have used a Panasonic stacked film capacitor of the same rating. C907 is a 1 uF / 50 V bipolar capacitor that was replaced with a Nichicon ES series counterpart of the same rating.
The relay can be ordered by the OMRON MY4-02-DC24 part number. This fits perfectly in place of the old relay. In fact it is identical despite the 40+ years that have passed.
Relays. Old and new OMRON parts.
And this is after.
F-1215-A is very easy to work on because there is exactly one capacitor to be replaced. Removal is easy as well. Only one screw to remove. Here is a picture before the job.
I have replaced the old 220 uF / 35 V capacitor with a modern Nichicon KZ MUSE series counterpart rated 220 uF / 50 V. And here is after the job.
While putting this board back, pay attention to the metal fixture. It has a tendency to run off and misalign with the screw hole. Nothing to worry about. With minimal care I have secured it back in place in no time.
F-2034 Driver Board Restoration
The driver board is just near the power transistors array. Very easy to service if you turn the unit upside down to have access to the soldering side while holding parts with your left hand from beneath the chassis.
This is the board prior to restoration.
Input coupling capacitors C801 and C802 were originally rated 4.7 uF / 50 V. I have used same rating Nichicon FG series replacements. C805, C806, C811, and C812 were rated 47 uF / 50 V and were replaced with same rating Nichicon KZ MUSE series modern capacitors. The bipolar capacitors in positions C807 and C807 were rated 47 uF / 10 V while in my unit were 68 uF / 10 V. I have replaced these with Nichicon ES bipolar series capacitors rated 47 uF / 16 V. Power supply rail decoupling capacitors C815, C816, C821, and C822 were rated 100 uF / 50 V and were replaced with the same type modern parts from the Nichicon KZ MUSE series. C819 and C820 are used for emitter decoupling of transistors TR805 and TR806 for static functioning point stabilization. These were 220 uF / 6.3 V and were replaced with Nichicon KZ MUSE series parts rated 220 uF / 25 V.
I have also replace all four 2SA726 transistors in positions TR801, TR802, TR803, and TR804. hFE matched Fairchild KSA992 transistors were used instead.
And here it is after.
I have reapplied solder on the component terminals. Here is a view on this.
On the underside of this board there are the two 3.3 uF / 50 V capacitors mounted directly on the emitter and collector terminals of each bias transistor. I have changed these with Nichicon FG series modern parts of the same rating.
This is a view before the replacement.
And after the restoration.
I really enjoyed working on this board as everything worked flawlessly and clean. If only there were more of these vintage units built like this. Normally with vintage amplifiers you face a deep forest of wires. Think about Pioneer for instance.
F-2028 Equalizer Board Restoration
The equalizer board is socketed and can be found in the rightmost side of this amplifier. It has a thick steel Faraday cage protecting the circuit of electromagnetic radiation. Removal of the shield is done by removing the two screws securing it in place.
Gently pull the board out of its socket presents you with this.
Input capacitors C601 and C602 were of tantalum type, rated 2.2 uF / 25 V. I have replaced these with Nichicon FG series modern parts rated 2.2 uF / 50 V. C603 and C604 in the RIAA correction loop were polarized parts rated 10 uF / 10 V. They were replaced with Nichicon ES series bipolar parts rated 10 uF / 16 V. C609 and C610 are rated 47 uF / 16 V on the schematic while on the board were 47 uF / 50 V. I have measured the voltage drop on their terminals and it is about 7 V. So I have replaced these with Nichicon KZ MUSE series capacitors rated 47 uF / 25 V. C611 and C612 were rated 33 uF / 10 V and were replaced with Nichicon KZ MUSE series modern parts rated 33 uF / 25 V. C623 and C624 were marked as 10 uF / 50 V on the schematic while on the printed circuit board were 1 uF / 50 V. Output coupling is done through C625 and C626, rated 1 uF / 50 V. I replaced all these with Panasonic stacked film capacitors rated 1 uF / 50 V. All fault-prone transistors were replaced with modern Fairchild parts, hFE matched.
Here is the board after the restoration.
I have reapplied solder on the component joints.
F-2014 Tone Control Board Restoration
On this board things are more complicated than on the other ones. This is because of the wires soldered directly on the solder-side of the board. Nothing to worry about as I could work my way on avoiding damaging or desoldering them. However extra care should be taken. There are a lot of capacitors and a handful of transistors subject to replace. In particular there is the big local filtering capacitor that is very hard to remove due to the fact that it has some ground wires soldered directly onto its terminals. These are the wires that I had to desolder.
Here is a picture of this board prior to servicing.
C703 and C704 were rated 47 uF / 16 V and were replaced with Nichicon FG series parts rated 47 uF / 16 V. C707 and C708, rated 33 uF / 16 V were replaced with Nichicon KZ MUSE series capacitors rated the same as the originals. Coupling capacitors C709, C710, C741, and C742 were rated 4.7 uF / 50 V and were replaced with same rating Nichicon FG series parts. C723 and C724 which are emitter decoupler were rated 47 uF / 16 V and I have replaced them with Nichicon KZ MUSE parts rated 47 uF / 25 V. C719, C720, C729, C730, C739, and C740 were 1 uF / 50 V polarized capacitors. I have replaced these with Panasonic stacked film 1 uF / 50 V capacitors. C721 and C722 were 10 uF / 16 V and were replaced with 10 uF / 100 V Nichicon KZ MUSE series parts. Why the high voltage? Because this is what I had available at the moment. Finally C743 which is also the hardest to remove due to the fact that two wires are soldered directly on one of its terminals, was originally rated 220 uF / 50 V. It was replaced with the same rating Nichicon KZ MUSE series modern part. Faulty transistors were replaced with modern Fairchild parts, hFE matched.
Here is the board after the restoration.
This is the hardest part to work on. You can see various soldered wires that I didn't want to mess around with. So I carefully worked my way through them. Keep in mind that the notion of easy is relative however to your experience. What I could find easy to service, you could find hard. And recto-verso.
An overview of the soldering side shows that I have applied new solder over the component terminals.
The restoration cannot be complete without performing the electrical settings as illustrated in the service manual. Thus, there are two main settings that I need to touch before putting the case back on.
- Output of Power Amplifier Section
- Current Alignment of Power Amplifier Section
The procedure for setting the output of the power amplifier DC voltage is very simple. First, commute the Speakers selector to SYSTEM-A position. Then connect a DC voltmeter to the SYSTEM-A left terminals. Set the scale of the DC voltmeter to 20 mV. Adjust VR801 up until the voltmeter reads 0 mV ±10 mV. Repeat this step for the right channel. This time adjust VR802.
Adjusting the idling current of the power amplifier is done by removing the speaker fuse cover on the back of the unit. The next step is to remove fuse F002. Turn Speakers selector to OFF position. Connect DC ammeter on the terminals of F002 fuse. Adjust VR803 for a reading of 30 mA ±2 mA. Repeat the procedure for the right channel. This time remove fuse F003 and adjust trimmer VR804.
Needless to say that you need to power the unit off each time you remove the fuses. It can be done with the unit powered but there could be a risk to short-circuit terminals and blow off the nice output power transistors.
On my unit, there is a slight variation of the DC output voltage, particularly on the left channel. It is close to 0 mV but it fluctuates repetitively around ±15 mV. This is subject for another further investigation.
Cleaning the Front Panel
Dismantling the front panel requires first removal of all knobs and switch caps. It is fairly easy but you need to be careful in order not to damage the front aluminum mask. I cleaned everything with window cleaner based on ammonia.
Knobs and felt between switches and the frontal mask come next.
There was a lot of dirt and I used about twenty cotton sticks for the buttons alone. The frontal mask came up pretty clean but there are some oily stains deep within the aluminum ridges that I cannot remove no matter how hard I tried. But I am happy the dirt is now a matter of the past.