Marantz 2275 - trouble in mono

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I have a wood-encased Marantz 2275. Both it and its manual look as if they just came off the assembly line. The receiver works that way, too. Except that, in the phono mode, when I play a record and push in both "mono in" buttons, the channels seem to cancel each other out, with practically no sound -and even that distorted at best. With these buttons out, or only one pushed in, the sound is fantastic. This doesn't happen on any of the other inputs.

Where should I begin in looking for why this is happening?
 
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This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.