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Bias problem on a KA-7100, new trimmer does nothing

After replacing the resistors and removing the outputs, measure the voltage across R57 and R59.
 
I'm good with replacing R57 + 59, that makes logical sense to me. You have to put some sort of load on the circuit to make it think its operating, I get that far without theory. I was afraid of frying little 1/2 watts where there was a 3 watt, and of course I'm needing to protect the main outputs. If I can do this after pulling out the mains I feel better about it, and I do trust you, I've read your posts for a while. This is my tenth restoration, and most of my knowledge of working out the issues has come from AK forums, specifically Kenwood. You have answered questions in here for me before, dr. and always been helpful.

This has ben a frustrating problem, and I greatly appreciate all the help!

I'll take some voltages before the change, too.
 
yeah, ummm, electronic theory...well I try! It's funny, I remember stuff from college 50 years ago, but trying to learn and apply things now seems a lot harder! I think my electrolytic caps could use replacing...

I'm also trying separating the control board ( #2, #3) to check for influence on the dc offset there. Maybe I reversed something in the control board, and I have overlooked that until now.
 
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petehall347 said:
how about simple voltage readings of drivers base emitter junctions ? and if they change adjusting the bias pot . ? sorry if i am rambling on again .
Click to expand...
Sorry to act dumb again, petehall, but the only Vbe measurements I ever knew were on a bare transistor. I have never thought about trying to measure that in place in circuit, if that is what you mean. Apparently so if I need to adjust the bias pot at the same time.

Previous measurements =
In circuit, powered on full AC Qe23 E= 5.9mW (spec is .6V) C= 50.0 V B= -120mW (spec is 1.15V)
Qe25 E= -1.24V (spec is -.6V) C= 50.1V , the dropped to 44.5V B= -.63v (spec is -1.15V)

Can I measure across B-E safely while operating the amp? Which lead, or try both directions? Any other combinations? Anything to avoid, ie, anything harmful or dangerous? I have a small stock of common trans., but don't want damage to travel through other components. i also have the tips of the probes covered with a cut piece of wire insulation, but the tip itself could short across close connections.

I shut down the amp when the rail voltage dropped. I'm looking for the cause of that now. Checking diodes, pass regulators in PS. Looking for strange grounds
 
whoa, new surprise. I have been operating the amp on full AC for 1-2 minutes at a time to get a measurement, then turning it off. I went back to using the DBT for measurements, and I expect some lower readings some times, right? Well the first measurement I try to take clicks opens the relay and flashes the bulb. Turn off. Try again, turn back on, DBT dims, relay clicks closed. I touch the solder point and relay clicks open, DBT flashes. Turn off. Now what?

I'm hoping the current is low because of the DBT, so any change in the voltage trips the relay, who is already seeing a different working voltage. The differential of adding the DVM to the circuit throws it over the threshhold and trips it open, right? er, no? At least I know that circuit is working! I'm getting a little bit nmore anxious...
 
Qe23 was C1567 (npn) ECB replaced with KSC2690ays ECB
Qe25 was A794 (pnp) ECB replaced with KSA1220ays. ECB

Checked the silkscreen top and bottom of board, same. I also noted that Qe24 and Qe26 are the same configuration, and that side (right) works.
 
Re43 = 269.7 ohms. spot on. I'm still confused by having no voltage readings at the Left bias trim pott itself. How can I check that further?
 
yeah, i confuse lots of stuff any more, you're absolutely right! i wish this was some of that, but because i go over it about ten times and in different ways, then compare to the right side, I try hard to get it correct. I want to find it wrong, too, maybe that would be the solution this time

All of the voltages from the L bias trim pot to the main (L) outs are suspect, except rails, but wait - the rail dropped to 44.5V a couple of times, too. My review of the PS was unremarkable, i could not find anything wrong there.
 
Qe7 E 43V C 7.47V B 42.7V
Qe9 E 43V C -131mV B 42.9V

these readings are from the last time I had the amp powered full AC. Since then I tried measuring while on a DBT and the relay opens and I shut down the amp.
 
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there's like, no voltage at the trimmer. i have changed trimmer twice, changed Qe13 twice, checked resistors, soldering, pcb traces.
 
i warned ya, pete, it's quite a puzzle! and it will probably be something stupidly simple, like a missing ground somewhere, all the while I'm thinking there's something shorting to ground that shouldn't be. Why else would voltages just die? Love your help! Thanks!
 
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