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

[pa_200]

Sometime it’s better to recheck everything twice. Looking very close to my picture, you can see the mistake I made with D2. The leads have been clipped to pin 23 and cathode of D2.
Having changed it to the correct legs, now I have a voltage of 2.1 across D2 with the STV-sub. So that looks much better now.
Unfortunately, the right channel is still way down low.

 

[merlynski]

Are you reading the diode voltage cathode to anode or referenced to ground?

 

[merlynski]

Sometime it’s better to recheck everything twice. Looking very close to my picture, you can see the mistake I made with D2. The leads have been clipped to pin 23 and cathode of D2.
Having changed it to the correct legs, now I have a voltage of 2.1 across D2 with the STV-sub. So that looks much better now.
Unfortunately, the right channel is still way down low.

Set the Stereo/Mono switch to Mono. If you are using the Tuner as a signal source, try something else. Radio as a source is not good for testing unless you are troubleshooting the tuner.

 

[pa_200]

Made a second test this morning with my notebook playing mp3 via aux, but same result on the right channel, very quiet.
Across D1 I still got 2.2 vdc and across D2 2.13vdc, dc offset on both side around 0.5 mVolt.

The signal input from the control amplifier comes on pin 7 for the left side and on pin 15 for the right side so I tried something different a friend learned me years ago, just to see what’s coming in via each line to the outputs. With volume at very low level I touched these two pins with an isolated electric screwdriver, left side was only a decent klick but right side was a much louder klick. What ever that indicates.

According to my little knowledge, think there are four options right now.

1. In the meantime I got another STV-4H which I can trop in instead of the sub. But I think that wouldn’t change anything
2. Checking the parts in the area of pin 15 and in line to the outputs Q12/Q14
3. Change the transistors and caps on the control amplifier first.
4. Using a better way to test the input signal on the power amp.

I guess it will be the best to wait for help and advice before I continue on the power amp.

 

[merlynski]

Made a second test this morning with my notebook playing mp3 via aux, but same result on the right channel, very quiet.
Across D1 I still got 2.2 vdc and across D2 2.13vdc, dc offset on both side around 0.5 mVolt.

The signal input from the control amplifier comes on pin 7 for the left side and on pin 15 for the right side so I tried something different a friend learned me years ago, just to see what’s coming in via each line to the outputs. With volume at very low level I touched these two pins with an isolated electric screwdriver, left side was only a decent klick but right side was a much louder klick. What ever that indicates.

According to my little knowledge, think there are four options right now.

1. In the meantime I got another STV-4H which I can trop in instead of the sub. But I think that wouldn’t change anything
2. Checking the parts in the area of pin 15 and in line to the outputs Q12/Q14
3. Change the transistors and caps on the control amplifier first.
4. Using a better way to test the input signal on the power amp.

I guess it will be the best to wait for help and advice before I continue on the power amp.

1. Where did you find an STV-4H?
2. OK
3. Before changing any more parts we should figure out what is wrong.
4. Yes, definitely.

Did you set the Stereo/Mono switch to Mono and still have the weak right side?
Can you set your notebook to play a continuous tone into both aux channels at the same level at the same time? May need to download an audio signal generator program. Then you can follow the signal through the receiver, seeing where the levels start to differ. If you use about 400Hz your DMM on AC volts should be good enough for simple comparisons. You don't need speakers on for the whole time doing this test, listening to a continuous tone is pretty irritating.
Compare pins 7 (left) and 15 (right) on the power amp pcb. If they are different levels there the problem is before the power amp.

 

[pa_200]

The STV-4H is a used one and I got it private from someone who had a 737 for spare parts. But I prefer to leave the STV-sub installed as long there is no other reason for a change.

The unit was set to MONO both speakers deselected only headset on. Than with an audio signal generator on my notebook connected to AUX I played a 400Hz on both channels.
With my DMM on AC volts I got on both sides the same value on pins 7 (left) and 15 (right) on the power amp pcb. It was a bit strange, for comparison I used three different DMM and none of them showed the same value, but always very close values while switching from pin 7 to pin 15 with minus connected to the ground.
The only difference with the noise when I put the clip on the pin 15. It was much higher than on pin 7.
But in general there was no change on the volume, left is good and right side still low.

Something else I found out while testing. On the SX-737 the tuner is always on power. You can see always the signal, tuning meter and stereo light if you are tuned in on a stereo frequency. Never had that before.

 

[merlynski]

The STV-4H is a used one and I got it private from someone who had a 737 for spare parts. But I prefer to leave the STV-sub installed as long there is no other reason for a change.

The unit was set to MONO both speakers deselected only headset on. Than with an audio signal generator on my notebook connected to AUX I played a 400Hz on both channels.
With my DMM on AC volts I got on both sides the same value on pins 7 (left) and 15 (right) on the power amp pcb. It was a bit strange, for comparison I used three different DMM and none of them showed the same value, but always very close values while switching from pin 7 to pin 15 with minus connected to the ground.
The only difference with the noise when I put the clip on the pin 15. It was much higher than on pin 7.
But in general there was no change on the volume, left is good and right side still low.

Something else I found out while testing. On the SX-737 the tuner is always on power. You can see always the signal, tuning meter and stereo light if you are tuned in on a stereo frequency. Never had that before.

I would leave the sub in place unless it is determined to be a problem.
Just yesterday I learned that 'tuner always on' is normal for Pioneer SX-x2x and SX-x3x receivers so that radio music can be recorded on tape using the Record Switch.
What were the actual voltage readings on pins 7 and 15? I do not expect them to be identical but they should be pretty close. This indicates that the problem IS in the power amp section. You may need to increase the input signal level to get a better measurement. Did you disconnect the speakers? If not, please do so, and then measure the output ac voltages (pins 3 and 17) to get an idea of the difference between them.
I am suspecting a solder bridge or a transistor installed wrong or a backwards electrolytic cap to be the problem. Did you replace any resistors?

 

[pa_200]

Speaker have been disconnected and deselected, so there was only headset selected. The Voltage was 66mv AC on pin 7 and 67 mV AC at pin 15.
No resistors replaced so far. I will recheck all transistor and electrolytic cap again for their pin position installed.

 

[merlynski]

Speaker have been disconnected and deselected, so there was only headset selected. The Voltage was 66mv AC on pin 7 and 67 mV AC at pin 15.
No resistors replaced so far. I will recheck all transistor and electrolytic cap again for theire pin position installed.

That looks close enough to call it the same to me, 1mV difference is not enough to be audible.

 

[pa_200]

Have made just another recheck, but can’t find anything wrong. Later I will make a kind of “forensic recheck” between the pictures before the recap and after. Lucky I made enough from the topside.

One thing to remember. During the first test run I could bring the right channel alive with increased volume or as well with full bass level, but the volume went down on the right side again after a while. That worked during the first 40 minute test, but never again with the later tests.

 

[merlynski]

Let's start the trouble shooting process for the right channel with checking DC voltages. No input signal, volume to minimum. Check:
PCB pin 25: -36vdc
PCB pin 22: +36vdc
PCB Pin 12: -40vdc
PCB pin 6: +35vdc
Emitters of Q2, Q4: +0.6vdc
Base of Q10: -1.2vdc
Emitter of Q10: -0.6vdc
Base of Q8: +1.2vdc
Emitter of Q8: +0.6vdc
Measure collectors of Q1 and Q2, should be very close to the same.
Q1C:
Q2C:
Measure bases of Q4 and Q2, should be very close to the same, and close to 0vdc (dc offset).
Q4B:
Q2B:

 

[pa_200]

Here are the actual values after the slash

PCB pin 25: -36vdc / -37.9 vdc
PCB pin 22: +36vdc / +38 vdc
PCB Pin 12: -40vdc / -40.4 vdc
PCB pin 6: +35vdc / +34.2 vdc
Emitters of Q2, Q4: +0.6vdc / + 0.59 vdc
Base of Q10: -1.2vdc / -1.2 vdc
Emitter of Q10: -0.6vdc / -0.59 vdc
Base of Q8: +1.2vdc / + 1.19 vdc
Emitter of Q8: +0.6vdc / + 0.6 vdc
Measure collectors of Q1 and Q2, should be very close to the same.
Q1C: / -39.3 vdc
Q2C: / -39.2 vdc
Measure bases of Q4 and Q2, should be very close to the same, and close to 0vdc (dc offset).
Q4B: / 19.3 mv
Q2B: / 25 mv

The same is on Q1B and on Q3B
The dc offset was around 2mv on pin 3 and 17 after 3 minutes warmup.

 

[merlynski]

Here are the actual values after the slash

PCB pin 25: -36vdc / -37.9 vdc
PCB pin 22: +36vdc / +38 vdc
PCB Pin 12: -40vdc / -40.4 vdc
PCB pin 6: +35vdc / +34.2 vdc
Emitters of Q2, Q4: +0.6vdc / + 0.59 vdc
Base of Q10: -1.2vdc / -1.2 vdc
Emitter of Q10: -0.6vdc / -0.59 vdc
Base of Q8: +1.2vdc / + 1.19 vdc
Emitter of Q8: +0.6vdc / + 0.6 vdc
Measure collectors of Q1 and Q2, should be very close to the same.
Q1C: / -39.3 vdc
Q2C: / -39.2 vdc
Measure bases of Q4 and Q2, should be very close to the same, and close to 0vdc (dc offset).
Q4B: / 19.3 mv
Q2B: / 25 mv

The same is on Q1B and on Q3B
The dc offset was around 2mv on pin 3 and 17 after 3 minutes warmup.

Those all look good to me.
Let's work on ac signal next.
With the same conditions you used for ac measurement before, 400Hz, no speakers, etc compare power amp pins 7 and 15 again on ac volts. Can you increase the input and/or the volume to get 150mvac at those inputs? According to the level chart in the Service Manual 1v at pins 7 and 15 drives the power amp to 18vac, 40 Watts@8Ω. That 150mv should be a safe but easier to measure ac voltage.
Then compare the ac signal at the + side of C1 to the + side of C2:
C1:
C2:
Then the - side of C10 to the - side of C9:
C10:
C9:
Then pins 3 and 17, the amps outputs:
Pin 3:
Pin 17:
With NO speakers connected switch on one of the speaker outputs and measure the ac voltage there.
Left:
Right:
I am expecting about 2.7vac at the 'good' output (with 150mv at inputs), that would be just under 1W into 8 ohms if a load was connected.

EDIT: more info

 

[pa_200]

I will check the ac-volts today after work. One information on how I made the measurements, just to reconfirm with you, that it is done properly. If not, please correct me.

All values from the transistors were done from on the top side with black connected to ground. Every transistor leg that could be reached without the risk of a short circuit, was measured direct. On all others I followed the traces on the pcb going away from that leg and check the voltage on an easy to reach position, which had a direct contact to the desired transistor leg.
For example, to check the voltage on the base of Q10, I set the probe to the cathode on D2 (STV-4H).
For the emitter voltage of Q10, I set the probe to the leg of R30, being on the same trace as the emitter.

 

[pa_200]

One other unusual thing on the receiver I found out yesterday with FM-Tuner selected on the function switch.
When I select Tape monitor 2 (the one left of loudness) the system goes quiet because there is no signal connected, the same is happening with Tape monitor 1, that is good on both. I always select one at a time, never both together.
BUT if I move the switch up again for Tape monitor 1 the tuner sound is switched on right away. Using Tape monitor 2, it take several seconds before the tuner sound/signal can be heard.
Looking on the circuit layout, there is nothing close to both Tape monitor switches what could be responsible for such a delay.
So one question could be, is there some wear and tear inside the switch for Tape monitor 2 that has to be cleaned?

 

[pa_200]

Here are my ac- volts. Setup with 400 Hz, no speakers.

Volume turned up until 150 mv-ac on pin 7 but only 5.5 mv-ac on pin 15 with balance centered. I didn't want to risk some damage to the power amp, so I left it at that volume level.
All my values in ac after the slash

Then compare the ac signal at the + side of C1 to the + side of C2:
C1: / 165 mv
C2: / 630 mv
Then the - side of C10 to the - side of C9:
C10: / 120mv
C9: / 3.19 Volt
Then pins 3 and 17, the amps outputs:
Pin 3: /3.19 V
Pin 17: /118 mv
With NO speakers connected switch on one of the speaker outputs and measure the ac voltage there.
Left: 3.16 Volt
Right: 118 mv

Looks quite odd to me!

 

[merlynski]

I will check the ac-volts today after work. One information on how I made the measurements, just to reconfirm with you, that it is done properly. If not, please correct me.

All values from the transistors were done from on the top side with black connected to ground. Every transistor leg that could be reached without the risk of a short circuit, was measured direct. On all others I followed the traces on the pcb going away from that leg and check the voltage on an easy to reach position, which had a direct contact to the desired transistor leg.
For example, to check the voltage on the base of Q10, I set the probe to the cathode on D2 (STV-4H).
For the emitter voltage of Q10, I set the probe to the leg of R30, being on the same trace as the emitter.

That is very good thinking, getting the concept of 'nodes' is important in electronics, that all physically connected locations are part of the same 'node'. The next part of that is to remember that a cracked or broken trace on the pcb, broken wire in the harness, or bad solder joint, breaks the paradigm and splits what the eyes see as one node on the schematic into two (or more) physical nodes on the actual pcb. That kind of failure can be very hard to troubleshoot, especially if it is intermittent.

 

[merlynski]

Here are my ac- volts. Setup with 400 Hz, no speakers.

Volume turn up until 150 mv-ac on pin 7 but only 5.5 mv-ac on pin 15 with balance centered
All my values in ac after the slash

Then compare the ac signal at the + side of C1 to the + side of C2:
C1: / 165 mv
C2: / 630 mv
Then the - side of C10 to the - side of C9:
C10: / 120mv
C9: / 3.19 Volt
Then pins 3 and 17, the amps outputs:
Pin 3: /3.19 V
Pin 17: /118 mv
With NO speakers connected switch on one of the speaker outputs and measure the ac voltage there.
Left: 3.16 Volt
Right: 118 mv

Looks quite odd to me!

I agree. it appears that the problem with the AC signal comes from before the power amp, the signal at pin 7 is good, the signal at pin 15 is weak. We have another problem, but I believe the power amp is fixed.
Let's compare ac voltages back through the signal path. Setup remains the same.

Control Amp A'ssy AWG-030
Pin 3, left out:
Pin 1, right out:
Pin 8, left in:
Pin 5, right in:

Switch and Volume A'ssy AWX-071 1/5
Pin 32, left out:
Pin 28, right out:
Pin 21, left Aux in:
Pin 22, right Aux in:

Switch and Volume A'ssy AWX-071
Pin 33, stereo/mono, left:
Pin 35, stereo/mono, right:
Change Stereo/Mono switch to Stereo and recheck:
Pin 33:
Pin 35:

EDIT: correction

 

[pa_200]

Let's compare ac voltages back through the signal path. Setup remains the same.
Here are my ac- volts for the comparison of the ac voltages back through the signal path.
Setup with 400 Hz, no speakers, Volume turn up until 150 mv-ac on pin 7, with balance centered.
All my values in ac after the slash.

Control Amp A'ssy AWG-030
Pin 3, left out: / 150mv
Pin 1, right out: / 6mv
Pin 8, left in: / 17.2mv, every time I touched pin 8 it triggered the protection relay to open and a view seconds later back to close.
Pin 5, right in: / 16.7mv, with no effect on the relay

Switch and Volume A'ssy AWX-071 1/5
Pin 32, left out: / 17.4mv
Pin 28, right out: / 0.5 mv
Edit: Can you please recheck on pin 28 for right out? According to my sm it must be pin 29 for for right out. Pin 28 goes to the FM-tuner Function Switch S1-3F
Pin 29 reads / 16.5 mv

Pin 21, left Aux in: / 289 mv
Pin 22, right Aux in: / 286 mv

Switch and Volume A'ssy AWX-071
Pin 33, stereo/mono, left: / 280 mv
Pin 35, stereo/mono, right: / 278 mv
Change Stereo/Mono switch to Stereo and recheck:
Pin 33: / 278 mv
Pin 35: / 278 mv

 

[pa_200]

With my limited knowledge, it looks to me at this stage, the problem lies probably inside the Control Amplifier Assembly (AWG-030) with lots of critical SA726 and SC1344. If there is no other advice and nobody calls “wrong conclusion”, I will recap that board as next step and report back with the new ac-values for pins 1, 3, 5, 8.