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SX-737 power amp AWH-033 issue, blown transistors

So what shall I do next? Q5, 7, 9, as well as Q11 &13 are all out and rechecked. None of them is indicated as broken or damaged.The only one I have not at hand for replacement are the Q11 and Q13.
 
Let us see if the right channel will balance and is adjustable. Put Q6, Q8 and Q 10 back in if they were removed. Output transistors are disconnected? Q11, Q13, Q12, Q14.
Without the D1 STV in place we cannot check the left channel.
Leave Q7 and Q9 out for now.
Power up on DBT and check pin 17, it should be 0vdc +/- a few millivolts. If it is close to 0 adjust VR4 and see if the voltage on pin 17 varies + and - around 0 and is stable when not being adjusted. If it is not close to 0vdc let me know. This test is to see if the differential amp Q2 and Q4 are balanced and the dc feedback loop is closed.
 
SORRY SORRY, everything back, my bad handwriting tricked me and I mixed it up with the picture of the foil side. The D1 is the good one and D2 is broken.

I have just rechecked it. If you lock on the topside, it is die STV sitting between pin 22 and 23.

Ok I will/can to it tomorrow, when I’m home from work and report back.
 
pa_200 said:
SORRY SORRY, everything back, my bad handwriting tricked me and I mixed it up with the picture of the foil side. The D1 is the good one and D2 is broken.

I have just rechecked it. If you lock on the topside, it is die STV sitting between pin 22 and 23.

Ok I will/can to it tomorrow, when I’m home from work and report back.
Click to expand...

No problem, we will sort it out tomorrow.
 
So I’m back and ready to continue work on the SX-737 AWH-033.

First I found another mistake in the SM on page 47 where you can find the foil side of the AWH-033. I was looking for the Q3 and couldn’t find it first. But Q3 has to sit close to Q1, above Q1 there is Q8 printed instead of Q3. The real Q8 is on the fare left side. So if anyone is interested you better change the number on the SA726 above Q1 to Q3.

Back to the requested test. As you can find above, it mixed D1 with D2, so I had to change the give Q’s for the test.

With a good D1 I put Q5, Q7 and Q9 back, Q10 + Q8 are still out. VR 1+2 were set to minimum, VR 3+4 were centered. The DMM with red on pin 3, black to ground, setting 200mV.

Powered the unit up on DBT, nice dim, than DMM showed around 50mV and with little steps on VR 4, it could be adjusted down to 0.0mV, balanced nicely with a very little move around 0.03-0.00mV.
 
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pa_200 said:
So I’m back and ready to continue work on the SX-737 AWH-033.

First I found another mistake in the SM on page 47 where you can find the foil side of the AWH-033. I was looking for the Q3 and couldn’t find it first. But Q3 has to sit close to Q1, above Q1 there is Q8 printed instead of Q3. The real Q8 is on the fare left side. So if anyone is interested you better change the number on the SA726 above Q1 to Q3.

Back to the requested test. As you can find above, it mixed D1 with D2, so I had to change the give Q’s for the test.

With a good D1 I put Q5, Q7 and Q9 back, Q10 + Q8 are still out. VR 1+2 were set to minimum, VR 3+4 were centered. The DMM with red on pin 3, black to ground, setting 200mV.

Powered the unit up on DBT, nice dim, than DMM showed around 50mV and with little steps on VR 4, it could be adjusted down to 0.0mV, balanced nicely with a very little move around 0.03-0.00mV.
Click to expand...
That is good news. Do you have good transistors to put in place for Q11 and Q13 outputs?
 
pa_200 said:
The test was done without any outputs installed, as you wrote in #22.
Click to expand...
Yes indeed, the next step is install output transistors on this (left) channel only to test, first on DBT, check for dc offset adjust, then full power, check for dc offset adjust. Since D2 is bad and removed we cannot test the right channel so the outputs must remain out on that side.
 
I made some attempts to find some STV-4 diodes, but apparently the whole world is out of stock. You will have to use the alternate option from Post 25.
 
Merlynski I will to the next test first thing tomorrow morning (after breakfast) ;ö)))
Complete family dropped in today and requests attention.

Second thing will be a rebuild for the STV-4H, hope it will work.

A question about a good replacement on the outputs. The BOM list recommends MJ21194G and MJ21193G which are a bit expensive over here and I can get it only with high shipping cost. My preferred supplier has MJ15024 and MJ15023 in stock, what about these?
Has anyone some experience with these two as outputs with pioneer?
 
I looked over the datasheets for those transistors. The MJ21194 and MJ21193 have a DC current gain of 25, and the MJ15024 and MJ15023 have a current gain of 15, at Ic of 8Amps (both). Otherwise they are the same, or close enough. I have not tried them but would not be afraid to. The 023 and 024 datasheet says they are intended for "high power audio and other linear applications" so give them a try.
And have a good day with your family!!
 
Let’s continue. Today was the first test with the outputs Q11 and 13 in place and on DBT. Voltage did balanced at 0,3mV, than next step without the DBT on full power, first reading was around 5mV and it could adjust it down to very stable 0,2mV. What a relieve at that time.

Maybe I’m lucky and can get some STV-4H from another pioneer owner in our country. Until the parts will arrive, I’m working on a replacement with 3x 1N4148.

Anything else I can do on the board in the meantime?
 
Did you already replace the Q5, Q7 and Q9 transistors? Did you replace VR1? Is it set to minimum resistance? I know we discussed this before, just making sure. If so we can set the bias on the left channel. Power down, clip your DMM onto pin 1 (black lead) and pin 2 (red lead) on a scale that will read millivolts accurately, power up again and adjust VR1 (remember the SM has wrong VR in the instructions) carefully and slowly for 20mV (0.020vdc). Setting this too high will cause the unit to run hot and maybe kill the output transistors again. Monitor the voltage and readjust after 10 minutes to 20 mVdc. If it stabilizes, power down, connect a speaker to that channel only, connect an audio source and try playing some music through that channel. Turn the volume up slowly, don't try to play too loud.
 
In the link to echowars' STV-4 replacements he found that the UF1004 was a better choice than 1n4148, using 4 to replace the STV-4, if you can get them.
 
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merlynski said:
Did you already replace the Q5, Q7 and Q9 transistors? Did you replace VR1? Is it set to minimum resistance? I know we discussed this before, just making sure. If so we can set the bias on the left channel. Power down, clip your DMM onto pin 1 (black lead) and pin 2 (red lead) on a scale that will read millivolts accurately, power up again and adjust VR1 (remember the SM has wrong VR in the instructions) carefully and slowly for 20mV (0.020vdc). Setting this too high will cause the unit to run hot and maybe kill the output transistors again. Monitor the voltage and readjust after 10 minutes to 20 mVdc. If it stabilizes, power down, connect a speaker to that channel only, connect an audio source and try playing some music through that channel. Turn the volume up slowly, don't try to play too loud.
Click to expand...
What about the 8 Ohm dummy resistor that is mentioned in the SM?
I did replace the Q5, Q7 and Q9 transistors as well as all four VR’s. For the test VR1 and 2 have been set to minimum resistance.
The next test will be done tomorrow. Now there will be a cozy family barbeque. :beerchug:
 
pa_200 said:
What about the 8 Ohm dummy resistor that is mentioned in the SM?
Click to expand...
You can put one on if you like. On the older output capacitor coupled amps a dummy load was required to keep dc leakage from the output cap from building up at the output terminal. This is a direct coupled amp. The only benefit I can see would be to reducing risk to a speaker for the first trial of sound reproduction on full power. I use a pair of 'sacrificial' speakers for this purpose. If they get fried it is not big loss (I have fried one in the past). The other purpose for dummy loads is for high power output testing where speaker volume would be too loud, and for measuring distortion, as a resistor is a linear load compared to a speaker. Sorry if that is too much information :blah:.
 
Every information is highly appreciated. It’s the best way to learn during work.
I have the two dummy loads at hand because I also needed them before for other purposes.
 
Next test was this morning with a little change. I got two DMM so connected the first for the bias onto pin 1 (black lead) and pin 2 (red lead) set to automatic DC-Scale.
The second one was clipped with red on pin 3, black to ground.

Then power up on DBT. The bias was at ZERO in the beginning, after a few minutes I increased VR1 careful from 0 to 20mV. With this increase the voltage at pin 3 changed to -26mV, I guess that is correct and had to be readjusted at a later stage if everything else work correct.

Second test without DBT on full power, bias was at 21mV and pin 3 at -25mV. With the bios adjustment to 20mV the voltage on pin 3 got down to -24mV

Now if I hear nothing else, I will reinstall the Q6, Q8 and Q10 for the D2 Side and test it later with my rebuild stv-4h replacement.
 
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