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

Thanks for the idea, gort. As you know, that might find a problem or it might spread a problem. I have numbered, removed, cleaned, tested and measured the original outputs, then reinstalled them with new grease and clean mica. I did not record these, but I did note they passed the cheap chinese tramsistor tester with similar values L to R. I used Ceramique 2.

I wonder if I can get voltages with no outputs in place? I imagine it could affect some readings, but it would guard the outputs if I could test without them.
 
You could always insert a low ohm resistor (e.g., 1R or so) into the collector feed to Qe27 (or Qe29) - just to check the current going into it.
 
I'm not an engineer, so I can't read a schematic and tell the actual current paths - particularily to the L bias pot. I get voltage on the right pot, but no voltage on the left. Where do I look? Things (component values, voltages from s/m) measure reasonably until the pot, then they get off. I have removed this pot and changed it twice after close inspection of pcb, but I can't tell why there is no voltage. I believe this is key to solving the mystery. From the L pot on to the outputs most voltages are off.
 
ah, thanks Goldie! But, as I was just answering Gort, I/m not an engineer. How does inserting a resistor into the collector feed allow me to check the current? How do I measure that, just the usual DVM to ground?
 
OK, don't worry. Was just an idea to try and confirm what the outputs are doing, e.g., are they actually conducting or not - sound, & especially heat, would suggest they are, but your bias measurement suggests not.
 
You don't need to insert a resistor to measure the current; there are already emitter resistors to do that. When you measure the voltage drop across a resistor, you divide the voltage by the resistor value to calculate the current. This is Ohm's Law.
 
Just for fun, with the power off and A speakers selected, measure from + to - on the speaker terminals, both channels on Ohms.
 
yeah, Goldie, they seem to be working in some fashion. As noted earlier, I get sound through the amp, headphones or speaker A right and left. I do not leave it on long for fear of the heating problem, and assuming (dangerous!) that's related to having a 000.0 bias measurement on the L channel.
 
Probably unrelated, but I'm trying to follow everything through this thread (again...) - can you just confirm the Ohms values of Re27 & Re31 ? I think you reversed them in post #7, but just to be sure....
 
The 0 bias is not related to the heating up, it works the exact opposite. Something is drawing current on the left channel in order for the heating to take place. I need to think about this some. Do you have any 1K resistors? There is a test we can run.
 
Hmmm, so much to learn. I believe yes, I have a 1K, probably 1/2W. Let's run that test!

I've been reviewing other 7100 threads, and I found more things to check. Like any voltage coming from the control board, even backwards or bad parts in the control board. those tantalums are suspect, as are the tone and sub sonic switches. The films i wanted were not available whenevr I had placed the order, and I skipped the tantalums as a result. I need to check some resitors in the PS too.
 
i can only guess 1k resistors instead of output transistors b to e to complete the circuit although not needed . so for drivers emitters instead.
sorry i missed a lot of this but surely output emitters resistors should show high current if outputs are getting hot ? high frequency has been ruled out from what i read .
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 .
 
Hi petehall, welcome to the puzzle. Thanks for your idea - where do I measure that? Q23 B?
 
My idea is just to protect the outputs and drivers while we are working on it, to limit the current. So, replace R69, R71, R57 and R59 with 1K resistors. this should keep the heat sink cool, by limiting the current. then we can safely make measurements without risk of it blowing up. Also, put tape on the meter probe so only the very tip is exposed. This will help prevent shorting things out when you slip.
 
That's a lot for me to comprehend quickly Dr.A. I'm sure you understand what changes you are suggesting, and I could use a little more explanation please. My mechanical brain thinks putting a high resistance lower watt resistor in place of the emitter resistor might be risky, but I don't know what to expect for electrical results. Meanwhile I'll keep trying a few more things to find the culprit.

Is it possible to get any of these measurements with the outputs removed?
 
You can remove the outputs, but you still have to replace R57 and R59 with 1K resistors. The purpose of the resistor change is to limit the current so you don't blow up transistors with the amp not operating properly. If you don't understand, you'll just have to trust me on this or start reading on electronics theory.
 
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