Hello again.
All measurements below are without +/-16v jumper connected.
So, I took the other 2SA1837, which is the removed Q6 and measured it. hFE of 160, so closer than the other. However, the numbers for Q11 still have a low emitter number:
Q11 64.6, 47.8, 51.2
Q9 -69.3, -46.5, -70
I measured the 2SC4793 from Q18 and it measures as two resistors. So I assume defective/blown/junk.
Q11 75.9, 61.8, 62.8
Q9 -81, 58.7, -69.5
Well... maybe this is useful? Q11 emitter is now closer to where Q9's emitter is.
with a 12k resistor between ground and plus on the output, Q11 and Q9 collectors didn't change.
All measurements below are without +/-16v jumper connected.
So, I took the other 2SA1837, which is the removed Q6 and measured it. hFE of 160, so closer than the other. However, the numbers for Q11 still have a low emitter number:
Q11 64.6, 47.8, 51.2
Q9 -69.3, -46.5, -70
I measured the 2SC4793 from Q18 and it measures as two resistors. So I assume defective/blown/junk.
With U1 short removed:
Q11 75.9, 61.8, 62.8
Q9 -81, 58.7, -69.5
Well... maybe this is useful? Q11 emitter is now closer to where Q9's emitter is.
with a 12k resistor between ground and plus on the output, Q11 and Q9 collectors didn't change.
Hi Peter,
With the U1 jumper removed, the expectation is that bias to Q30, Q31, Q28, Q26 is disabled and therefore they won't influence subsequent stages.
The bias current through Q37, Q38, Q24, Q25 should drop dramatically, and the spreader voltages seen at C26 should be centered near 0V, clearly at odds with what you observe.
I have the impression that Q18 and Q6 are removed, thus disabling the output transistors. Is this true?
Would you measure voltage drops across R71, R76 and R46, R43 ? I believe the drops should be a few tenths of a volt at most. Nominally, the drops should be similar at each rail, but they may differ significantly. If these measurements don't lead directly to a culprit, would you also also measure voltages re ground for transistors Q37, Q38, Q11, Q9, Q25, Q24 ?
Thanks.
With the U1 jumper removed, the expectation is that bias to Q30, Q31, Q28, Q26 is disabled and therefore they won't influence subsequent stages.
The bias current through Q37, Q38, Q24, Q25 should drop dramatically, and the spreader voltages seen at C26 should be centered near 0V, clearly at odds with what you observe.
I have the impression that Q18 and Q6 are removed, thus disabling the output transistors. Is this true?
Would you measure voltage drops across R71, R76 and R46, R43 ? I believe the drops should be a few tenths of a volt at most. Nominally, the drops should be similar at each rail, but they may differ significantly. If these measurements don't lead directly to a culprit, would you also also measure voltages re ground for transistors Q37, Q38, Q11, Q9, Q25, Q24 ?
Thanks.
Something wrong near R76. Open resistor, trace, or defective Q38, etc. Be mindful of how you’re measuring. Measuring directly across a part vs. measuring V to ground can give deceptive symptoms because meter provides a leakage path to ground.
Both R76 and R43 measure correctly, in circuit. If R76 were defective, wouldn't that more affect the emitter of Q38 than the collector, which is what seems to be the mismatch with the other rail?
I pulled Q38 and it appears to measure as a transistor, with hFE 121, Vbe 6.33mv, Ic, 6.1ma.
With Q38 removed, no U1 jumper:
Q11 76.7, 62.3, 63.7
With Q38 removed, with U1 jumper:
Q11 66.6, 50.2, 53.5
Across, without U1: With U1:
R43 1.1 6.3
R46 0.04 6.4
R71 0.07 6.2
R76 3.5 7.4
C26 3.0 1.9
Would playing with the bias adjuster R53 have some interesting info? If possible, what if I adjust it so there is 0v across C26, would that give us a better picture of the symmetry of the two rails? If so, there might another imbalance shown that we don't see currently.
I pulled Q38 and it appears to measure as a transistor, with hFE 121, Vbe 6.33mv, Ic, 6.1ma.
With Q38 removed, no U1 jumper:
Q11 76.7, 62.3, 63.7
With Q38 removed, with U1 jumper:
Q11 66.6, 50.2, 53.5
Across, without U1: With U1:
R43 1.1 6.3
R46 0.04 6.4
R71 0.07 6.2
R76 3.5 7.4
C26 3.0 1.9
Would playing with the bias adjuster R53 have some interesting info? If possible, what if I adjust it so there is 0v across C26, would that give us a better picture of the symmetry of the two rails? If so, there might another imbalance shown that we don't see currently.
Good morning, Peter.
I think one channel is working properly, correct? If so, I would like to confirm my assumptions about how the amp biases when the frontend is in standby. Confirm the good channel is in standby, i.e. U1 is open--- roughly 30V between pins 3 and 4. Measure voltage C26 and at the amp output with no load connected. I anticipate voltages will be near 0, but what do you find?
Back to the problem channel and your latest post.
With Q38 absent, the voltages you report are probably reasonable. In its absence, you can confirm reasonable nearby node voltages. But to get proper bias, Q38 has to be present and you will have to have Q38 in place to get more data.
Not much to be learned from the bias pot with the frontend in standby.
Remember you can use the working channel as a guide to nominal voltages. Just be careful to preserve the healthy channel.
I think one channel is working properly, correct? If so, I would like to confirm my assumptions about how the amp biases when the frontend is in standby. Confirm the good channel is in standby, i.e. U1 is open--- roughly 30V between pins 3 and 4. Measure voltage C26 and at the amp output with no load connected. I anticipate voltages will be near 0, but what do you find?
Back to the problem channel and your latest post.
Maybe a typo re Vbe? I would presume maybe 633mV?
With Q38 absent, the voltages you report are probably reasonable. In its absence, you can confirm reasonable nearby node voltages. But to get proper bias, Q38 has to be present and you will have to have Q38 in place to get more data.
Not much to be learned from the bias pot with the frontend in standby.
Remember you can use the working channel as a guide to nominal voltages. Just be careful to preserve the healthy channel.
Getting the other amp out of the chassis is a chore, so it'll take some time. I would hook it up with the same value current limiting resistors.
You mention to measure it in standby, without U1 jumped... but can I control that? I mean, when does it come out of standby? And how? If I power it on, it does a system check, then the relays click and it's on. Not what I would consider standby... more like ready to go. I assumed we did something to the bad channel to defeat this so we could investigate and control it?
You mention to measure it in standby, without U1 jumped... but can I control that? I mean, when does it come out of standby? And how? If I power it on, it does a system check, then the relays click and it's on. Not what I would consider standby... more like ready to go. I assumed we did something to the bad channel to defeat this so we could investigate and control it?
Maybe the test isn’t with the effort. It’s almost a given that the amp has to work as imagined. Does the amp have a Mute switch? That would make the matter a nonissue.
I think think you have to reinstall the Q38 and resume the hunt. Did you replace the transistor you suspected earlier?
I think think you have to reinstall the Q38 and resume the hunt. Did you replace the transistor you suspected earlier?
Yes, it has a mute function. I am still thinking of pulling the other side, if it saves us time, then it's a great idea. So resistors installed, amp turned on, mute engaged... then check U1 and C26. I'll also check the Q38/Q11 junction and Q25/Q9 just to see where it's at.
I will reinstall Q38 and replaced the suspect transistor with the other one with a closer match.
Thanks again for your time.
I will reinstall Q38 and replaced the suspect transistor with the other one with a closer match.
Thanks again for your time.