bradfro
Active Member
I've been working at restoring this kenwood amp. When I got it, Q19 was completely melted along with the surrounding resistors. I went through and replaced all the burned out components and have been systematically fixing issues that came up. I seem to be stuck on the bias circuit in the left channel. The right channel works. I can't adjust the bias. The reading now is .1mv, it should be 40mv when adjusted properly. The trim pot has no effect on this. The trim pot does measure correctly with an ohm meter and changes value when adjusted. Q7, which is part of the bias circuit was bad, so I replaced it, but the bias still won't adjust. The left channel does produce audio fine at low level. When the volume is increased, there is noticeable distortion. A 1khz signal looks fine at the base of Q1at any volume. But turning up the volume, half the sine wave flattens out if you look at the collector. I would guess that Q1 and Q3 are the issue, but these are brand new and HFE matched. So I'm suspecting the bias circuit is causing this. Needless to say, all the voltages are correct.
Here is the schematic, all the green transistors have been replaced with new ones. Also, all the electrolytic caps in the left channel have been replaced. I also swapped the two output transistors Q21 and Q23 from another spare 3500. Maybe I should just replace the remaining 4 transistors, but I'm hoping someone may have helpful insight in this and point me to the likely cause.
Here is the schematic, all the green transistors have been replaced with new ones. Also, all the electrolytic caps in the left channel have been replaced. I also swapped the two output transistors Q21 and Q23 from another spare 3500. Maybe I should just replace the remaining 4 transistors, but I'm hoping someone may have helpful insight in this and point me to the likely cause.
