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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 1st Oct 2019, 1:35 pm   #41
Andrew64
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Default Re: Revox A77 refurbishment advice please.

Sorry I missed that bit in post 33, I will probably do similar but will get the re-cap and trimmer change done first.

I have toyed with the idea of ppm's, converting the existing VU's to semi ppm's at least should not be to onerous, will have to be careful about calibration though, again a job for later on.

I've done the boards on the transformer and replaced the assembly, all seems to be working fine, I've had to whip the power board out again to incorporate Rev 1 of the 21v supply mod, it's a fiddle but I could just get the board out by itself. I've put it to one side now while I sort a TC377 motor for Michael M.
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Old 6th Oct 2019, 3:13 pm   #42
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Default Re: Revox A77 refurbishment advice please.

Any update on PSU mod?
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Old 6th Oct 2019, 6:10 pm   #43
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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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Old 6th Oct 2019, 9:26 pm   #44
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Default Re: Revox A77 refurbishment advice please.

You only need the rotor, so a pot of any case style or value that has the same rotor and shaft would do.... which probably means any of the A77 front panel pots.

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Old 7th Oct 2019, 11:44 am   #45
Andrew64
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Default Re: Revox A77 refurbishment advice please.

Thanks for that tip David, I have it sorted now, a set of pots is on its way from Switzerland , a spare or two may be useful, never know if any of the others are suffering fatigue. There is a set of motor caps as well, shame I've just bought new ones, they are secondhand but if anybody is needing some I will have them.

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Old 19th Oct 2019, 10:30 pm   #46
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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.

Regards,
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Old 23rd Oct 2019, 11:31 am   #47
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Default Re: Revox A77 refurbishment advice please.

P504 sets the VU meter level,

P503 sets the record level,

P501/502 set the equalisation. all should be set using an oscillator and meter and suitable tape.
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Old 9th Nov 2019, 12:21 am   #48
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Default Re: Revox A77 refurbishment advice please.

Hi all,

I have just got back to this as life has kept me busy.

Can anybody help me with a calibration tape or where to get one?

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