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

[merlynski]

Check for bad soldering on the pin 8 of the control amp, reflow solder.
Pins 8 and 5 measure close to the same (inputs) but 3 and 1 (outputs) are very different. I think there is a problem on the Control Amp
Stereo/Mono pins look ok.
Switch and Volume Aux pins 21, 22 look ok.

Control Amp pin 5 right in, (16.7mv) should be the same as Switch and Volume pin 28 right out, (0.5mv) but pin 28 is lower. Please recheck the reading for pin 28. Verify 0Ω between them (power off) also.

Then we will compare ac voltages on the control amp.

 

[merlynski]

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.

My conclusion as well. Faderlube on the tone pots as well as some exercise end to end, then leave them centered.

 

[pa_200]

Control Amp pin 5 right in, (16.7mv) should be the same as Switch and Volume pin 28 right out, (0.5mv) but pin 28 is lower. Please recheck the reading for pin 28. Verify 0Ω between them (power off) also.
Then we will compare ac voltages on the control amp.

Did you see my edit at no.79 regarding pin 28 vs 29?
I just recheck it on both boards. The pink wire goes from pin 5 control to pin 29 on the switch board. The same with the white wire from pin 8 control to pin 32 on the switch board.

 

[merlynski]

Did you see my edit at no.79 regarding pin 28 vs 29?
I just recheck it on both boards. The pink wire goes from pin 5 control to pin 29 on the switch board. The same with the white wire from pin 8 control to pin 32 on the switch board.

I did not see your edit. You are correct, it is pin 29, on my schematic displayed on my monitor the 9 looks very much like an 8 until I looked again closely after your post. Just one of the difficulties of remote intercontinental trouble-shooting. Thanks for figuring it out.

 

[pa_200]

Just one of the difficulties of remote intercontinental trouble-shooting. Thanks for figuring it out.

I wouldn’t call it “difficulties”. It is rather the proof that the remote intercontinental trouble-shooting can work very well, if at least one has the patience and the professional knowledge to give step by step instructions so one can work together on the same issue.

 

[pa_200]

I took out all transistors and Electrolytics on the Control amplifier assembly and check every single piece. I expected a big difference between the parts from left and right channel, but there wasn’t, at least I cannot see one. The hfe on the small caps (orange ones 4.7 and 47µF) is out of limit, but equal on both channels. Nothing special so far that might be an indicator for the low output of the right side at pin 1.
Will see what happens if the new parts are in, hopefully tomorrow late evening. So wait and see, if one of you has another clue, you are always welcome.

That are my values from the original parts.
Left channel
Q1 = 2SA726 hfe 524, 658 mv
Q3 = 2SC1344 hfe 398, 660 mv
Q5 = 2SC1344 hfe 412, 667 mv

Right channel
Q2 = 2SA726 hfe 517, 659 mv
Q4 = 2SC1344 hfe 519, 656 mv
Q6 = 2SC1344 hfe 450, 673 mv

Electrolytics, Left channel
C1 = 1/25 = 0.988 µF
C5 = 100/16 = 115 µF
C9 = 4.7/25 = 6.17 µF
C17 = 4.7/25 = 5.62 µF
C19 = 4.7/25 = 6.00 µF
C21 = 47/10 = 55.1 µF
C23 = 2.2/25 = 2.15 µF

Right channel
C2 = 1/25 = 1.06 µF
C6 = 100/16 = 117 µF
C10 = 4.7/25 = 6.17 µF
C18 = 4.7/25 = 5.85 µF
C20 = 4.7/25 = 6.06 µF
C22 = 47/10 = 56.7 µF
C24 = 2.2/25 = 2.23 µF
C29 = 100/35 = 109 µF

 

[merlynski]

While you have those parts out, measure the resistors on the board.
I think it is just as likely to be one of the pots or the switches as it is a cap or transistor, especially after seeing your old parts specs.
The signal has to pass through S6 =>on the main schematic (=>S1 on pcb schematic) no matter what and a little corrosion there could put a high resistance in the signal path. Measure ohms across that switch's contacts.
Deoxit that switch again also.

EDIT: more info

 

[pa_200]

Thanks merlyn very good plan. Before I put in the new parts, I try to follow the signal path backwards from pin 1 to pin 5 and compare it with the good left channel.
So far I found no visible sign of a damaged part or burned resistor. But will have a special view on those switches and potis.

 

[pa_200]

Lots of work today on my pioneer, mainly measure and comparing especially around the control amplifier and switch board. Hope I can finish the next step until Saturday and see a success, keep my fingers crossed.

Something special as a little extra help for every SX-737 owner. I was lucky and got an unused full size block diagram for the SX-737. So I took the chance and made a new three page full scal scan, covering the left side, middle part and right side.
If I find a way to upload a pdf-file, would like to share it with other users.

Edit: pdf is attached now!

 

[merlynski]

Lots of work today on my pioneer, mainly measure and comparing especially around the control amplifier and switch board. Hope I can finish the next step until Saturday and see a success, keep my fingers crossed.

Something special as a little extra help for every SX-737 owner. I was lucky and got an unused full size block diagram for the SX-737. So I took the chance and made a new three page full scal scan, covering the left side, middle part and right side.
If I find a way to upload a pdf-file, would like to share it with other users.

You can post it in this thread by using the "Upload a File" button next to "Post Reply" =========================>>
I am looking forward to hearing your results!

 

[merlynski]

Something special as a little extra help for every SX-737 owner. I was lucky and got an unused full size block diagram for the SX-737. So I took the chance and made a new three page full scal scan, covering the left side, middle part and right side.
If I find a way to upload a pdf-file, would like to share it with other users.

Edit: pdf is attached now!

Very nice, thank you!

 

[pa_200]

There is light at the end of the tunnel.

After I have changed all caps and the 6 transistors, even it was only necessary for 8 caps, I’m closing in at the two switches for high and low filter. Probably the balance switch as well. Still some minor problems.
The first round of Deoxit worked very well, was thinking first about taking the switches out, dismantle and cleaning the inside contacts. But postponed that for the next two Deoxit runs.

We will be away for two weeks vacation, so I can’t work on this lovely machine.
But will write a longer conclusion about that job when all is sorted out, so everyone with the same problem can use the information and lessons learned.
Merlyn your knowledge and experience is PRICELESS. Big thank to you already at this time.

 

[merlynski]

Das ist schon gut! Auf Wiedersehen .

 

[pa_200]

Holiday will start on sunday

Did some more checks on the control amplifier AWG-030
According to the sm there should be at Q5 Collector 8.5 vdc, but I have 10 vdc. The others on Q5 are, base 1,9 vdc and emitter 1.3 vdc.
Same voltage on Q6 for the right side.
Pin 7 has 30,2 vdc and behind r45 I have 23.2 vdc

Any idea why I have 10 vdc on Q5C instead of the 8.5 required in the sm?

Short feedback on the major issue between left and right side. As you wrote, the signals runs through all the switches. For now I worked only on the AWG-030 and the problem on that board was inside the high filter switch but only on the bank for the right side. 3 proper Deoxit runs did the job.

For now it works fine again and I plan to continue in 3 weeks with the tuner board. Will write more about the whole project after my holiday.

 

[merlynski]

Pin 7 has 30,2 vdc and behind r45 I have 23.2 vdc

Those look ok to me, R45 and C23 are a power supply decoupling filter.

Any idea why I have 10 vdc on Q5C instead of the 8.5 required in the sm?

Q5 (and Q6) is a basic class A ac coupled amplifier with C17 as the ac input, C19 as feedback, and C23 as the output. DC levels are set by the bias voltage divider R29 and R31 (about 2.5vdc on the base), and the emitter resistor R35. if the transistor you replaced Q5 with has a different dc current gain than the original the dc voltage at the collector will be different. I do not see this as a problem as there should be plenty of headroom with 23vdc on the + side of R33, and there probably won't ever be a greater than 3vac p-p signal on the collector. The Level Diagram on SM pages 9 & 10 shows the output of the Control Amp (AWG-030) as 1v (nominal). A 1v rms sine wave has a p-p value of ≈2.8v. Unless you end up with a distortion problem it should be ok.

 

[pa_200]

What I can tell about the new transistor on the AWG-030 is, that I replaced the sc726 with sa992 (Q1+2) and the sc1344 with sc1845 (Q3-6).
The original sc 726 hat a hfe around 520 and the sc1344 a hfe between 398 and 519.

The new replacement 992 (for 726) I had a hfe of 390 and the sc1845 (for 1344) a hfe between 398 and 438. So some of the hfe are quite a bit lower than the measured 500+
The good news, the required voltage for Q1 til Q4 are nearly spot on.

 

[pa_200]

We are back from vacation and I started work again on the SX-737.

During a test run I check the temperature on some parts, especially on the Power supply. I had at outside temperature around 20°C the following values.
Q1 = 54°C, with now heatsink
Q3 = 63°C, with the big heatsink
R3 = 90°C
Is the high temperature at R3 in limit or is there a way to reduce that temperature?
The Q1 and Q3 had been changed before.

 

[merlynski]

We are back from vacation and I started work again on the SX-737.

During a test run I check the temperature on some parts, especially on the Power supply. I had at outside temperature around 20°C the following values.
Q1 = 54°C, with now heatsink
Q3 = 63°C, with the big heatsink
R3 = 90°C
Is the high temperature at R3 in limit or is there a way to reduce that temperature?
The Q1 and Q3 had been changed before.

Is the +13.5vdc power supply voltage correct (±1vdc)?
Measure the voltage across R3 and calculate the power it is dissipating using Ohm's Law (P=E²/R). That is a wire wound resistor, and is rated at 5W so Pioneer expected it to dissipate significant power. The current load for the +13.5vdc supply indicated on the schematic is 75mA (near pins 19. 20 and 21) is about 1W on R3. That is quite a bit of power for a small component, however wire wound resistors are pretty tough. Further calculation shows an expectation of about 13.5vdc across R3. You could replace it with a higher wattage resistor, which would have a greater surface area to dissipate the heat, but if it really is dissipating 1W it is well within its power rating.

 

[pa_200]

At pin 19/20 I have 12.68 vdc, but on pin only 5.93 vdc.
When the unit is cold it has on Q4C and Q3C 34.6 vdc and on C7+ 49.6 vdc, giving across R3 around 15vdc.
But after a short while the collector voltage on Q3/Q4 goes down to 26.5 vdc increasing the voltage across R3 up to 22.3 vdc.

Edit Correction, change Q3 and Q4 E was initially wrong, should be collector voltage

 

[merlynski]

At pin 19/20 I have 12.68 vdc, but on pin only 5.93 vdc.
When the unit is cold it has on Q4E and Q3E 34.6 vdc and on C7+ 49.6 vdc, giving across R3 around 15vdc.
But after a short while the collector voltage on Q3/Q4 goes down to 26.5 vdc increasing the voltage across R3 up to 22.3 vdc.

The voltage on Q3E, Q4E is way too high, it has probably damaged C18 (100µF, 16vdc).
Measure vdc at the cathode of zener diode D9, sb +14vdc ±5%. If it is open (or shorted) the Q3-Q4 regulator will not function correctly. Test Q1, 2, 3, 4 with meter for shorts and opens. If any of those semiconductors is bad replace them all (Q1-4, D9) and the electrolytic caps around them that may have been subjected to over-voltage (C16, C17, C18).
Check the voltage at pin 14 of the PS board (+31.5vdc), its regulator also uses D9, through Q2 and Q1.