SA-8800 Trim Pots

Ok. I just re-read your first post.
Was this the issue that was mentioned there? Or was it something else?
If there are no glaring defects, then your only choice is to very methodically work your way through the circuit to find the issue.
So, to start, individually measure the voltage drop across each emitter resistor. They should all be very close to the same.
 
The high bias voltage discussion starts on Page 6, Post #104.

As for methodically working through the circuit; I thought that this was what Mark was having me do?

I will measure the voltage drops and report back.
 
I saw your post on page 6. Was that also the issue you were referring to in your first post?
 
No. I got this amp and upon initial examination, found a couple of swollen caps on the EQ board, which I replaced. Before testing the amp and doing basic service work, I decided to replace the bias pots. That's what started the thread.

It was only after replacing the pots, turning it up on DBT with no issues (but NOT checking bias voltages while on DBT), and then turning it up on full power, that I discovered the high bias in the left channel. Since then, all work has been with DBT in place.
 
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What was that issue then?

??? See above (Post #145). Is that what you are asking about?

FWIW, I have found two other issues with this amp:

1. Broken speaker selector switch, which I will be replacing (assuming I can resolve the bias issue), and

2. Two capacitors (47uf/25V NP) apparently missing from the main amp board.

Both of these issues were mentioned earlier in this thread. Based on my low level of expertise, it would not seem that either of these problems plays a role in the high bias issue.
 
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I must have responded before your edit. I don't suppose you had a chance to check the bias before replacing anything?
 
I must have responded before your edit. I don't suppose you had a chance to check the bias before replacing anything?

I certainly had the chance, but did not do so. AK is full of admonitions about the risks of faulty bias pots to NSA amps, and the need to replace them preemptively...which is what I did. I saw no reason to check bias beforehand, since I knew I would be starting from zero/near zero with the new pots.
 
did you disconnect speaker out wires before they go into the broken switch ? and did you check emitter resistors resistance yet ?
 
I certainly had the chance, but did not do so. AK is full of admonitions about the risks of faulty bias pots to NSA amps, and the need to replace them preemptively...which is what I did. I saw no reason to check bias beforehand, since I knew I would be starting from zero/near zero with the new pots.
Just trying to figure out whether this was a pre-existing problem or not.
At any rate, check the voltage drops across the emitter resistors individually. If they are all essentially the same on that channel, we can fairly safely assume that they, the output transistors, and base resistors are all OK.
 
OK, here is a new piece of information. I emailed the fellow I got the amp from, who picked it up at an estate sale.

As background - when I bought the amp, he said that it was working, and then showed that to be the case by playing it with an ipod as the source and headphones for output. He also mentioned that he had to replace the larger fuse on the power board when he first got it.

In the email I just got from him, he says that he took it to his friend who does repair work. The friend apparently found the blown fuse, and also told him the bias was "not right" on one channel, but that this was nothing to worry about. I think that's the extent of the history we're going to get (prior to my ownership).
 
OK, here is a new piece of information. I emailed the fellow I got the amp from, who picked it up at an estate sale.

As background - when I bought the amp, he said that it was working, and then showed that to be the case by playing it with an ipod as the source and headphones for output. He also mentioned that he had to replace the larger fuse on the power board when he first got it.

In the email I just got from him, he says that he took it to his friend who does repair work. The friend apparently found the blown fuse, and also told him the bias was "not right" on one channel, but that this was nothing to worry about. I think that's the extent of the history we're going to get (prior to my ownership).

That's very useful to know. Now go check those voltage drops.
 
Reading from left to right (looking down on board from topside), on the left (hot) channel:

20.1mV / 19.9mV / 20.4mV / 19.9 mV

EDIT: FWIW...I measured resistance across the new left channel 100ohm bias pot at .2 ohm with pot turned to minimum resistance, and I also verified, yet again, that resistance increases as I turn the pot CW. At this setting, with DBT, bias voltage is around 50mV.

Did same with right side, which is set at 4.1ohms and yielding 14mV or thereabouts. This side was set to 56mV when unit was on full power (prior to me realizing I had a problem in the other channel).
 
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Now it's all down to process of elimination.The good thing is that you have a good channel to compare against.
What's your voltage drop across D7?
Check R95, R97 (it has a thermistor in parallel with it). Compare it with the good channel. Test them with power off.
You can test the function of the thermistor by alternately heating and cooling.
 
It's just a resistor. How does it compare with the one on the other channel? Does it change value when heated/cooled?
 
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