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Yamaha CR-420 Intermittent Bias Voltage

tube-it

tube-it

Tubes and Tubes
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I just finished restoring a CR-420 I picked up a working but ugly CR-420 to replace the guts in a cosmetically perfect yet blown unit. I did a full re-cap while I was in there. I am now getting intermittent distortion in the right channel that will go away if I crank up the volume a bit. I am not sure if this was a pre-existing condition since I only tested the parts unit for a monent to confirm it was working. I found that the bias voltage on the right side drops to zero once the unit warms up. Could this be a bad transistor? I used some component cooler and found when I chill the ASC458 the problem seemed to go away. I replaced the transistor from the old main board, since it seemed to test fine, and the problem seemed to go away for a moment but now it's back. My Guesses so far are a still a bad replacement transistor (Used ASC458), bad trace, cold solder joint, or could this be a bad trimmer pot? Would a bad transistor show as a voltage drop at the test points and fail intermittently?
 
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You can't test a transistor with a multimeter. If it's dead it's dead. A meter will always show it as working, unless it is shorted.

2sc458 is always bad. It's the worst transistor in the world.

Replace the speaker relay also, it is causing your bad sound upon power up.
 
I do have a transistor tester and the c458 I used to replace the existing one tested OK on that, the old one tested bad at first then tested with a higher hfe than the replacement transistor :dunno:. I ordered a pack of sanyo asc458s that just came in, I may as well give it shot. Is there a better replacement I should look into? The sound is ok on power up. It's after it gets warm the right channel goes bad. With the old transistor this happened sooner, but hat could be coincidence. Are you talking about the plastic enclosed Relay towards the front? What else should I look at?
 
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tube-it said:
I am now getting intermittent distortion in the right channel that will go away if I crank up the volume a bit.
Click to expand...
A couple possibilities.
1. fouled protect relay contacts
2 cold solder connection
3 dirty speaker select switches
 
tube-it said:
Would a bad transistor show as a voltage drop at the test points and fail intermittently?
Click to expand...
It could. Check your solder/mechanical connections on the output transistors and there sockets , as well as the output emitter resistors. .
 
Thanks avionic. I'll report back. Switches should be good as they were cleaned extensively. It does look like the trace started to lift right at one of the pins for the ASc458 transistor. I may jump the connection and try that.
 
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