First, confirm presence od +/- 80V rails.
Then measure voltage between pins 3 and 4 of U1, just so we know. Then short across, temporarily with small insulated tool so that you can release quickly if needed. If nothing alarming, then make a more permanent connection.
If nothing obvious presents as a failure, then I suggest taking data outlined in post 19. I'd probably start with measurement of the supply rails for a big-picture view, then drill more deeply for the other nodes.
Good luck!
Then measure voltage between pins 3 and 4 of U1, just so we know. Then short across, temporarily with small insulated tool so that you can release quickly if needed. If nothing alarming, then make a more permanent connection.
If nothing obvious presents as a failure, then I suggest taking data outlined in post 19. I'd probably start with measurement of the supply rails for a big-picture view, then drill more deeply for the other nodes.
Good luck!
Pins 3&4 show 49VAC. 0.0VDC.
No obvious surprises when shorting.
Will document measurements next.
Thanks again,
Peter
No obvious surprises when shorting.
Will document measurements next.
Thanks again,
Peter
Hello again... been busy with life.
*Forget below, they both go to ground.
I got back into it and found continuity between pins 3&4 of U2. I realize it's hard to determine if it's the PC817 that's bad or something else in circuit, but I wanted to mention it anyway.
Any thoughts?
Take care,
Peter
*Forget below, they both go to ground.
I got back into it and found continuity between pins 3&4 of U2. I realize it's hard to determine if it's the PC817 that's bad or something else in circuit, but I wanted to mention it anyway.
Any thoughts?
Take care,
Peter
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Hello again,
Results below. Q6/7 and Q17/18 look questionable.
TP5-8 : -0.01
Peter
Results below. Q6/7 and Q17/18 look questionable.
TP1&3 : 0.01, TP2&4 : 0.0
TP5-8 : -0.01
Thanks again!
Peter
I wonder if data above was taken without the 2W current-limit resistors. Below is from post 20. The combined C3+C12 voltage may be enough to reveal what the output stage is doing.
And yes, you should short across pins 3 & 4 of U1 to activate bias of the power amp if U4 isn't already conducting.
And yes, you should short across pins 3 & 4 of U1 to activate bias of the power amp if U4 isn't already conducting.
This is with the 20W resistors, otherwise I'm sure it would blow the fuse again.
This is, however, with shorting pins 3 & 4 of U2. Doing this jumped Q32 up to 3.1V.
This is, however, with shorting pins 3 & 4 of U2. Doing this jumped Q32 up to 3.1V.
So, are measurements in post 24 still correct? Rail voltages seem lower than what you reported in post 20. Are rail voltages lower due to shorting U1?
Yes! When removing the short, C3 goes from to 10.4V from around 0.95V. When adding the short again, it drops all the way into -V for a bit, then eventually stabilizes at 0.95V-ish.
C12 is also at 0.95V stabilized with short.
4.41V without short.
C12 is also at 0.95V stabilized with short.
4.41V without short.
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If I interpret correctly, the right channel is powered independently from its own power transformer, so it's working fine. We can compare measurements from it if needed. I may have to ask for comparative measurements in the right channel.
The supply voltages with U1 shorted are so low that it's hard to draw any conclusions. But I believe those voltages are also abnormal with U1 short absent. Would you report voltages of C1 and C17 near the +/- 80V rails. Then remove the U1 short and check for any LED current flowing by measuring across R42. If any voltage present, install a short across U1 pins 1 and 2; I want to ensure U1 is not conducting.
Then remeasure at C1, C17, across Q32 collector to emitter, and all points listed in post 19.
Thanks!
The supply voltages with U1 shorted are so low that it's hard to draw any conclusions. But I believe those voltages are also abnormal with U1 short absent. Would you report voltages of C1 and C17 near the +/- 80V rails. Then remove the U1 short and check for any LED current flowing by measuring across R42. If any voltage present, install a short across U1 pins 1 and 2; I want to ensure U1 is not conducting.
Then remeasure at C1, C17, across Q32 collector to emitter, and all points listed in post 19.
Thanks!
Hi, so during the brief testing of R42 (1 minute, maybe) I noticed R13 & R14 getting pretty warm/hot.
C1 & C17 show almost no voltage. 0.03 or so, usually showing 0.0. My meter might not be the best...
The same with R42, pretty much nothing.
C1 & C17 show almost no voltage. 0.03 or so, usually showing 0.0. My meter might not be the best...
The same with R42, pretty much nothing.
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I don't understand C1 and C17 not showing voltage of more than +/-60VDC. R2 and R23 should be smoking hot if 80V rails are working and C1 & C17 are truly at 0V. Double check +/-80V rails and probe R2 and R23 resistor leads on both sides of the resistor bodies. Once things make sense, please attempt post 29 readings again.
Thanks.
Thanks.
Hello,
80V rails with U1 jumper= 86V. This might move a bit if I kept it on but with the jumper, those resistors mentioned earlier get hot so I did this quickly.
R2 without jumper on U1 3&4= 2.5
R23 as above = 2.0
R2 with jumper = 8.6
R23 as above= 8.5
Thanks again for your help!
Peter
80V rails with U1 jumper= 86V. This might move a bit if I kept it on but with the jumper, those resistors mentioned earlier get hot so I did this quickly.
R2 without jumper on U1 3&4= 2.5
R23 as above = 2.0
R2 with jumper = 8.6
R23 as above= 8.5
Thanks again for your help!
Peter
The voltages across R2 and R23 seem reasonable; 8.6V across 150 ohm is less than 0.5W dissipation and those resistors are specified at 3W.
2.5V across R23 implies about 13mA, not a lot of current, but I think we should try to determine where it's flowing. Would you do a bit of signal tracing and see how much voltage is dropped across R69, R71, R76 and R43, R45, R46. And also voltages across R14 and Q32.
Thanks.
2.5V across R23 implies about 13mA, not a lot of current, but I think we should try to determine where it's flowing. Would you do a bit of signal tracing and see how much voltage is dropped across R69, R71, R76 and R43, R45, R46. And also voltages across R14 and Q32.
Thanks.
Okay, some numbers!
R13 136.7V
R14 135.3V
R69 6.41V
R71 5.98
R76 5.86
R43 6.75
R45 5.7
R46 6.74
Q32 3.37/3.49
So yeah, those hot resistors are hot for a reason!
Any ideas?
R13 136.7V
R14 135.3V
R69 6.41V
R71 5.98
R76 5.86
R43 6.75
R45 5.7
R46 6.74
Q32 3.37/3.49
So yeah, those hot resistors are hot for a reason!
Any ideas?
So, let's install a jumper wire across C26 or Q32, where convenient. That should shut down all the power output transistors (i.e. Q32 and all transistors to the right), but may not if there's short in the output stages. So with luck the voltages across C1 and C17 should rise dramatically and collector voltages in the outputs. Let me know what what voltage you observe at the amp output and at the +/- 60V rails.
Amp output = speaker connection? If so, with speakers 1 turned on, 0.0V
C1 78.5
C17 79.0
60V rails 2.54 and -7.43
What should be the vintage of the 60V rail when using the limiting resistors?
C1 78.5
C17 79.0
60V rails 2.54 and -7.43
What should be the vintage of the 60V rail when using the limiting resistors?
One thought... I tested C1 and C17 from their side of R2 and R23 to ground. Maybe this isn't correct! It's difficult to get to the bottom of the board where they are, but not impossible. Let me know if this is wrong.
If so, I'm sorry to have mislead you with incorrect information.
If so, I'm sorry to have mislead you with incorrect information.
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Your latest C1 and C17 measurements look fine.
Amp output should be open circuit for the suggested test, measured at "OUTPUT", eg. R9,R21. Sorry I wasn't more explicit.
I was mistaken in referring to expected dramatic rise in C1, C17 voltage; I intended to refer to +/-60V rails, eg. C12, C3. The rise in their voltages will be the big reveal when a short is installed across C26; that should turn off all output devices, i.e. Q32,Q18,Q17,Q12,Q14,Q15 and Q6,Q7,Q2,Q1,Q3. With those devices off, the hope is that the 60V rails will slowly charge toward 60V via the current limit resistors. How they actually behave should be a big clue, as well as base-emitter voltages in the output devices.
Amp output should be open circuit for the suggested test, measured at "OUTPUT", eg. R9,R21. Sorry I wasn't more explicit.
I was mistaken in referring to expected dramatic rise in C1, C17 voltage; I intended to refer to +/-60V rails, eg. C12, C3. The rise in their voltages will be the big reveal when a short is installed across C26; that should turn off all output devices, i.e. Q32,Q18,Q17,Q12,Q14,Q15 and Q6,Q7,Q2,Q1,Q3. With those devices off, the hope is that the 60V rails will slowly charge toward 60V via the current limit resistors. How they actually behave should be a big clue, as well as base-emitter voltages in the output devices.
The 60V rails, with pin 3&4 U1 jumper and C26 jumper, are stable at 2.5/-7.4.
What part of the output devices are turned off? It's there something I can measure to check?
What part of the output devices are turned off? It's there something I can measure to check?