Hi.
In current configuration:
R76=8.4
R24=7.0
When connecting to Q24 base, the other meter jumps up about 0.6V. Nothing of note when probing the other emitter.
In current configuration:
R76=8.4
R24=7.0
When connecting to Q24 base, the other meter jumps up about 0.6V. Nothing of note when probing the other emitter.
Would you report observed voltages. When probing Q24 emitter, does it match Q25 base? When you probe Q24 base, Q25 base shifts more negative by 0.6V? What are voltages at bases of Q24 and Q25? They should differ by 0.6V
Q24 emitter: -71.8
Q25 base: -71.8
When probing Q24:
Q24 base: -72.6
Q25 base: -72
Both if these hunt around a bit.
Q24 base to emitter: 0.6V
Q25 base: -71.8
When probing Q24:
Q24 base: -72.6
Q25 base: -72
Both if these hunt around a bit.
Q24 base to emitter: 0.6V
All these readings look ok. So there's no indication of that mysterious 4V base-emitter observed earlier?
Would you report the voltages on Q9? I expect Q9 collector to be near the negative rail. Now would be a good time to check the bias spreader. What voltage variation do you find across C26 as you adjust R23 through its range?
Would you report the voltages on Q9? I expect Q9 collector to be near the negative rail. Now would be a good time to check the bias spreader. What voltage variation do you find across C26 as you adjust R23 through its range?
Q9: -69.8, 46.3, -70
C26 1.7V-3.2V across when turning R53.
Sorry about the lost hope... there is still ~4V difference between Q24 base and emitter to ground. My battery died in my second DMM just now, so I guess it gave me a strange result that doesn't correlate. I'll get a new battery and repeat.
C26 1.7V-3.2V across when turning R53.
Sorry about the lost hope... there is still ~4V difference between Q24 base and emitter to ground. My battery died in my second DMM just now, so I guess it gave me a strange result that doesn't correlate. I'll get a new battery and repeat.
Poking around, carefully, I see that D9 measures 0.7 across, but D20 measures 2.7V.
Funny it's on the other side from Q24, but maybe it means something?
Funny it's on the other side from Q24, but maybe it means something?
D7 may be forward biased, D20 be may be reversed biased. Remeasure with red lead on anode, black on anode, consistent orientation on each diode.
The diode voltages suggest that D9 is near 0V and not conducting any significant current. D20 appears to be reverse biased.
Your Q9 voltages in post 248 seem to indicate base emitter voltage is only 0.2V, not enough for Q9 to be conducting any significant current. Something odd, still.
Did replacing meter battery change anything?
Your Q9 voltages in post 248 seem to indicate base emitter voltage is only 0.2V, not enough for Q9 to be conducting any significant current. Something odd, still.
Did replacing meter battery change anything?
Hi!
If I pull the rest of the unit apart and get the other channel powered with current limiting resistors, as the other side, what would you like to see checked?
I was thinking of the numerous test points that are all over the board... the problem is I don't have them in the SM pages so I'd have to try to figure out what they are connected to.
If I pull the rest of the unit apart and get the other channel powered with current limiting resistors, as the other side, what would you like to see checked?
I was thinking of the numerous test points that are all over the board... the problem is I don't have them in the SM pages so I'd have to try to figure out what they are connected to.
Hi,
I suppose having more current limit resistors on hand would give you that option, but I'd be reluctant to subject a working channel to that much added jeopardy. My instinct is to retain the working channel as a reference to nominal voltages. And in the interest of caution, keep all but the tip of your meter probe shrouded with heat-shrink to minimize mishaps, and be very cautious about possible .slips of a probe that could wreak havoc.
I would keep searching for the apparent defect in the front end. The approach is to conduct experiments and look for behavior that doesn't make sense. Explore there until you understand what's happening and why. That eventually leads you to the problem. Easier said than done.
Many of the experiments we've contrived should force the front end into a rail. The most recent in posts 248/249. I would return to that point and try to learn why Q9 didn't conduct. Whenever when thinks don't make sense, it make sense to worry about oscillation making a scope the instrument of choice. I doubt oscillation is the explanation on this occasion, but can't be sure.
Take readings to get again acclimated.
Thanks.
I suppose having more current limit resistors on hand would give you that option, but I'd be reluctant to subject a working channel to that much added jeopardy. My instinct is to retain the working channel as a reference to nominal voltages. And in the interest of caution, keep all but the tip of your meter probe shrouded with heat-shrink to minimize mishaps, and be very cautious about possible .slips of a probe that could wreak havoc.
I would keep searching for the apparent defect in the front end. The approach is to conduct experiments and look for behavior that doesn't make sense. Explore there until you understand what's happening and why. That eventually leads you to the problem. Easier said than done.
Many of the experiments we've contrived should force the front end into a rail. The most recent in posts 248/249. I would return to that point and try to learn why Q9 didn't conduct. Whenever when thinks don't make sense, it make sense to worry about oscillation making a scope the instrument of choice. I doubt oscillation is the explanation on this occasion, but can't be sure.
Take readings to get again acclimated.
Thanks.
Hello again BSST! It's been a while, and I'm ready to have at it again.
I really hope you are willing and able to share more expertise, and time.
I changed the two Thermaltrak transistors as the bias offset instability on this amp was the reason I got into this situation in the first place. I haven't turned it on yet, but was planning on getting all the measurements again to start.
Do you feel you have the energy for some more investigations and guidance? I'd really appreciate it.
Peter
I really hope you are willing and able to share more expertise, and time.
I changed the two Thermaltrak transistors as the bias offset instability on this amp was the reason I got into this situation in the first place. I haven't turned it on yet, but was planning on getting all the measurements again to start.
Do you feel you have the energy for some more investigations and guidance? I'd really appreciate it.
Peter
Hi Peter,
I’m game if you are!
I understand that the Thermal track transistors have been discontinued?
I’m on a cruise and web access is erratic, so my replies may be sporadic.
Best,
Steve
I’m game if you are!
I understand that the Thermal track transistors have been discontinued?
I’m on a cruise and web access is erratic, so my replies may be sporadic.
Best,
Steve
Hi Steve,
They might be but Mouser had the two I needed.
I guess unit is okay for baseline testing? Here is the current state:
Current limiting resistors in place
C21 & C22 caps are bypassed (post #179)
A 22k connects between R56 and -16V (post #229)
Is there anything I need to consider before powering up and getting readings? Should I leave the above mods in place?
A cruise huh? Sounds nice. Where are you headed/leaving?
Take care,
Peter
They might be but Mouser had the two I needed.
I guess unit is okay for baseline testing? Here is the current state:
Current limiting resistors in place
C21 & C22 caps are bypassed (post #179)
A 22k connects between R56 and -16V (post #229)
Is there anything I need to consider before powering up and getting readings? Should I leave the above mods in place?
A cruise huh? Sounds nice. Where are you headed/leaving?
Take care,
Peter
If you're reasonably confident the amp is in a safe state for measurements, fresh data would be good. I have to revisit where we were.
Cruise from Sydney to Hong Kong. My wife lives to cruise...
Cruise from Sydney to Hong Kong. My wife lives to cruise...
Hello Steve, how was the cruise?
Here are the current measurements, BCE:
Q22 8.1,7.3, 8.7
Q34 -8.1, -7.7, -8.9
Q23: 7.7, 1.5, 8.1
Q35 -7.4, -1.2, -8
Q36 15.1, 71.3, 14.6
Q29 -15, -71.3, -14.4
Q31 0.1,14.7, -0.5
Q28 0.1, -14.5, 0.7
Q33 15.1, 66.7, 14.5
Q27 -15.1, -72, -14.6
Q30 0.6, 14.9, -0.1
Q26 0.5, -14.5, 1.1
Q37 70.6, 75, 70.3
Q24 -67.7, -76.5, -67.5
Q38 70.3, 47.9, 65.2
Q25 -67.7, -67.4, 68.2
Q11 47.9, 43.7, 61.2
Q9 -67.4, 42, -66.8
Whew...
Still some big discrepancies after about Q33/Q27, then a larger one at Q38/Q25.
Any thoughts?
Here are the current measurements, BCE:
Q22 8.1,7.3, 8.7
Q34 -8.1, -7.7, -8.9
Q23: 7.7, 1.5, 8.1
Q35 -7.4, -1.2, -8
Q36 15.1, 71.3, 14.6
Q29 -15, -71.3, -14.4
Q31 0.1,14.7, -0.5
Q28 0.1, -14.5, 0.7
Q33 15.1, 66.7, 14.5
Q27 -15.1, -72, -14.6
Q30 0.6, 14.9, -0.1
Q26 0.5, -14.5, 1.1
Q37 70.6, 75, 70.3
Q24 -67.7, -76.5, -67.5
Q38 70.3, 47.9, 65.2
Q25 -67.7, -67.4, 68.2
Q11 47.9, 43.7, 61.2
Q9 -67.4, 42, -66.8
Whew...
Still some big discrepancies after about Q33/Q27, then a larger one at Q38/Q25.
Any thoughts?
Hi Peter,
I'm still trying to get re-assimilated to the present state of your amp.
Given that voltages fluctuate as you measure, a 'scope would be the instrument of choice for troubleshooting. There's a possibility that oscillation is obscuring accurate measurement. But you have only a couple of DVMs, right?
The C21 & C22 caps-bypass disables the servo, so confirm that the opamp output is 0V. This tactic still seems appropriate, as it removes the servo as a contributing unknown.
Your data from #256 implies the bias spreader voltage (i.e. voltage across C26) is about 1.7V (43.7V-42V). It takes about 3.6V spread to bias the output stages into conduction, so the outputs shouldn't be in conduction if the 1.7V number is trusted. I don't know why the spreader voltage is so small. Did we modify its configuration in an earlier test?
The 22k connects between R56 and -16V. If everything were nominal, the output would be ~ (-16V*R55/22k) = +7.3V, but bias near C26 suggests it's roughly ~+40V. What output voltage do you observe? What are voltages on the +60V & -60V rails (currently current limited)?
I'm still trying to get re-assimilated to the present state of your amp.
Given that voltages fluctuate as you measure, a 'scope would be the instrument of choice for troubleshooting. There's a possibility that oscillation is obscuring accurate measurement. But you have only a couple of DVMs, right?
The C21 & C22 caps-bypass disables the servo, so confirm that the opamp output is 0V. This tactic still seems appropriate, as it removes the servo as a contributing unknown.
Your data from #256 implies the bias spreader voltage (i.e. voltage across C26) is about 1.7V (43.7V-42V). It takes about 3.6V spread to bias the output stages into conduction, so the outputs shouldn't be in conduction if the 1.7V number is trusted. I don't know why the spreader voltage is so small. Did we modify its configuration in an earlier test?
The 22k connects between R56 and -16V. If everything were nominal, the output would be ~ (-16V*R55/22k) = +7.3V, but bias near C26 suggests it's roughly ~+40V. What output voltage do you observe? What are voltages on the +60V & -60V rails (currently current limited)?
Hi Steve,
C26= 2.2v
U1 pins 3&4= 27,7v. Normally shorted.
60V rails = 45v. One observation regarding the 60v rails... the positive rail jumps to 45v when powering on, but the negative climbs slowly over 30seconds or so.
Configuration:
C21&C22 bypassed
R55 out of circuit
Q6 & Q18 out of circuit
22k between -16v and R56
Q17 shorted to ground with resistor
C26= 2.2v
U1 pins 3&4= 27,7v. Normally shorted.
60V rails = 45v. One observation regarding the 60v rails... the positive rail jumps to 45v when powering on, but the negative climbs slowly over 30seconds or so.
How do I test for this?
Configuration:
C21&C22 bypassed
R55 out of circuit
Q6 & Q18 out of circuit
22k between -16v and R56
Q17 shorted to ground with resistor
This is concerning. With U1 not shorted, pin 4 should be roughly +16V, pin 3 roughly -15V. Please double check.
2nd thought, if there's 27.7V between the pins, that's OK.
U1 should be shorted to establish operating bias. You may have to remeasure voltages.