Bias is good, but only get distorted sound

[rulerboyz]

Now that I've worked out the bias problem I tested the sound on the KR-4140 receiver. Alas it isn't really there (yet). What I am getting is a somewhat subdued distorted sound that only comes through when I crank it 3/4 of the way up. Also, when I am listening with earphones I get a moment where the sound clears up and comes in louder at the moment just after I press and release the on switch. I have noticed this exact same behaviour before several months ago on an different KR-4140. Before I worked on solving the bias problem I was getting good sound in one channel, and not too bad on the other (the channel with the bias problem). Has anyone encountered a similar distortion behaviour (where sound gets better temporarily as the power turns off)? Any ideas what the cause may be? The small transistors all appear to pass the bipolar junction transistor testing, at least while embedded in the circuit board (I didn't pull any out to test).

Update:

Something I remembered. I suspect that it is probably two small npn transistors on either side which were both directly involved (on separate occasions) in a blowout of a power transistor (replaced the bad power transistors but I overlooked checking the smaller ones). I'll pull them out and do a transistor test on them.

 

[rulerboyz]

I replaced a good deal of these transistors and still haven't found the problem.

 

[Rockmonton]

most simple: did you give the pots and switched a good cleaning?

 

[EchoWars]

Feed it a sine wave and look at the output with a scope. That would be a picture worth a thousand words (or a thousand hours of guessing anyway).

 

[rulerboyz]

I have replaced the pots with sealed bourns multiturn, so nothing wrong with them.

Looks like I might consider some more testing equipment.

I've got my 1kHz sine wave on my computer, now I just need a scope. Any recommendations?

Interestingly the amplifier circuit board comes directly from a different KR-4140. It was that other KR-4140 that had this same problem. Given that I am using the same circuit board from that unit and I initially was able to get it working, it points to the possibility that the problem could have been solved in the same way (but I forget what I did exactly).

 

[rulerboyz]

I pulled out a 2SC971 NPN transistor and tested the Hfe on it. It reads as 71. I'm not sure if that is unusually low or not (nothing to compare it to). I also pulled out a 2SC734 and found that the current gain on it was much lower than the replacement transistor. After replacing that I am hearing a little bit more of the highs coming through. I'll have to wait for the other transistor to arrive.