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Old 6th Oct 2019, 6:10 pm   #43
Andrew64's Avatar
Join Date: Jan 2016
Location: Ramsgate, Kent, UK.
Posts: 137
Default Re: Revox A77 refurbishment advice please.

Yes John, just finished it!

I have finalised and tested the 21v Power supply mod, not so much a revision more a re-design, see circuit.

Operation for this assume +21v rail is reference, 0v is output and rising means -ve going.

At switch-on R107 starts things going, current down R105/P106/R102 starts Q101 conducting, in turn starting to turn on Q102. Dx clamps R105/P106 jcn to Cx, this starts charging and also via Rx. The output rail quickly rises to 12v. D104 then starts to pull up Q101 emitter, the rate of output rail rise then tracks the rate on Cx. When the output is near 21v, conditions start to stabilise around Q101/Q102, Cx charging via R105 ceases but charging continues via Rx, Dx then reverse biases effectively disconnecting Cx from R105/P106 jcn. The power supply then regulates per the original design.

At switch-off Cx discharges via Dy, mostly into the power supply load thus resetting the circuit, a second or so extra is required for C101/C103 and Cx to fully discharge.

In practice the power supply turn-on/turn-off times are slowed sufficiently such that the VUs swing to 100% only and no hitting the end stops!, If switching speeds a pause for a few seconds at off is required to allow a full reset otherwise needles will bounce off the ends stops. I hope this mod will also extend the life of the VUs. I have only modded my A77, others may need adjustment of Cx. The extra 4 components are fairly easy to get on the board.

I hope this is all clear and makes sense!

Any observations will be welcome.

I still need to do 6 more of the plug-in boards but I have done a test to see if my ministrations and mod have introduced faults, during testing I noticed one of the record level controls became intermittent at around the no.5 point. I removed it for examination and repair, it's riveted together so careful work was needed with the Dewalt to open it up. The fault was obvious, see photo. The sliding contact is that which connects the wiper to the middle terminal. After being under tension for decades metal fatigue has set in, a similar fate has befallen the other contact of the pair, you can just see it in the upper left of the photo. No easy repair here so a replacement is required, a want has been placed!

Otherwise all is working so the other boards next!
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