Pioneer SX-838 stays in protection

[tonefoto]

You would not be able to set the bias correctly because there would be no current through the output transistor emitter resistors. The driver emitter resistors (150Ω) do connect to the feedback path so you could power up without the output transistors and see if the dc offset adjusts with its VR, and you might get the voltage across the both driver emitter resistors to increase and decrease with the bias adjustment, just to check functionality. Be sure to set the bias adjust back to minimum before installing the outputs. No load should be connected.

Minimum bias = min resistance read on trimmer VR3/VR4? would powering up without output transistors confirm it comes out of protection?

 

[merlynski]

Minimum bias = min resistance read on trimmer VR3/VR4? would powering up without output transistors confirm it comes out of protection?

Yes to minimum resistance on VR3/4. If all the other conditions are correct, including dc offset, going to the protection circuit, I would expect (but not guarantee) the relay will click. It might not while still on DBT. If it was a bad output transistor causing the problem having them removed should allow the amp to balance. Check the output transistor's emitter resistors, (0.5Ω) and connections, as an open one might cause a similar problem after the outputs are re-installed.
EDIT: more info

 

[tonefoto]

Yes to minimum resistance on VR3/4. If all the other conditions are correct, including dc offset, going to the protection circuit, I would expect (but not guarantee) the relay will click. It might not while still on DBT. If it was a bad output transistor causing the problem having them removed should allow the amp to balance. Check the output transistor's emitter resistors, (0.5Ω) and connections, as an open one might cause a similar problem after the outputs are re-installed.
EDIT: more info

Ok, I'm back with a final report on this money pit. At this stage I should've built myself a nice tube amp from scratch. With output transistors removed, 150W DBT, I could read and adjust DC offset on left channel. I couldn't get the right channel to adjust from -21VDC. So there I am.

 

[tonefoto]

View attachment 1573998 View attachment 1573999 View attachment 1574002 View attachment 1574004 View attachment 1574002 View attachment 1574002 View attachment 1573998 View attachment 1573999

Ok, I'm back with a final report on this money pit. At this stage I should've built myself a nice tube amp from scratch. With output transistors removed, 150W DBT, I could read and adjust DC offset on left channel. I couldn't get the right channel to adjust from -21VDC. So there I am.

 

[merlynski]

Ok, I'm back with a final report on this money pit. At this stage I should've built myself a nice tube amp from scratch. With output transistors removed, 150W DBT, I could read and adjust DC offset on left channel. I couldn't get the right channel to adjust from -21VDC. So there I am.

Does final report mean your are done with it or want to keep working on it? You are making progress . . .

 

[tonefoto]

Does final report mean your are done with it or want to keep working on it? You are making progress . . .

I don’t know if I should keep going, I thought I’d have licked it by now. What’s your suggestion? I read the dc offset at -21 VDC, I think it was pin 20, green wire

 

[merlynski]

I don’t know if I should keep going, I thought I’d have licked it by now. What’s your suggestion? I read the dc offset at -21 VDC, I think it was pin 20, green wire

SO FAR, the only thing that has stopped me (and the AK community) from helping somebody fix their issue is when the original poster decides not to continue. FWIW, I do understand the money pit issue, if you are trying to fix it to resell it it probably is not worth it. But for myself, the satisfaction of having brought something back to life is very rewarding. In my younger days I bought used vehicles and fixed them up and drove them till they just fell apart. Having the skills and the tools saved me time and again. It is entirely up to you, I will continue if you wish to, what looks like an insurmountable obstacle sometimes becomes a great success when that final problem is fixed.

 

[tonefoto]

SO FAR, the only thing that has stopped me (and the AK community) from helping somebody fix their issue is when the original poster decides not to continue. FWIW, I do understand the money pit issue, if you are trying to fix it to resell it it probably is not worth it. But for myself, the satisfaction of having brought something back to life is very rewarding. In my younger days I bought used vehicles and fixed them up and drove them till they just fell apart. Having the skills and the tools saved me time and again. It is entirely up to you, I will continue if you wish to, what looks like an insurmountable obstacle sometimes becomes a great success when that final problem is fixed.

I agree with it being great to resurrect a dead amp with persistence and skill. I obviously am missing at least one of those elements of success. The advice I've been given here is appreciated, thanks again.

 

[tonefoto]

SO FAR, the only thing that has stopped me (and the AK community) from helping somebody fix their issue is when the original poster decides not to continue. FWIW, I do understand the money pit issue, if you are trying to fix it to resell it it probably is not worth it. But for myself, the satisfaction of having brought something back to life is very rewarding. In my younger days I bought used vehicles and fixed them up and drove them till they just fell apart. Having the skills and the tools saved me time and again. It is entirely up to you, I will continue if you wish to, what looks like an insurmountable obstacle sometimes becomes a great success when that final problem is fixed.

I've tested everything so far suggested. I'm reading about -18 VDC on pin 1 of the protection board and the same at pin 20 of the amp board and cannot adjust it on the trimmer. The other channel on pin 5 adjusts to 0 VDC offset. Replaced every transistor so far, and cap, and checked polarity and transistors orientation, testing with 150W DBT and output transistors removed but still won't get the relay to click. If anyone has suggestions please let me know.

 

[merlynski]

In your post a page back, you recommended testing; "Also test D5, VR4 (100 Ω) and D3 (STV4). You cannot test D3 on the ohmmeter or diode range, but if you connect a 9v battery with a 1kΩ resistor in series across D3 to forward bias it you will see about 4 pn junctions voltage, ≈2.4vdc if it is good".. I don't understand the procedure for testing the diodes D5, D3 as outlined. Is it not possible to test them with a transistor analyzer? Can you explain what setting the meter should be reading from when testing with the 9V battery and 1KΩ resistor in series please?

The inexpensive transistor analyzers most people (hobbyists) have does not put out enough output voltage or current, nor do most DMMs. If you have a higher end analyzer that allows you to pick higher voltage or current use it.
Here is a schematic of the test circuit, you can use one or two nine volt batteries, and chose R1 as a 1k to 10k resistor to get approximately the current from the unit. One battery and a 1k resistor will yield about 6.5 mA and your meter will read about 2.4 vdc across the STV-4. The most likely failures are shorted (0 vdc) or open (full battery voltage). If it reads between 2 and 3 vdc it is probably ok. Lift one end to eliminate interference from other components in the circuit. Meter should be on a dc voltage range that is a maximum of 20vdc, or autorange.

The other thing that can be done is to measure the voltage across the STV in the good channel and compare it to the bad channel STV. Then you just test with the power on, not using the test jig.

Verify power supplies: pin 24 has about -44 vdc, and pin 25 has about +44vdc, these may be 10-15% lower on DBT. If pin 20 is -18vdc (and the output transistors are disconnected) I would test (power off) Q8 and Q10, one may be shorted or open. Also measure R32 and R34, 150Ω.
Be careful, the leads break off the STVs real easy, right next to the body.

EDIT: after review of previous posts: The adjustment will never correct for that much offset, the protection circuit is working by not allowing the relay to pull in with that much dc at the power amp stage output (pin 20). Looks like there is still a failure in the power amp stage.

 

[tonefoto]

The inexpensive transistor analyzers most people (hobbyists) have does not put out enough output voltage or current, nor do most DMMs. If you have a higher end analyzer that allows you to pick higher voltage or current use it.
Here is a schematic of the test circuit, you can use one or two nine volt batteries, and chose R1 as a 1k to 10k resistor to get approximately the current from the unit. One battery and a 1k resistor will yield about 6.5 mA and your meter will read about 2.4 vdc across the STV-4. The most likely failures are shorted (0 vdc) or open (full battery voltage). If it reads between 2 and 3 vdc it is probably ok. Lift one end to eliminate interference from other components in the circuit. Meter should be on a dc voltage range that is a maximum of 20vdc, or autorange.
View attachment 1576011
The other thing that can be done is to measure the voltage across the STV in the good channel and compare it to the bad channel STV. Then you just test with the power on, not using the test jig.

Verify power supplies: pin 24 has about -44 vdc, and pin 25 has about +44vdc, these may be 10-15% lower on DBT. If pin 20 is -18vdc (and the output transistors are disconnected) I would test (power off) Q8 and Q10, one may be shorted or open. Also measure R32 and R34, 150Ω.
Be careful, the leads break off the STVs real easy, right next to the body.

EDIT: after review of previous posts: The adjustment will never correct for that much offset, the protection circuit is working by not allowing the relay to pull in with that much dc at the power amp stage output (pin 20). Looks like there is still a failure in the power amp stage.

Thank you for the tips;
With a 9V and 1K resistor in series with the STV4 D3 reads about -6.5 VDC. Pin 24 -40VDC, Pin 25 40VDC, with 150W DBT, output transistors removed. R32/R34 are good. Q7 through Q10 drivers were just replaced with Larry's recommendation

 

[merlynski]

Thank you for the tips;
With a 9V and 1K resistor in series with the STV4 D3 reads about -6.5 VDC. Pin 24 -40VDC, Pin 25 40VDC, with 150W DBT, output transistors removed. R32/R34 are good. Q7 through Q10 drivers were just replaced with Larry's recommendation

You are welcome!

-6.5 vdc is not what I expected. Was the polarity of the battery and the meter and the STV hooked up just like the schematic? If the battery was hooked up to the STV backwards you were seeing a leakage current voltage drop rather than a forward bias voltage. If the the STV and the battery were correct polarity (+ to anode) and the meter backwards I think the voltage is a bit high. How do they compare in circuit with power on, if you checked that way?

Did you replace Q2, Q4 and Q6?
Next step is some dc voltage measurements with power on, DBT ok. Measure DC voltages:
Base of Q2:
Base of Q4:
Base of Q6:
Collector of Q6:

 

[tonefoto]

You are welcome!

-6.5 vdc is not what I expected. Was the polarity of the battery and the meter and the STV hooked up just like the schematic? If the battery was hooked up to the STV backwards you were seeing a leakage current voltage drop rather than a forward bias voltage. If the the STV and the battery were correct polarity (+ to anode) and the meter backwards I think the voltage is a bit high. How do they compare in circuit with power on, if you checked that way?

Did you replace Q2, Q4 and Q6?
Next step is some dc voltage measurements with power on, DBT ok. Measure DC voltages:
Base of Q2:
Base of Q4:
Base of Q6:
Collector of Q6:

I re-read 2.3 VDC with the battery and resistor following the schematic provided, they read about 2VDC powered on with DBT. I replaced Q2, Q4, Q6 with Larry's list. Voltages below read with DBT
Base of Q2 = 0.00 VDC
Base of Q4 = 0.32 VDC
Base of Q6 = -39VDC
Collector of Q6= -22VDC

 

[merlynski]

I re-read 2.3 VDC with the battery and resistor following the schematic provided, they read about 2VDC powered on with DBT. I replaced Q2, Q4, Q6 with Larry's list. Voltages below read with DBT
Base of Q2 = 0.00 VDC
Base of Q4 = 0.32 VDC
Base of Q6 = -39VDC
Collector of Q6= -22VDC

I would be happy with the voltages you read the second time on the STV4s !!
Base of Q2 is where it should be.
Base of Q4 should be the same as Q2 if the diff amp is balancing correctly.
The collector of Q6 should be -1.2vdc, measure pin 17 of the pcb.
Did you replace VR4? <==

Turn off power and test VR4, to be sure it is 100Ω at maximum resistance and 0Ω at minimum resistance. You may have to unsolder it. Those old pots have a reputation for getting flaky with age, if they go open, even momentarily, then bias can go through the roof and damage output and driver transistors.

Test D5.
It looks to me like the problem is right in that area. Look for broken traces (sometimes they crack so small you can't see them, use ohmmeter to check point to point) and/or solder bridges, measure R22, R24, R26, R28 and R30. Re-verify Q6 is oriented correctly, we have all installed them wrong in the past.

EDIT: correct 'VR1' to VR4 <==

 

[tonefoto]

I would be happy with the voltages you read the second time on the STV4s !!
Base of Q2 is where it should be.
Base of Q4 should be the same as Q2 if the diff amp is balancing correctly.
The collector of Q6 should be -1.2vdc, measure pin 17 of the pcb.
Did you replace VR4? <==

Turn off power and test VR4, to be sure it is 100Ω at maximum resistance and 0Ω at minimum resistance. You may have to unsolder it. Those old pots have a reputation for getting flaky with age, if they go open, even momentarily, then bias can go through the roof and damage output and driver transistors.

Test D5.
It looks to me like the problem is right in that area. Look for broken traces (sometimes they crack so small you can't see them, use ohmmeter to check point to point) and/or solder bridges, measure R22, R24, R26, R28 and R30. Re-verify Q6 is oriented correctly, we have all installed them wrong in the past.

EDIT: correct 'VR1' to VR4 <==

V4 behaves very near to what V3 does, with the exception that V4 reads a min of 5Ω to a max of 84Ω, V3 reads min 6Ω to max of 103Ω.
Pin 17 reads -40 VDC.
R22/R24/R26/R28/R30 all read good tested with one leg removed.
Q2/Q4/Q6 are all oriented correctly.
I'm going over the traces on Q2/Q4 and installing new copper pads. The board has been worked over so much before I got it, I'm afraid it could be bad connections at these traces.

 

[merlynski]

V4 behaves very near to what V3 does, with the exception that V4 reads a min of 5Ω to a max of 84Ω, V3 reads min 6Ω to max of 103Ω.
Pin 17 reads -40 VDC.
R22/R24/R26/R28/R30 all read good tested with one leg removed.
Q2/Q4/Q6 are all oriented correctly.
I'm going over the traces on Q2/Q4 and installing new copper pads. The board has been worked over so much before I got it, I'm afraid it could be bad connections at these traces.

Those look like acceptable values for VR4 and VR3.
I did not realize it had been 'worked over' before, that just makes checking pads and traces even more important.

 

[merlynski]

Pin 17 is an unregulated -45vdc supply, -40vdc is a reasonable reading on DBT.

 

[tonefoto]

Pin 17 is an unregulated -45vdc supply, -40vdc is a reasonable reading on DBT.

OK, here's what I have so far. I installed new pads on the Q2/Q4 traces and got a cleaner contact solder. I soldered the wire wrap that was frayed on pin 5.
Checked voltage on pin 5 with DBT and its now reading about 0 VDC like pin 20. No audible relay click with DBT

 

[merlynski]

So your power amp stages' outputs are both reading 0vdc? Does the dc offset change on pin 20 when you adjust VR1? Does the dc offset change on pin 5 when you adjust VR2? Did the relay click?

 

[tonefoto]

So your power amp stages outputs' are both reading 0vdc? Does the dc offset change on pin 20 when you adjust VR1? Does the dc offset change on pin 5 when you adjust VR2? Did the relay click?

Output transistors not installed. On DBT I can adjust both VR1 and VR2. No audible relay click