ADCOM GTP 500 II - Negative DC voltage on RCA inputs

[Erin Brigham]

Hi Everyone,

This is my first post. I'm measuring negative DC voltage on the RCA source inputs. It moves to different sources as the source selection changes. Anywhere from -2.5 to -13 volts DC. It does not appear on the phono inputs though. I can listen to the phono input through the headphone jack (GTP 500 II has a built-in amp for the headphones) though it does not sounds right, very scratchy as I turn up the volume and cuts out completely if I turn it up all the way. Both channels are working though when I turn the balance control. The bass and treble controls have no effect.
Any ideas on where to look would be greatly appreciated. I do have the service manual from HiFiEngine but having trouble tracking down what the issue is.

 

[Ray Gianelli]

Hi Everyone,

This is my first post. I'm measuring negative DC voltage on the RCA source inputs. It moves to different sources as the source selection changes. Anywhere from -2.5 to -13 volts DC. It does not appear on the phono inputs though. I can listen to the phono input through the headphone jack (GTP 500 II has a built-in amp for the headphones) though it does not sounds right, very scratchy as I turn up the volume and cuts out completely if I turn it up all the way. Both channels are working though when I turn the balance control. The bass and treble controls have no effect.
Any ideas on where to look would be greatly appreciated. I do have the service manual from HiFiEngine but having trouble tracking down what the issue is.

I'd take a look at the +15 volt supply feeding the analog switch IC's, since you're only seeing negative offset. Notice that the phono inputs don't go into the IC's:

 

[Erin Brigham]

Thanks Ray, I'll check it out. Much appreciated.