Okay, well some progress! I did as you said and it turned on!
Unfortunately, the TP1 to speaker ground measures 60V, when it should be adjusted to only 38mV. R53 was at the full 200ohm adjustment. I didn't try adjusting as I suspect something is still wrong.
I measured the speaker terminals and no DC or AC was present.
Any idea what I could have possibly done wrong?
Unfortunately, the TP1 to speaker ground measures 60V, when it should be adjusted to only 38mV. R53 was at the full 200ohm adjustment. I didn't try adjusting as I suspect something is still wrong.
I measured the speaker terminals and no DC or AC was present.
Any idea what I could have possibly done wrong?
All preliminary tests checked out, so you’ve installed the output stages? Are the current limit resistors still present? Anything hot?
Some broad diagnostic voltages:
Is U1 active, i.e. voltage between pins 3 and 4 near 0V, so amp is enabled?
Voltages across R43 and R76?
Voltages re ground:
Spreader voltages at collector and emitter of Q32
input at base Q31
feedback at base Q30
Output
supply voltages at U4, Pins 4 and 8
U4 pin 1
Thanks!
Some broad diagnostic voltages:
Is U1 active, i.e. voltage between pins 3 and 4 near 0V, so amp is enabled?
Voltages across R43 and R76?
Voltages re ground:
Spreader voltages at collector and emitter of Q32
input at base Q31
feedback at base Q30
Output
supply voltages at U4, Pins 4 and 8
U4 pin 1
Thanks!
This was done without the current limiting resistors. Nothing noticeably hot, but I didn't leave it on long.
So, I assume I do those tests with the limiting resistors?
So, I assume I do those tests with the limiting resistors?
No need for limit resistors apparently since you survived without them. 🙂
But probe carefully, perhaps with heat shrink to minimize exposed lead.
But probe carefully, perhaps with heat shrink to minimize exposed lead.
Is U1 active, i.e. voltage between pins 3 and 4 near 0V, so amp is enabled? Nope, 27V. I assume this is because I don't have the ribbon cable J1 connected?
Shorting U1 as before, I get 35V bias instead of 60V.
Voltages across R43 and R76? 1.1/2.6
Voltages re ground:
Spreader voltages at collector and emitter of Q32 59.4/62.3/59.2
input at base Q31: 0.5V
feedback at base Q30: 2.1V
Output
supply voltages at U4, Pins 4 and 8: -15.0/0.0
U4 pin 1: 16V
All tested without the U1 jumper and J1 disconnected.
Shorting U1 as before, I get 35V bias instead of 60V.
Voltages across R43 and R76? 1.1/2.6
Voltages re ground:
Spreader voltages at collector and emitter of Q32 59.4/62.3/59.2
input at base Q31: 0.5V
feedback at base Q30: 2.1V
Output
supply voltages at U4, Pins 4 and 8: -15.0/0.0
U4 pin 1: 16V
All tested without the U1 jumper and J1 disconnected.
Last edited:
Hi Peter,
It should be OK for J1 to be disconnected, but U1 has to be shorted for input bias stages to be enabled, but disconnected J1 disables U1 drive.
You saw no smoke when you temporarily shorted U1? Then restore short at U1 as before. Supply voltages at U4 look good, but remeasure the following, as I anticipate results will be different:
Voltages across R43 and R76?
Voltages re ground:
Spreader voltages at collector and emitter of Q32
input at base Q31
feedback at base Q30
Output
U4 pin 1
Output voltage will likely be +37V as you measured earlier, but we have to find why this the case.
It should be OK for J1 to be disconnected, but U1 has to be shorted for input bias stages to be enabled, but disconnected J1 disables U1 drive.
You saw no smoke when you temporarily shorted U1? Then restore short at U1 as before. Supply voltages at U4 look good, but remeasure the following, as I anticipate results will be different:
Voltages across R43 and R76?
Voltages re ground:
Spreader voltages at collector and emitter of Q32
input at base Q31
feedback at base Q30
Output
U4 pin 1
Output voltage will likely be +37V as you measured earlier, but we have to find why this the case.
Voltages across R43 and R76? 10.6/11.3
Voltages re ground:
Spreader voltages at collector and emitter of Q32: 36/36.8/35.3
input at base Q31: 0.5
feedback at base Q30: 1.5
Output: 35.2
U4 pin 1: -14.9
Voltages re ground:
Spreader voltages at collector and emitter of Q32: 36/36.8/35.3
input at base Q31: 0.5
feedback at base Q30: 1.5
Output: 35.2
U4 pin 1: -14.9
Output : the bias adjustments points.
Or did you want be to check for DC on the speaker output? If so, that still measures 0.0V.
Or did you want be to check for DC on the speaker output? If so, that still measures 0.0V.
The bias spreader is back to being too low, so that's disappointing, but we'll ignore for now.
I'm surprised to find input at base of Q31 at +0.5V--- I was expecting ~ 0V.
Check for same 0.5V at R57 and 0.5V across R57 leads. Confirm with power off that R57 is in fact 22k and not open. (R57 on schematic is drawn near the J1 connector on page page 109. Now that I reflect, it's probably sufficient to measure resistance from Q31 base to ground, Should be about 23k.)
If these tests suggest that Q51 base is real and not a leakage reading, then measure voltages across R39 and R75. Confirm +/- 16V rails near these resistors.
Measure voltages at:
collector of Q23
emitter of Q31
emitter of Q30
emitter of Q28
emitter of Q26
collector of Q35
DC output at the test points is OK for measurement point.
I'm surprised to find input at base of Q31 at +0.5V--- I was expecting ~ 0V.
Check for same 0.5V at R57 and 0.5V across R57 leads. Confirm with power off that R57 is in fact 22k and not open. (R57 on schematic is drawn near the J1 connector on page page 109. Now that I reflect, it's probably sufficient to measure resistance from Q31 base to ground, Should be about 23k.)
If these tests suggest that Q51 base is real and not a leakage reading, then measure voltages across R39 and R75. Confirm +/- 16V rails near these resistors.
Measure voltages at:
collector of Q23
emitter of Q31
emitter of Q30
emitter of Q28
emitter of Q26
collector of Q35
DC output at the test points is OK for measurement point.
Last edited:
R57/Q31 to ground= 22.8k
R57 0.48V
R39: 7.44
R75: 7.46
16V rails = 15V
collector of Q23:0
emitter of Q31:1.71
emitter of Q30:1.72
emitter of Q28:0.04
emitter of Q26:2.2/-14.6/0.04
collector of Q35:-15
R57 0.48V
R39: 7.44
R75: 7.46
16V rails = 15V
collector of Q23:0
emitter of Q31:1.71
emitter of Q30:1.72
emitter of Q28:0.04
emitter of Q26:2.2/-14.6/0.04
collector of Q35:-15
A problem narrowed:
Q35 collector is -15V, but Q31, Q30 emitters are at +1.72V, so there's an open somewhere in this area, most likely between the Q35 collector and junction of R63, R64. (Since emitters have the same voltage, the R63 to R64 path is likely intact.)
Once repaired and base of Q31 = ~0V, remeasure points in post 109.
Q35 collector is -15V, but Q31, Q30 emitters are at +1.72V, so there's an open somewhere in this area, most likely between the Q35 collector and junction of R63, R64. (Since emitters have the same voltage, the R63 to R64 path is likely intact.)
Once repaired and base of Q31 = ~0V, remeasure points in post 109.
While checking I noticed that touching ground and my steel sink gives me a mild shock. Kinda tickles. Probably not by design.
Obviously, you shouldn't do that! Be safe. Try one handed probing. You don't want any currents flowing through your chest!
The voltage at Q35 collector should be roughly -1.2V, so there's still something wrong in this area. Measure voltages across R63, R63. Should be about 0.6V if balanced. My guess no drops.
A hint--- probe voltage between R63,64 and Q35 collector. It should be 0V but probably isn't. Or more safely, probe for continuity or the lack of. Try to find the open.
Okay, so I have no idea what happened yesterday, but while checking for continuity and voltages on the resistors, I'm not getting the same readings. I don't know if my jumper list connection or what, but please ignore yesterday's numbers.
Here is a fresh list. I'm sorry again for wasting your time.
Continuity looks fine between Q35 and R63,R64.
Q35 to R63 1.2/0(continuity)
Q35 to R64 0.2V/0
BCE of Q23: 6.9/2.3/7.6
BCE of Q31: 0.5/14.7/-0.1
BCE of Q30: 1.5/14.7/0.9
BCE of Q28: 0.5/-14.7/1
BCE of Q26: 1.5/-14.7/2.0
BCE of Q35: -7/-0.3/-7.6
Just in case this might help:
BCE of Q36: 15/70.4/14.5
BCE of Q29: -15/-62.6/-14.5
BCE of Q33: 15.2/61.5/14.7
BCE of Q27: -15.1/-71.3/-14.6
16V rails: 15V
Voltages across R43 and R76: 10.4/11.2
Here is a fresh list. I'm sorry again for wasting your time.
Continuity looks fine between Q35 and R63,R64.
Q35 to R63 1.2/0(continuity)
Q35 to R64 0.2V/0
BCE of Q23: 6.9/2.3/7.6
BCE of Q31: 0.5/14.7/-0.1
BCE of Q30: 1.5/14.7/0.9
BCE of Q28: 0.5/-14.7/1
BCE of Q26: 1.5/-14.7/2.0
BCE of Q35: -7/-0.3/-7.6
Just in case this might help:
BCE of Q36: 15/70.4/14.5
BCE of Q29: -15/-62.6/-14.5
BCE of Q33: 15.2/61.5/14.7
BCE of Q27: -15.1/-71.3/-14.6
16V rails: 15V
Voltages across R43 and R76: 10.4/11.2
The input voltage at base of Q31 seems stuck at 0.5V for no good reason that I can think of. The readings near Q28 are enough to make me suspicious. Would you remove and test Q28? In the event is tests good, remove and test Q31. I don't recall--- do you have a transistor tester?
Thanks.
Thanks.