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8116A Code 42

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Mick B:
Hello Mark, down for parts on the Saab. Well, I was a lot more careful with the measurement's a lot of them not directly connected to the power rails had fluctuating readings as if caps charging. I tried to get the first reading. Where it says mv, that is right off my meter. HP34401A its readings match my Keithley 2015 to within microvolts.
I replaced Q501 because I had a new one and it was already out. I have the TO-93pkg heatsinks so at some point I will replace Q502 also. Looking at the resisters around Q501 and Q503 readings made with 4wire, R516. 10.962k.  R524. 100.437Ω   R525. 2.0012K.  R533. 19.635Ω   R534. 19.599Ω those numbers on the schematic sure look like an 8 but definitely a 6.
I'm going to pull CR503. right now.  Oh ya. notice the polarity difference around U501 & U502?
Thanks, MarkL. 

Mick B:
For some reason it didn't post my schematic here it is.

UPDATE: CR503 checks fine out of circuit!
AND: found a bad solder joint AT the topside of the MAIN board where Q510 Collector meets the board making a sketchy +24v connection. It was soldered on the bottom but not the top where is gets the +24v Makes me want to test output transistors with heatsinks where you can't see the legs from the bottom of the board at the legs what a pain that will be. perhaps Mark you see some transistors I might want to recheck looking at new voltages? 

MarkL:
The voltages around Q503 that you're showing on the schematic are very different than those you posted before.  I can no longer conclude there's anything wrong with Q503, CR503, or R533.

The polarity difference wrt. U501 is not a major concern.  U501 is providing an offset current to the amplifier input to keep the amplifier's virtual ground at 0V to compensate for imbalance due to component variability.  So, it could come to rest at any value ideally close to 0V.  But because the amplifier is currently way out of balance due to whatever failure (note mid-point between Q501.E and Q502.E), U501 is pinned to one side as a side effect.  I've learned to never rule anything out completely, but I don't see anything at the moment that leads me to believe there is  a problem with the way U501 is behaving.

I'm seeing some voltages around Q512 that don't make sense: How can it have +0.36152 on its emitter when the base, collector, and other side of the emitter resistor are all higher than that?

Before we go any further, can you describe the "fluctuations" you're seeing?  Or maybe put your scope on a couple of the nodes where you're seeing a lot of fluctuation?  It's not out of the question that in its current failure mode the whole amplifier is oscillating.  Obviously it shouldn't be, but that would make it impossible to obtain stable voltage measurements.

Are the power supply rails stable according to your scope?

MarkL:
In addition to the above, fluctuations notwithstanding, the voltage on the collector of Q506 is not making a lot of sense.  I'd like to know what kind of current is flowing through the emitter.  Can you take a look at the emitter voltage on Q505 and Q506?  We can determine the current from the emitter resistors R534 and R537.  Thanks.

Mick B:

--- Quote from: MarkL on February 18, 2024, 10:19:04 pm ---In addition to the above, fluctuations notwithstanding, the voltage on the collector of Q506 is not making a lot of sense.  I'd like to know what kind of current is flowing through the emitter.  Can you take a look at the emitter voltage on Q505 and Q506?  We can determine the current from the emitter resistors R534 and R537.  Thanks.

--- End quote ---

Hi Mark, Ok I spent all day yesterday looking over voltages I am dyslexic and i have to make multiple measurements and read them over and over to make sure I have not transposed numbers and since my first set of numbers, I have been diligent in this. and trying to make since of these voltages. I left question marks at some of the fluctuating numbers. I circled just 3 the numbers were any ware from 0.32??? to what you see in the circles Just an hour or so apart, and it had been warming up well over an hour. This is what I mean by fluctuating the last 3 digits do not stop sometimes changing 2nd digit. the Power supply is rock solid. I looked at a few nodes with my scope really nothing to see just DC voltage, I saw no AC until I got down in the weeds @ 50mv = noise. (I had that thing running all day the 2 ICs on the A2 board haven't caught on fire yet). :-+
When it rains, went to post this yesterday and I internet went down. the same network I had up 5yrs or so no issues, I saw ATT workers at one of their junctions where a lot of their stuff meets. I'm sure they did it. My whole house is wired with Ethernet, using a Cisco 48 port switch & an ATT, fiber router/modem with typical default IP address, long story short. all of a sudden my switch has the same IP add. as the router. That took a min to get to the bottom of and fix. 

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