Don’t you mean moving coil input?
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I don't know how any of us could hope to give you advice that is as informed as, let alone better than, that of the person who actually did the mod and apparently has your unit in his possession.
There are any number of shots in the dark that any one of us could take. Is there a mute switch? Could it be somehow engaged or if that is not the case, has he checked the resistance between hot and ground of either the input side or the output side? Like Totem said, the MM/MC switch is a viable culprit, because both channels go through it. Could be a short, due to any number of other causes. Next I would check the gain devices in the MC circuit. If there is no short to ground, then the likelihood is that one of the gain devices bit the dust, but it would be odd to lose both channels at once that way. I bet it's something obvious.
Check the Selector/Bypass/Mode switches are set correctly.
They intereact with each other - reread the manual.
Bypass mode shorts inputs.
Otherwise get another "tech" - any competent tech should be able to figure it out - faulty caps, resistors, relays, latching circuits - this is basic stuff. Go to your local high end store and ask if they know a good tech if you don't.
Thanks all, good points. We may have inadvertently changed some toggle switches on the front panel in handling the preamp. I have always kept the bypass switch off and the bypass mode on balanced.
The "tech" is a friend of mine who did this kind of work in his youth and whose skill at switching out the resistors we trusted would be much better than mine. He didn't expect to run into any problems like this. I will discuss the front panel switches with him in the AM. He is concerned about both the MM/MC switch and the age of the electrolytic capacitors in the MC output area. I do have a pro at a high end shop nearby who can probably diagnose the problem quickly.
Looking back on this, it's true that it wasn't realistic of me to present you all with such an open-ended situation. I will report back.