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Technics SU-8044 repair

Folkas

Folkas

New Member
Hello everyone!

So, for a couple of days now I've been trying to repair Technics SU-8044 stereo amplifier.

When I first got it, there was no audio coming out. Only the LCD was turning on. When I opened it up, two resistors (R436 and R421) were burned. After replacing them, the right channel came back to life! To check if both STK0039 (IC401 and IC402) were working, I swapped them and when I turned the amp back on, the right channel was silent and the left one had a horrible static sound. After few moments R436 and R422 burned. This wasn't the best idea. :biggrin: After some testing with a multimeter, I found that the IC401 was shorted. I replaced the resistors again, removed both STK modules and started testing the voltages without the STKs. For some reason on transistors Q407 and Q409 collectors the voltage at first is -40 V, but after few seconds it drops to 0 V, while on transistors Q408 and Q410 the voltage stays on -40 V. I replaced the Q407 and Q409, but that didn't solve the problem.

I don't have much experience in electronics, so I'm kind of lost now.

I hope someone can help identify the cause.

Schematic: http://i.imgur.com/4booJbL.png
 
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Can't really see the detail of the schematic but suspect 407, 409 are a differential pair and replacements would need to be well matched. Try and get back to having a good right channel again to reference against for the left. By the way I bought one of these new as my first good amp out of high school. I think they are a good looking and sounding integrated, wish you well fixing this.
 
Right clicking on the image and opening it in a new tab should show it in full resolution.
I replaced transistors on both channels (Q407, Q409, Q408 and Q410), but the right channel works fine.
Few days ago STK0039 replacement arrived. When I reinstalled IC401 and IC402, the right channel was working, but the left one had loud static noise again.
 
First state, right worked, left didn't, two burnt resisters. Current state right is once again working, and left works but is noisy. Have the two resisters stopped blowing now with new output transistor and have you verified/ set bias and D.C. Offset after replacing and moving everything around?
 
Uploaded the schematic to gdrive: https://drive.google.com/file/d/0B60n0KxxwwEfYk82aV9aVi0zdjg/view

Don't know if the resistors are still blowing since I don't want to leave it on for too long. The new STK0039 migh break again.
IC402 (R) / IC401 (L) voltages:
Pin 1: -1.3 V / ~29 V
Pin 2: -40.1 V / -40.5 V
Pin 3: 0.05 V / ~12 V
Pin 8: 0.06 V / ~15 V
Pin 9: 39.5 V / 39.5 V
Pin 0: 1.2 V / ~16 V

IC401 voltages might be not accurate, since they were jumping all over the scale and I only did quick measurements to not burn anything again.
 
Your schematic gives reference voltages, I see them just can't read them. It should also say in a note the conditions the unit should be set up with in order to get them. I believe I can make out a .0# volts on schematic, so right channel readings make more sense, rail voltage is at ~source, w/ low gate voltage , so a low drain voltage. The Left side is driven, but shouldn't be, but again you need to make sure initial conditions are set correct. I don't know if you could be feeding a source, have the volume cranked and the balance set all left for example. If none of this makes sense you may want in person help from some one, not remote.
 
I've measured the voltages with no input connected, the volume is set to 0 and balance is in the middle. Since nearly all measured voltages are the same as in schematic, the conditions should be correct. The static noise on left channel is loud on any volume setting.
 
Only thing I see the same between the left and right in your chart are the two rail voltages, the others readings are several orders of magnitude different. Right likely most correct.
 
Looks like Q401 is bad. This is two transistors in one package (5 leads). There are no longer available, standard practice is to match (with the hfe measurement available in most meters) a pair of PNP trnasistors, and install them with the two emitters in the middle hole, and the bases on the outer most holes. Usual sub is KSA922 or 2SA970. There are others available, but life is easiest with ones where the collector is the center lead .There still may be other issues.. B-C-EE-C-B Good luck

Also, it is possible to trouble shoot with out the STK0039 in circuit. With the STK removed, put a 1k resistor between pins 0 and 8, and between 1 and 3 and the resT of the circuit is happy (but no speakers or load!)
 
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That 10 volts coming out of Q409 makes me suspicious of it. It should be close to 1.2v. Q401 is being driven by the 10v. Definitely don't hook speakers to this. You have a lot of D.C. Going out on that channel.
 
Thanks for suggestion, sregor. Should I replace Q402 as well, to match both channels?

Schmidlapper, is it possible, that those 10 volts coming form pin 1 to the Q409? When the IC401 was removed, the pin 1 showed -40 volts , but after few second it dropped to 0 (on the right channel pin 1 stayed on -40 volts), while pin 0 had constant 36 volts. And yes, it was 10 volts positive.
 
Folkas said:
Thanks for suggestion, sregor. Should I replace Q402 as well, to match both channels?
Click to expand...
Not if its not causing a problem.
Folkas said:
Schmidlapper, is it possible, that those 10 volts coming form pin 1 to the Q409? When the IC401 was removed, the pin 1 showed -40 volts , but after few second it dropped to 0 (on the right channel pin 1 stayed on -40 volts), while pin 0 had constant 36 volts. And yes, it was 10 volts positive.
Click to expand...
Without IC401, the voltages will be all over the place. That is why you should put 1k resistors between 0 and 8, and between 1 and 3 where the IC mounts.
It is hard to pinpoint problems based on voltages because of everything being in a loop. (Feedback) My advise is to either test individual components, and to look at operating voltages to see if any indicate a problem with that component (like large difference between bases and emitters, or conditions which should generate a different voltage than you are measuring.) A lot of this comes with experience - it is difficult to formulate.
 
Schmidlapper, yup, that's the voltage I've got. Checked it just right now just to be sure. D405 showed 0.692 V foward and 0 V backwards on diode test.

sregor, well, Q407 and Q409 were replaced, so they should be good. Q401 and Q403 showed nothing wrong in diode test.
 
Check the equivalent diode as comparison please. You need to sort out why you have these wrong voltages. The two burnt resisters were victims as well.
 
So, today I've started testing the components again. Transistors at start. Everything seemed fine on the diode test. While testing them, I also swapped them between left and right channels and just as sregor suggested, the Q401 was the culprit! As soon as I swapped it with the Q402, the noise jumped to the right channel. The voltages also jumped on IC402 pins 1, 3, 8 and 0.
Tomorrow I'll go and get some replacement parts.
Now what's left to find out is why there's such high reading on Q503.
 
sregor also gave you some good suggestions towards substitutes also. Once you have it functional you may want to do general preventative maintenance that a forty year old electronics device usually needs. Keep us posted on progress.
 
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