Technics SU-8077 Rebuild

[taineasy]

Jeromach, I have another question for you. First, I purchased a better multi-meter with a transistor tester included. I have tested all the KSA992FTA and KSC1845FTA transistors and found two sets of 4 exact matches for each which will be used to replace the double transistors. I then took a couple of transistors that I will not be using, and tried the "heat shrink together" thing, with a small bit of thermal paste between them. That went well enough.

Then I tried to bend the legs to get them into the correct position to fit them in the board. (Keep in mind this is all a trial to see how it is done). That brought me to the realization that in such a tiny space, it is very easy for the legs to cross, and touch when they should not. So, I tried adding some heat shrink to the legs, and found that the size that I have is too large. I believe I have found some other heat shrink that will be fine enough to stay on the legs. Bonus, it is also colored so that I can identify the B, C, and E legs more clearly.

My question is this, before I do this work. Will the heat shrink on the legs of the transistors affect the performance of the transistors, especially in regard to heat? I plan to leave a bit of the legs exposed next to the board so that I might use a heat sink when soldering to protect the transistor. How much of an issue would I be creating, though, using heat for shrink wrap? I plan to use minimal heat, but maybe this is still not a good thing to do. I would love to hear your experienced thoughts on this.

Sorry to jump in on this but I was wondering why the double transistor replacement? Were the originals shorted or is this a preventative thing? I have had one of these since new and have never had any issues with the transistors. Others have posted that the trim pots should be replaced (50kohms) on the power supply should be changed but I have never been able to find drop in replacements. In the manual it mentions TP1-TP4 for adjusting Bias/idle current but I have never been able to locate these touch points on the board. Are they marked?

 

[taineasy]

Does the amp still work? And if so, is it ok (or having some minor glitches, which mostly is no real issue to fix)?

There are some things that need to be replaced/done without any question;

To start with, a word of warning; be very very careful with the thermal diodes mounted on the two rear power transistors. Do not bend the leads, they tend to break and the part is nearly impossible to replace (the same goes for the SU-8088, but there they are mounted to the heat sink)

The trimmers right in front of the large filter caps need to be replaced (do NOT - I repeat NOT - try to adjust the old ones, this might blow up the power transistors).
The double transistors 2SA798 and 2SC2291 need to be replaced (Q301, 2, 3 and 4) - you need to use two normal transisors per double transistor and match the new ones (when you're there I can explain how to construct them)
Check all solder connections for bad ones, remove the old solder and apply new.
All switches need to be treated with de-oxit. This does not always work satisfactorily, they then need to be taken apart, cleaned and greased (ask for instructions before dismounting them).
The contacts of the relay need to be cleaned or the relay just replaced

As for the electrolytics, using Nichicon gold for all of them, except the large filter caps, will be ok. Panasonic FM will also do the job nicely.
The large filter caps is optionial if they still measure ok, but if you do not have the tools to do so, it is better to just replace them. Panasonic will be fine, but stocks run out or have done so already, since they stopped producing them. Cornell Dubilier also has nice large filter caps. The diameter is important to fit them in the existing brackets, new caps are smaller, but by just taking a cap with a higher voltage it must be possible to get the correct diameter.

Here's a cap list;

Column 3 and 4 is what is stated in the service manual, column 5 and 6 is what actually came out.

Has anyone found a replacement for the trim pots (50kohms?) that will drop in without too much modification? I have had no luck finding a replacement for those. The manual mentions TP1-TP4(touch points?) to adjust idle but have not been able to locate them. Are they marked on the PS or Main board somewhere? My DC offset has always measured 1mv left/2mv right so I never messed with it.