Tascam 688 repair questions

Hermanlifer

New member
I'm experiencing a puzzling issue with this 688 midistudio. Everything seems to work fine, except an intermittent popping through channels one and two, when no inputs are used and no tape is in the deck. The sound can be heard through the monitor and the meters bounce accordingly. I've got electronic repair experience, I'm just looking for tips to speed up the troubleshooting. So far I've cleaned up all the pots, faders and connections. I've also cleaned the heads of the tape deck. I'm thinking this it's an output issue, because signals are good on their way into these channels. Any advice would be welcome. I'm new to tape machines.
 
Sounds like the problem is either bad parts on the ch boards,or a grp 1/2 problem.Do the ch pop when assigned to different groups?
 
There are a number of 4066-quad bilateral switches,used to rout signals to and from the DBX portion of the circuitry.When they fail it puts DC voltage onto the audio path.The 4066s are being run right at their rated max voltage.They were replaced with the 4966,a high voltage version of the 4066.Pressing the record enable button on a track and seeing the meter peak and then drop back is a indication of a DC voltage/4066 failure.
 
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