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Yamaha CA-810 Pre-Amp issue - DC Voltage on plus side of C329 (Left Channel) is much too high

T

tkflage

New Member
I have recapped the signal path for both channels, and replaced the transistors for the left channel, on the Tone Control Circuit Board. The Amp passed the dim-bulb test ok, but when measuring voltages I discovered 8.7 V DC on the + pin of C329. The voltages in the signal path before this cap is per specification. From C329 the signal goes through the bass and treble (and respective turnover switches) before going into the second half of the pre-amp at junction J313, where the voltage remains at 8.7 V DC. This leads to overheating of transistor TR311 causing distortion and weak signal after just a minute or two of playing.

Has anyone had similar problems before? Do you have any tips on what I should do next? I have checked for shorts, bad solder connections, etc. many times now without finding any obvious faults.

Regards,

Tor Kristian
 
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Consider moving thread to the yamaha forum, hit "Report" at bottom left of any post and ask moderators to kindly move.

The tone controls are passive so where does the voltage come from? I'd do further measurements moving in the direction of increasing voltage, ie, find the high point. Measure at LO and each side or R353(1k). Does adjusting the trebble or bass have any effect on the voltage. Yeah, don't know, haven't seen it so just fishing, TR309 suspect?
 
mbz said:
Consider moving thread to the yamaha forum, hit "Report" at bottom left of any post and ask moderators to kindly move.

The tone controls are passive so where does the voltage come from? I'd do further measurements moving in the direction of increasing voltage, ie, find the high point. Measure at LO and each side or R353(1k). Does adjusting the trebble or bass have any effect on the voltage. Yeah, don't know, haven't seen it so just fishing, TR309 suspect?
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Thank you for your rapid response! And good question that, thought at first it came from TR309 so replaced it along with 311 and 313, but will measure as per your suggestions, and I will pull TR309 to double-check.

Thanks for the tip of moving this thread.
 
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