200A crossover distortion repair & new issues?

Started by maestro, June 11, 2017, 08:15:11 PM

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maestro

I've got a 200A which had some annoying distortion happening during note decay.

I replaced TR 9/10/12/14 without success. TR9 base was sitting slightly above 0V.

I then re-based TR 9 with 240 ohms in place of R34 (nothing in R58's spot). I did this by placing a pot in series with the original R34, 160 ohm, and found that I eliminated crossover distortion at 240 ohms total. Better yet, the TR9 returned to 0 volts and my voltages at the output transistors are reading a bit better than before.

The new issue, potentially, is that I'm reading 40mV across R37/38 whereas the schematic only calls for a range of 1-10 mV. The heatsink which previously ran very cool is now warm to the touch. Is this a problem?

Jenzz

Hi .-)

Don't care about exact 0V on T9, the more important thing is to have the proper voltage accross R 37 / 38. I allways set this  to 5-10 mV. The annoying thing about the PCB design is that T9 should be attached to directly to the heatsink for optimum thermal coupling, since it works somewhat as a 'temperature sensor' and therefore regulates the bias / idle current.

So, with T9 in its original Position, bias current is very sensitive due to T9's actual temperature. After soldering anything around T9's area, allow 5 minutes to cool down before measuring, and re-adjust.

Jenzz

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maestro

What if you can't find a middle ground between bringing the output back to 5-10mV and getting rid of crossover distortion?

maestro

#3
Not sure what changed from last night until now but I was able to reduce R34 down to 205 ohm, parking the output stage voltage drop at 6-7mV. No crossover distortion. TR9 is at 50 mV and the amp output is at 30mV, idle with speakers disconnected. Thanks for sharing your thoughts!