VALVE Magazine Online! Tech tips and other unsolicited advice. The 75 Minute Restoration

From VALVE Volume 2 Number 7 July 1995

Bill called a couple Saturdays ago to tell me that he had traded a Mac 1700 (a hybrid receiver, with tube tuner and solid-state amp) for a Scott 340 receiver. The person he traded with acknowledged a greater value for the Mac than the Scott and offered to pay a set amount for service by yours truly to balance the deal. Would I be available this weekend to do check it out?

I asked Bill if it ran and he said yes. Knowing that the weak point on Scotts was their coupling caps, I suggested he run it for a while, constantly monitoring the output tubes for red plates. He called back a bit later and said that one tube's plate had started to glow red, at which point he turned the 340 off. I said bring it by and I'd get it going.

Well, he brought it by the next day. I knew Bill had to get back, but I told him I could probably get the receiver going while he waited. The only requirement was that he listen and evaluate my new speakers while I worked.

First the tubes came out for testing. The 7591 outputs were good, luckily. The tubes I suspected to be bad, however, were bad. These were the driver tubes, 6U8 triode-pentodes. Ask any technician who used Tektronix scopes in the 50's and 60's about 6U8's. They always wore out or shorted elements. Even worse, they can test OK in an emission tester even though bad. We put this pair on a TV-10 transconductance tester and they didn't look so hot. I suggested we replace them with a pair of new 6GH8A's, a pin for pin replacement, which seems a bit more reliable. Bill said OK.


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