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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 15th Jul 2019, 5:13 pm   #21
Welsh Anorak
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Default Re: Revox A77 refurbishment advice please.

Like Michael, I've had a couple of those large axial caps fail. The symptom of the right hand one when it shorts is simply no capstan rotation with the voltage missing. Took me longer than it should have done to find as I was convinced the fault was on the power supply board.
Confession time - to avoid removing the whole switch board I cut the legs by the capacitor and soldered in the replacement axial component.
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Old 16th Jul 2019, 9:03 am   #22
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Default Re: Revox A77 refurbishment advice please.

Quote:
Originally Posted by Welsh Anorak View Post
Confession time - to avoid removing the whole switch board I cut the legs by the capacitor and soldered in the replacement axial component.
If I'm charging for the job, I prefer to remove the board and solder the capacitors to the print. It saves the joint at the PCB track going dry jointed.

BTW, in the manual for my Korting tape recorder the manufacturer, the instructions for replacing parts on circuit boards is exactly as Glyn does.
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Old 16th Jul 2019, 3:44 pm   #23
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Default Re: Revox A77 refurbishment advice please.

I normally don't do this as it does smack of a bodge, but in this case it was a 'get you home' repair, so I just changed the faulty capacitor - plus the inevitable RIFAs, of course.
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Old 16th Jul 2019, 11:58 pm   #24
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Default Re: Revox A77 refurbishment advice please.

I have those caps on my list Michael & Glyn, 5 in total for that board + the trimmer, talking of trimmers, I can't find a Piher 20k for love nor money, plenty of 25k ones, I've tried at least a half-dozen sites and emailed Piher for a stockist, anybody know of somewhere?
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Old 17th Jul 2019, 10:14 am   #25
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Default Re: Revox A77 refurbishment advice please.

25K will be fine.
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Old 20th Jul 2019, 10:13 pm   #26
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Default Re: Revox A77 refurbishment advice please.

Thanks Michael - I'll get some of those on order.
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Old 7th Aug 2019, 10:59 pm   #27
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Default Re: Revox A77 refurbishment advice please.

I've bought the parts needed now, I managed to find 22k trimmers and as they were only 30p a pop I got enough to be able to sort them so I have four that are within 10% of 20k for the more critical locations.

I have nearly finished a move to a much better man cave so I'll get started on the refurb soon.
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Old 24th Sep 2019, 11:19 pm   #28
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Default Re: Revox A77 refurbishment advice please.

The refurb starts at last and I'm tackling the switch board first, I haven't taken it right out, too many wires to disconnect. I've taken the front metal plate off and taken the plugin boards out, first marking who came from where! I noticed the lamps though working have a green corrosion ring round their bases caused I suspect by storage in damp conditions, these were carefully teased out cleaned and lightly treated with Vaseline before replacement.

Now P305 trimmer, the existing one was 303k and it was set at 263.5k, as the new one was 263k to save having to set it right at the end I changed R305 to 100k, this should allow enough room if the crosstalk needs adjustment if the old P305 had drifted high.

A77s have a issues, the VUs banging the end stops at switchon offends my mechanical sensibilities and the VUs don't work on playback being two. I would like to correct this, now before I go attempting to reinvent the wheel has any of you modded your A77 to overcome these issues? I would be interested to know how if you have.

I'm doing the transformer assembly now and have it on the bench with the power and speed control boards removed ready to be worked on, a bit of a mission in one way but now easy to work on.
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Old 24th Sep 2019, 11:31 pm   #29
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Default Re: Revox A77 refurbishment advice please.

The Dolby A77 had the VUs operable on playback and tidied up a few funnies in the monitoring arrangements, but modification would not be simple.
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Old 24th Sep 2019, 11:55 pm   #30
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Default Re: Revox A77 refurbishment advice please.

The basic board (motherboard) had a number of modifications for the Dolby version which as I understand it are not simple to do. I think the print is different as is the printed switches.

Not something I'd try.

The VU meters are driven from the Record Amplifiers, to monitor the output, you'd have to disconnect the drive to them and reconnect them to the reproduce amplifier after the INPUT/TAPE switches
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Old 25th Sep 2019, 7:25 am   #31
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Default Re: Revox A77 refurbishment advice please.

I said it wasn't simple - the Dolby version did, however, raise the signal level at the tape monitor switch from the 9mv average of the standard version and eliminate the loading effect which causes a 1dB drop in recorded level when the switch is set to Input.

Angus McKenzie published some modifications which addressed these problems around 1970. These and re-connecting the VUs to the monitor output may be a way to go, but readings will be dependent on the monitor gain setting. Offhand, I can't remember whether it's possible to connect the meters before the gain control without additional buffering.

Last edited by Ted Kendall; 25th Sep 2019 at 7:37 am.
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Old 25th Sep 2019, 2:55 pm   #32
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Default Re: Revox A77 refurbishment advice please.

I would be interested in reading the Angus McKenzie article, a search turned up part 2 of the article on the American radio history site, Studio Sound April 1972 unfortunately the previous month March (part 1) is missing from the archive, can you point me in the right direction to part 1 Ted or anybody that may know?
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Last edited by Andrew64; 25th Sep 2019 at 2:57 pm. Reason: clarity
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Old 28th Sep 2019, 3:07 pm   #33
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Default Re: Revox A77 refurbishment advice please.

I have an old photocopy of the McKenzie article which I must have taken back in the 70's: I'll scan it. I modified my (then current) hi-speed Revox according to some of his recommendations (raising the level at the record-replay interface by 10dB, removing 6 dB pads).
By complete happenchance I've just bought (at great expense) a 4 track Revox from 1977 , partly for sentimental reasons (hi-speed one sold in the 80's). I hope you won't mind me muscling in on this thread for advice?

I did wonder if the 4u3 and 3u5 motor run caps should be replaced, as I can see a thin line of slightly guey black around some of the seals?

I'm a bit nervous about wholesale replacment of the trimmers, as it seems to be replacing carbon pots with carbon pots? Is there no source of cermet trimmers which can be made to fit?

John
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Old 28th Sep 2019, 3:28 pm   #34
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Default Re: Revox A77 refurbishment advice please.

McKenzie article attached. Apologies for the quality: it's an old photocopy which was probably already third generation in 1975.

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Old 28th Sep 2019, 6:43 pm   #35
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Default Re: Revox A77 refurbishment advice please.

Quote:
Originally Posted by John_BS View Post
I did wonder if the 4u3 and 3u5 motor run caps should be replaced, as I can see a thin line of slightly guey black around some of the seals?

I'm a bit nervous about wholesale replacment of the trimmers, as it seems to be replacing carbon pots with carbon pots? Is there no source of cermet trimmers which can be made to fit?
It would be best to change the motor caps if they are starting to ooze. The Piher presets available from Cricklewood Electronics are a great improvement on the standrad item and should see the machine out.
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Old 28th Sep 2019, 10:09 pm   #36
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Default Re: Revox A77 refurbishment advice please.

Thanks Ted.

Does anyone have a curve showing the bass lift on two & 4 track machines when playing a full track calibration tape? I've seen 2 or 3dB at 40Hz quoted, but no other frequencies.

John


PS am posting a request for a VU meter in the Wanted section.
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Old 28th Sep 2019, 11:25 pm   #37
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Default Re: Revox A77 refurbishment advice please.

Try this :

http://www.mrltapes.com/mcknight_low...alibration.pdf

There isn't an LF tweak on the A77 in any case, and it isn't usually a problem once you know what's going on. On my A810 (2mm tracks), it starts to kick in around 500Hz - only a needle's width, mark you...

Last edited by Ted Kendall; 28th Sep 2019 at 11:38 pm.
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Old 28th Sep 2019, 11:38 pm   #38
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Default Re: Revox A77 refurbishment advice please.

Many thanks for the article John, it is quite readable and I'll study it carefully and see which mods I will do.

Thanks for the heads up on the caps Ted, just had a close look at mine, see photo, they definitely need changing!, now where to get them...

I purchased a spare Piher trimmer and have sacrificed it to see what the construction is like, I would have preferred to see a carbon brush wiper but the track terminations look good. I had a look for cermet trimmers but couldn't find them in the same form factor.

I've found a way to stop the VUs banging against the end stops, after studying the regulator circuit and giving it a lot of thought I've bridged C103 (3.3 uF) with a 220 uF cap and this has slowed the rise time of the supply sufficient to stop the banging, it does not seem to have affected the stability. If anybody spots I've made a glaring error here please let me know.
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Old 29th Sep 2019, 11:58 pm   #39
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Default Re: Revox A77 refurbishment advice please.

I've ordered motor run caps, not quite sure how I missed the state of mine!, no fool etc.

I'm using service manual scan (0484 10th Edition), I presume 0484 means April 1984, page 24 describes the operation of the old speed control board with the inductor, page 41 has a description of how the later IC board works and at page 73 (according to my pdf reader) is the circuit followed by the parts list of the IC board, can anybody tell me if this is the latest version of the service manual?

I have come up with an improved way of slowing the 21v supply build up, I'll publish when I've tested it out.

Another question, has anybody modded their A77 as suggested by Angus McKenzies article?
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Old 30th Sep 2019, 10:38 am   #40
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Default Re: Revox A77 refurbishment advice please.

Quote:
has anybody modded their A77 as suggested by Angus McKenzies article?
Yes, see post 33 , but not the mods relating to 600 ohm studio output/input levels.

As I have a duff VU meter I'm thinking of modifying the "Radio" DIN connectors to feed a pair of external ppm's.

John

PS my scan is the same as yours.
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