Adcom GTP-500 II Phono section issue

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Hi all,

I have an Adcom GTP-500 II Preamp/Tuner that I picked up recently to swap in my system while I recap my Nikko Beta 30. The Adcom has an issue in the phono section. The output of this unit is fine in the CD and Tuner modes but when it is switched to the phono section the output is drastically reduced and seems to be at about 1/4 of what the other settings pass through.

Anyone out there have any thoughts on this issue?

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Hmmm. Its never that easy is it ;)

I assume both channels are behaving the same ? Gain faults are pretty rare, and on both channels......

Without hearing this its hard to say. Can you ultimately get the volume you need from the phono section or is it still to low, even with the volume on full. Your really knocking at the door of having to do specific tests on the unit and seeing how the quoted input sensitivity's compare to the spec.

Different sensitivities on different inputs can be frustrating but it may be 'normal' for the unit. Its very hard (impossible) to judge these things without doing exact measurement.

Could there be any user selectable gain options for the phono stage ? or could it have been modified in some way ?
Both channels are the same output. When connected to my Nikko Beta 30, the levels between all the different sections are nearly identical. The adcom levels are the same for the CD and Tuner sections and drop way off on the phono section.

There is no evidence of any external selectable gain adjustment which I would think would be the case if this was a feature of this preamp. I downloaded a service manual for this unit but the documentation is not very easy to read due to the quality of the scan... I have been trying to look through the schematic but it's a challenge. I am going to open the case up next and see if I can see any issues. I will post my results...
Without digging into it too much, someone pulled one end of R507 and R508 and soldered them on to another resistor's lead to apparently adapt the phono section to a MM cartridge... I will be verifying the values in this section and restoring the original circuit...
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