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AU-717 No signal on right channel

V

vmedia

Active Member
Hoping the experts here can point me in the right direction (I have scoured AK for clues, but so far no luck)

I am working on a 717 for a friend which has a dead quiet right channel. I have cleaned all switches including the back couple/separate switch. There is no signal at the right side power amp input (I have swapped inputs from left to right and the right channel amp is working, however distorted - that will be next after fixing the pre-amp issue.)

Power to 2720 is +34/-34. Replaced the four 82ohms fuse-able resistors. I have traced signal and it appears that signal is making it to the volume pot, with no right signal exiting. Hoping this is not a bad volume pot (?) I admit, I am not 100% clear on the path of audio on 2720. The block diagram seems to imply that the volume pot is the last step before sending signal to amp boards (I may be completely wrong on this)

Any advice on next steps or how to start over would be most appreciated.
 
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The number one suspect for a dropped channel in an AU-717 is the (rear panel) connected/separated switch, the switch contacts oxidise and fail to connect. The best treatment is to carefully dismantle the switch and clean the switch contacts with a non-abrasive contact cleaner. If you don't feel confident to do this without damaging the now 'unobtanium' switch, then externally applied switch contact cleaner (i.e. DeOxit D5 or D100), followed by exercising the switch several times, is the next best option.
 
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I have worked the switch with Deoxit already - can certainly try again (I've fixed a few 717's doing this)
Is there a point coming from the pre-amp board where I can check to see if signal is being sent to the rear panel switch? (Assuming this is the path the signal would take?

I should also mention that I have checked the pre and amp individual rca outputs (switched to separate) and there is no output from the right pre.
 
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Update: Turns out the voltages to the pre-amp suddenly were not correct to the right side.
Only guess I have is that they were intermittent due to the glue issue on the power supply board.
Some caps out of spec and a resistor with a (nearly) broken leg.
Fixed those issues, now I have sound out of both channels.
Fusistors on the amp board were causing the distortion.
Now to figure out why there's no bias on that channel...!
 
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