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Old 19th Oct 2019, 10:30 pm   #46
Andrew64's Avatar
Join Date: Jan 2016
Location: Ramsgate, Kent, UK.
Posts: 137
Default Re: Revox A77 refurbishment advice please.

I will describe how I did the trimmers on the Record Amp board, others may find it useful, see attached diagram. The values, total and from the wiper to one end were recorded before the trimmers were touched in case removal changed anything.

P504 is the easiest, just set the new installed one to the same value as the old one.

P503 is more difficult, it's total value was 22.5k and it was set at 13.44k from earthy end, this works out at 59.7% rotation from earthy end, the new on was 21.4k so 59.7% of that is 12.8k so I set it to that not 13.44K

P501 and P502 are more difficult again, the resistors in the circuit make it hard to accurately measure them, they also affect the value read at the wiper, in this case measuring them out of circuit and using the procedure above should get them nearish, well that's my theory. Tests later on should see how close I get them.

Notwithstanding the above I'm sure there will be other ways of doing this!

Boards all done now and replaced, the motor caps have come so there in as well. The level pot has arrived also and that has been replaced. I've run it up now and the capstan speed was within 1%, tweaked now to better than 0.1%. I've done a basic check, input levels at max, 1 kHz to both Aux inputs, levels sufficient to get 0db on one VU meter, the other channel was within db, I'm pleased with that!

More fettling of the brakes need doing, still get a bit of tape spill under certain conditions.

I will run some tests soon to check I get the graphs on pages 1.69 & 4.71.

Attached Files
File Type: odt Record Amp.odt (368.7 KB, 53 views)

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