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Need help on a 9090DB

E

ElectroJ

New Member
Hi all! I have been refurbishing a 9090DB for a friend. I have done an entire cap overhaul. I’ve replaced the matches transistors, the lights to LEDs, and every other upgrade I think possible. I reworked the Dolby board (caps, fed throughs, replaced ribbon cables). Everything works and sounds great. I have a problem though that I’m not sure about. When the Audio Muting -20db button is depressed ( sticking fully out) the left and right channel meters jump sporadically with no signal at all, even when the speakers are in the off position. When the button is pressed in ( the Audio Muting -20db) the meter’s work normal. Is this normal? I am not familiar with the operation of this unit. This is my first vintage radio restore. Most of the amps I work on are guitar and bass amps and other electronics. I’m not sure if the meters are just sensitive and picking up noise in that mode? I’ve combed through the schematic but can’t seem to find anything that is causing that issue. Has anyone seen this issue or have any suggestions of what I can look at? Any info would be appreciated. Thank you.
 
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Also it may be important to note on the above post when the audio muting -20db button is pressed it goes into protection mode for a few seconds then goes back to normal. Don't know if that helps with any more info.
 
Did you follow the DC offset adjustment and bias current settings per the SM? If not, read about the procedures in the FAQ sticky and use a DBT at all times. Also, try contact cleaning the mute push switch and the rest of them through the 2 or 3 tiny holes on their top, and once cleaner fluid gets inside, exercise them thoroughly.
 
Hi tnsilver. I did do all that. I found the problem. There is a wire that goes from the Mic board (2617) to the tone control board(2544) that broke and it was right up against the board where is should have been soldered. I thought it was connected but after double checking everything found it was broken. That was acting as an antenna picking up noise causing the meters to jump. Upon soldering it quieted the meter and the protection relay quit tripping. Everything is good. Can't believe i missed that. Thank you for the advice though.
 
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