Hi Peter,
Sorry for delay.
This data makes me suspicious of Q28 hfe.
Would you measure Q28 and Q31 gain in your tester?
And would measure Q31 base voltage in the good channel for comparison?
While Q28 and Q31 are out-of-circuit, you can confirm R57+R58 are about 23k from transistor pad to chassis. But I believe they and circuit traces are OK.
Thanks!
Sorry for delay.
This data makes me suspicious of Q28 hfe.
Would you measure Q28 and Q31 gain in your tester?
And would measure Q31 base voltage in the good channel for comparison?
While Q28 and Q31 are out-of-circuit, you can confirm R57+R58 are about 23k from transistor pad to chassis. But I believe they and circuit traces are OK.
Thanks!
Last edited:
Your hfe numbers from that post fit the data fairly well: Q28 base current ~ 1.8mA/145 = 12.4uA; Q31 base current ~2.2mA/495 = 4.2uA. Net bias current = 12.4 -4.2 = 8.2uA. And 8.2uA*23k = 0.19V which is in good agreement with observed base voltage. We had some suspicion at the time but didn't pursue fully. Measuring base voltage in the other channel would be a quick check of the this issue in the other channel.
I suggest replacing Q28, but it may not be the only problem.
With the perspective at the ~ 0.2V at the input is understood, post 195 seems to make sense. I suggest returning to the configuration in post 191. From there we can recheck the bias spreader, do some independent checks of output stage. Eventually reconnect everything.
I suggest replacing Q28, but it may not be the only problem.
With the perspective at the ~ 0.2V at the input is understood, post 195 seems to make sense. I suggest returning to the configuration in post 191. From there we can recheck the bias spreader, do some independent checks of output stage. Eventually reconnect everything.
Hello again!
So, I bought components for replacement, if needed.
I started with replacing Q23 & Q35 because the collectors were not matching. They now match much better:
Q23: 7, 1.4, 7.5
Q35: -7, -1.2, -7.5
Now, looking at Q36 & Q29 in the same light, their collectors mis-match too. 65v to 80v.
Are these good candidates to replace? It seems if these match better, the bases of Q37 & Q24 would be closer to reach other and that might help Q38 & Q25... Or do I have this backwards?
I can replace Q38 & Q25 as well.
What do you think?
So, I bought components for replacement, if needed.
I started with replacing Q23 & Q35 because the collectors were not matching. They now match much better:
Q23: 7, 1.4, 7.5
Q35: -7, -1.2, -7.5
Now, looking at Q36 & Q29 in the same light, their collectors mis-match too. 65v to 80v.
Are these good candidates to replace? It seems if these match better, the bases of Q37 & Q24 would be closer to reach other and that might help Q38 & Q25... Or do I have this backwards?
I can replace Q38 & Q25 as well.
What do you think?
Q28 and Q31 are by far the transistors most sensitive to matching of hfe (and low hfe), since those parameters control the bias current flowing through R57+R58 (=23k). Q26 and Q30 are less susceptible because they see much lower bias resistance (chiefly R56 = 270).
We had determined in post 203 that Q28 deserved replacement. Ideally, its hfe would be the same as Q31. Both would benefit from high hfe.
I suggest ensuring that the test configuration corresponds to post 195 and you'll likely see base voltage at "INPUT" similar to previous experiments. Then replace Q28 (and maybe Q31) with measured hfe. With luck, you will observe smaller bias voltage at INPUT. I think bias smaller than 100mV is probably acceptable, as bias servo will eventually null output to ~ 0.
Let me know how all this evolves. Eventually we will test bias spreader, do some separate checks of outputs, and finally rejoin everything.
We had determined in post 203 that Q28 deserved replacement. Ideally, its hfe would be the same as Q31. Both would benefit from high hfe.
I suggest ensuring that the test configuration corresponds to post 195 and you'll likely see base voltage at "INPUT" similar to previous experiments. Then replace Q28 (and maybe Q31) with measured hfe. With luck, you will observe smaller bias voltage at INPUT. I think bias smaller than 100mV is probably acceptable, as bias servo will eventually null output to ~ 0.
Let me know how all this evolves. Eventually we will test bias spreader, do some separate checks of outputs, and finally rejoin everything.
Hi! Okay, I replaced the following, hfe matching as you said and as close as I could.
Q28, Q26, Q31, Q24, Q37 in addition to Q23 & Q35 previously.
In config from post #195, so no resistor from R65, but still with the other resistor and jumping of the two caps.
I find it strange that Q23 & Q35 collectors don't match now. However, Q26 & Q30 emitters match.
Here are the results, which show the input where you want it, but still some others failed to get closer.
Q23 6.9, 1.3, 7.5
Q35 -6.9, -2.1, -7.5
Q29 -15, -76, -14.4
Q36 15, 66, 14.6
Q28 -0.7, -14.4, 0.5
Q31 -0.7, 14.6, -0.7
Q26 0, -14.4, 0.6
Q30 0, 14.6, -0.6
Q24-76.4, -70, -71.7
Q37 66.2, 76.6, 66
Q25 -71.7, -71.8, -72.7
Q38 66.1, 66.1, 66.9
Q28, Q26, Q31, Q24, Q37 in addition to Q23 & Q35 previously.
In config from post #195, so no resistor from R65, but still with the other resistor and jumping of the two caps.
I find it strange that Q23 & Q35 collectors don't match now. However, Q26 & Q30 emitters match.
Here are the results, which show the input where you want it, but still some others failed to get closer.
Q23 6.9, 1.3, 7.5
Q35 -6.9, -2.1, -7.5
Q29 -15, -76, -14.4
Q36 15, 66, 14.6
Q28 -0.7, -14.4, 0.5
Q31 -0.7, 14.6, -0.7
Q26 0, -14.4, 0.6
Q30 0, 14.6, -0.6
Q24-76.4, -70, -71.7
Q37 66.2, 76.6, 66
Q25 -71.7, -71.8, -72.7
Q38 66.1, 66.1, 66.9
The collectors are the outputs of constant current sources and their voltages go where dictated by the circuits attached to the collectors. Note that the Q26 and Q30 emitters have equal magnitudes but opposite signs. So the current sources have different output voltages.
I didn’t direct you to the best post. This connection is needed to establish bias on the front end (from post 191):
Please advise voltages at C26, Q30 base, and Q31 base.
Thanks again. I’m traveling so may have slow replies for a few days.
Steve
Hi Peter,
Im sorry for delay.
Now we need to discover why C26 voltage is so elevated, so let's make that the immediate focus.
One possible contributor might be via R65 from the amp output. What is voltage at output? If output is stuck high, would you remove R65 and see if this lowers voltage at C26?
But if amp output is near 0v, then R65 won't be a contributor. Please confirm servo amp is disabled, i.e. U3 pin 1 is ~0V.
Base voltage at Q30 is mysteriously large. In post 210, test gain was estimated as +82, but at the feedback node (base Q30), gain will be about -81, so expected voltage at C26 would be about 0.6V *(-81) = ~ -48V. But you observe +60V, so something is very wrong.
To get us started, would you take some initial readings:
C26, Q30 base, and Q31 base
Collector Q33
Collector Q36
Emitter Q37
Emitter Q38
Collector Q11
Collector Q27
Collector Q29
Emitter Q24
Emitter Q25
Collector Q9
Thanks!
Im sorry for delay.
Now we need to discover why C26 voltage is so elevated, so let's make that the immediate focus.
One possible contributor might be via R65 from the amp output. What is voltage at output? If output is stuck high, would you remove R65 and see if this lowers voltage at C26?
But if amp output is near 0v, then R65 won't be a contributor. Please confirm servo amp is disabled, i.e. U3 pin 1 is ~0V.
Base voltage at Q30 is mysteriously large. In post 210, test gain was estimated as +82, but at the feedback node (base Q30), gain will be about -81, so expected voltage at C26 would be about 0.6V *(-81) = ~ -48V. But you observe +60V, so something is very wrong.
To get us started, would you take some initial readings:
C26, Q30 base, and Q31 base
Collector Q33
Collector Q36
Emitter Q37
Emitter Q38
Collector Q11
Collector Q27
Collector Q29
Emitter Q24
Emitter Q25
Collector Q9
Thanks!
The most glaring problem I spot is Q24: assuming intact PCB traces, there is -69V at base but -64.8V at emitter. There should be only 0.6 V between base and emitter. Please explore directly on Q24 leads to confirm voltages and trace integrity. Also check for correct device type and correct insertion.
Thanks.
Thanks.
From your Q24 measurements, base is -69.5, emitter is -65.4 Difference is 4.1V
This cannot exist in a functional transistor. Would you arrange to get your meter leads across the base and emitter leads of Q24? Tack on hookup wires if necessary. This mystery has to be solved to make any progress.
This cannot exist in a functional transistor. Would you arrange to get your meter leads across the base and emitter leads of Q24? Tack on hookup wires if necessary. This mystery has to be solved to make any progress.
Hi Peter,
I've had a chance to become more awake.
Try this test procedure--- it may be easier.
Probe at collector of Q29 or on R46. You should observe about -69.5V.
Leave red probe in the same spot and explore with the black probe. There should be no difference voltage when you probe at base of Q24, and you should find 0.6V when you probe emitter lead of Q24. (You might find 0V or even a negative reading if there's an open trace at the emitter.) If you do find 0.6V at the emitter, trace along the path toward base of Q25, cathode of D10. Somewhere you should find a drop of about 4.1V noted in post 216.
Good luck.
I've had a chance to become more awake.
Try this test procedure--- it may be easier.
Probe at collector of Q29 or on R46. You should observe about -69.5V.
Leave red probe in the same spot and explore with the black probe. There should be no difference voltage when you probe at base of Q24, and you should find 0.6V when you probe emitter lead of Q24. (You might find 0V or even a negative reading if there's an open trace at the emitter.) If you do find 0.6V at the emitter, trace along the path toward base of Q25, cathode of D10. Somewhere you should find a drop of about 4.1V noted in post 216.
Good luck.
R46= 76v
Q24 base = 0.0
Q24 emitter no= 0.6
Q25 base = 0.6
D10= 0.6
D12= -1.3
I found D14 & D15 show 120v+ on their collectors to R46, and 57/58 to ground.
Okay .. also strange, but done differently. Before when I checked Q24, it was with power off and using the diode function... 0.6v. Now I tried with power on, and it's 0.6v between the base and emitter, but same 4v+ as before to ground! How is that happening?!
Q24 base = 0.0
Q24 emitter no= 0.6
Q25 base = 0.6
D10= 0.6
D12= -1.3
I found D14 & D15 show 120v+ on their collectors to R46, and 57/58 to ground.
Okay .. also strange, but done differently. Before when I checked Q24, it was with power off and using the diode function... 0.6v. Now I tried with power on, and it's 0.6v between the base and emitter, but same 4v+ as before to ground! How is that happening?!