SA-8800 Trim Pots

Thanks. Unfortunately, I still have a problem. The left channel has an idle voltage of 280mV or so, and that's at minimum resistance from the trimmer. Obviously, it only gets worse from there. The right channel dialed in to 56mV with no trouble.
 
Don't take it off the DBT until you get it sorted.
First thing to look for is solder bridges, caps in backwards (it happens), and check the pinouts on every transistor that you replaced.
I'm not gonna be much help otherwise until tomorrow afternoon/evening.
 
It is on DBT again. Boards have been thoroughly cleaned and inspected. New caps are all correct (the electrolytics on the power board and main amp board were replaced, matching polarity to the originals, which matched board silkscreens). No new transistors, other than some in EQ section.

FWIW, the protection relay closes when on DBT, but takes quite a while to do so....10-15 seconds.
 
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The initial settings are the same as the ones used on the working channel?

Check to make sure that the STV-2H diode on both channels ARE STV-2H diodes.
Visually (color = green) , then record the voltages across them.

Then compare the voltages on Q17 and Q18 e, b & c's, referenced to the +48v dc regulated power supply line on pin 25.
(that means that pin 25 gets the black dmm probe connected to it, not ground, use the red probe to measure)
The current through them sets the voltage drop across the STV-2H diodes.

Next do the same for Q27 and Q28, referencing to the +48v dc regulated at pin 25.
Then referencing to the - 48v dc regulated power supply line on pin 26, do the same with Q29 & Q30
(that means that pin 26 gets the black dmm probe connected to it, not ground, use the red probe to measure)
The current through these four control the turnon voltage of the transistors connected to the STV-2H diodes.

edit - you are correct, Q17 & Q18 NOT Q16 & Q17 - I must have been more tired than I expected.
 
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Mark - Will do post haste. I understand the instructions, but should I keep the unit on DBT for this testing?

As context...I verified (about three times) that the new trimmers were set at minimum resistance as the starting point, then turned unit up on DBT, which lit up and immediately dimmed down to a low level. I then did a check of DC offset (it is fine), put the unit on full power, set the right channel bias on the 100ohm trimmer to 56mV, and then connected to the left channel test points, saw that I had a problem there, and shut it down.
 
While waiting for instruction about whether the testing should be done with unit on or off of DBT, I tried to locate the STV-2H diodes. The board diagram in the service manual is very difficult to read, but it appears that these diodes are not mounted on the PA board, but rather on the bottom side of the heat sink, at either end of the row of output transistors. I assume these are the STV-2Hs, as they are strapped via wire to the PA board where the diagram shows them to be mounted. They are screw-mounted, green in color, and are printed with an ID code of "8DA".

Does anyone know where I can find a legible set of board diagrams and schematics? It is going to be very difficult to identify the transistors to be tested with the documents I've been using.
 
Tested what I believe are the STV-2H diodes - voltage across right channel diode is 1.09V, and across left channel diode is 1.1V. This was done while on DBT (60 watt bulb). I will hold off on transistor testing until I know whether to proceed with or without DBT (and hopefully can find a better board diagram).
 
Leave it on the DBT for now. I had expected more discussion and questions before action occurred, answering questions to fill in knowledge gaps, discussing procedures etc.

Not such immediate action.

It was just happenstance that I looked back in this morning before starting a detailed inspection of the inner surface of my eyelids..
 
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Thank you. I will do the transistor voltages this afternoon, if I can locate them on the board. As you know, the board isn't marked with component numbers (Q1, C5, R12, etc) and the quality of the service manual and board diagram I'm using is not good...very hard to make out part numbers.

EDIT: Pulled up the diagrams on our desktop PC with the big monitor, and can read it quite a bit better now. Unfortunately, the PC is on the 2nd floor and my workshop is in the basement...but I can use the exercise!

Mark, as I was locating the transistors to be tested, I just wanted to verify that the first pair to be tested is Q16 and Q17. The "matching" transistor for Q17 - in terms of position in the circuit - is Q18. And, the "matching" transistor for Q16 is Q15. So, just wanted to verify that you indeed want me to test Q16 and Q17, since they have different places in their respective circuits.
 
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Good Catch, you are correct - Q17 & Q18 - I have edited the post for both correctness as well as credit in spotting the error.

The only explanation was that I was more tired than I expected, and dyslexia slipped through my usual checks.
 
Ha...no problem...I just appreciate the help.

One step forward, two steps back. First, here are measurements for the first four transistors I just did (all negative):

Q17: E=1.36V / C = 36.5V / B = 1.97V
Q18: E = 1.35V / C = 37.4V / B = 1.94V
Q15: E = 78.5V / C = 60.1V / B = 76.3V
Q16: E = 78.1V / C = 42.7V / B = 76.4V

Now for the two steps back. While testing, I noticed the speaker switch looked a bit odd. Upon closer examination, I could see that it had a crack running all the way through it, and it had a slight concave shape to it. I toggled the switch a few times (power off), and the crack snapped. I've only seen one other wafer switch cracked like this...I guess it could have been some sort of stress fracture over time, as I'm pretty sure I didn't do it. I've never used this amp, and it is new to me, so I guess someone else could have cracked it while working on it, altho it's hard to imagine how.

Bizarrely enough, there is one for sale on ebay...so I guess I'm going to have to pay thru the nose for a new one. It would be great if I could figure out how serious this bias issue is before I spend another $40 on the switch. Is that possible, or should we stop here until I replace the switch?
 
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for safety you could cut the feeds to the switch and insulate to be safe .
on another note did you check emitter resistors values ? if these go high ohms you will see a false bias reading . was just a thought .
 
Sorry for the delay. Here are the measurements on the other transistors:

Q27: E = 1.61V / C = 38.55V / B = 2.02V
Q28: E = 1.58V / C = 38.05V / B = 2.18V

For Q27 and Q28, all voltages are negative.

Q29: E = 1.61V / C = 38.44 / B = 2.2V
Q30: E = 1.59V C = 38.2V / B = 2.2V

For Q29 and Q30, all voltages are positive
 
Look at all the transistors on the "hot" channel especially the heat sunk transistors and the output transistor.
See if any are not factory. Transistor markings, solder condition etc...
 
Will check them all again. Already checked solder joints very closely, re-flowed a few. FWIW, when I was checking Q16, the protection relay opened. I am quite sure I did not short anything when I was doing it. Turned amp off and back on, protection closed (slowly). Tested Q16 again, got same voltages. Since then, I've had it on and off a few times...protection sometimes closes, sometimes does not. Always on DBT.
 
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