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Tech Tips And Other Unsolicited Advice
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.
Next, I went far the coupling caps between drivers and outputs. This is the third or fourth Scott I've looked at for Bill (and about the tenth one I've worked on this year), so I know that Bill is sensitive to the sound of the coupling caps in these units. We put in some big black 600V rated film caps that I got from a Carver employee. They looked really serious and were supposed to sound great, but Bill's a really nice guy, so I let him have them instead of sticking them in one of my amps. I knew they had to sound better than the stock Ceracaps.
Note here, that I did not replace the filters. This was due to the fact that they checked OK, hum was very low on the scope, and there was both a monetary and a temporal budget to consider. I suggested to Bill that he monitor hum and maybe replace the filters in the future if it ever starts up.
Next, on to the AC balance. Driver tubes out, load across the outputs, and monitor output on the scope while turning the balance pot for minimum hum. Lastly, a speedy alignment of the tuner. A weak RF amp tube was replaced first. Then peak each IF, using the built in meter. Adjust the discriminator can for best audio, rocking the core for good centering. Then back to peak the first IF again. Repeat to get it just right, listening for bass distortion and separation. Now check pointer alignment. OK, so tweak RF and antenna trimmers at the high end of the dial, and once again toward the stations Bill likes to listen to. Compare separation with another tuner. OK, so don't fool with the MPX alignment. Yes, all this is done without a scope or MPX generator. You can get away with this if time is short and you've done a bunch of similar sets with the full test setup, so you know what your hearing.
The final result, one more totally unorthodox "speed restoration" by yours truly, one great sounding 340 (one of my favorite Scott pieces), and one happy customer, who made it to the ferry home in time.