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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 12th May 2020, 11:13 pm   #1
cgehansen
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Default Revox A77 issue

Hi, I've just acquired a Revox A77 high speed which has an odd issue with monitoring the input. When I set it to monitor the input I can only hear sound when the mute switch is disengaged (with the pinch roller up). With the pinch roller down there is silence. The play and record functions work perfectly but if a tape is playing and I select input using the rotary switch I can hear the input and also what is on the tape. When I got the machine the input/NAB rotary switch was broken and all the contacts had been removed. I've replaced it with another standard A77 rotary switch with three contacts. I noticed that the boards were very dirty so I've given them a good clean but it's not fixed the problem. Any advice would be greatly appreciated as I'm not sure where to look for the fault. Thanks, Christian
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Old 13th May 2020, 7:19 am   #2
Ted Kendall
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Default Re: Revox A77 issue

Welcome aboard!

Are you sure you've used a correct switch? I wouldn't swear to it without checking, but I think the input and monitor switches have different contact layouts. Or you may just have installed it the wrong way round.

If somebody's been at the original switch, they may have modifeid the main board as well, but I'd check out the switch first.
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Old 13th May 2020, 8:42 am   #3
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Default Re: Revox A77 issue

Hi Ted, Thanks for the reply. I've checked the switch and it is the correct one and correctly fitted. However what I didn't notice first time around is there appear to be some modifications to the board (the red wires at the bottom of the photo). I can't see them on the schematic.
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Old 13th May 2020, 10:46 am   #4
Ted Kendall
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Default Re: Revox A77 issue

Figures. Perhaps the phantom bodger wanted to bypass the monitor switch. Best to return it to standard and then try again. I think there is a manual on Hi Fi Engine.
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Old 13th May 2020, 11:25 am   #5
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Default Re: Revox A77 issue

Thanks for that. Got the manual. The switch had been bypassed. I’ve removed the wiring and it works as it should now.
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Old 13th May 2020, 11:29 am   #6
Ted Kendall
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Default Re: Revox A77 issue

Great stuff!
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Old 13th May 2020, 11:42 am   #7
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Default Re: Revox A77 issue

I'm inclined to agree with Ted, fortunately they haven't soldered onto the contact part of the board.
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Old 13th May 2020, 12:08 pm   #8
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Default Re: Revox A77 issue

If the phantom bodger has been around it, and it's 7.5/15ips you may want to check whether it's a real high speed one. They are fairly rare and a lot of them that turn up are after-market mods. Modifications verge from crude bodges to a proper job with all the correct parts needed. Real ones have a different capstan motor with a larger diameter capstan and most importantly a different offset to allow for that bigger capstan and keep the tape path in the same position.

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Old 13th May 2020, 1:09 pm   #9
Ted Kendall
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Default Re: Revox A77 issue

Good point. The quick check is for the position of the capstan motor bearing in the headblock cutout. In standard models it is offset towards the controls (larger gap at the back) and in high speed models it is offset towards the tape counter (larger gap at the front).
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Old 14th May 2020, 8:46 pm   #10
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Default Re: Revox A77 issue

I was wondering whether it was a conversion. The capstan offset appears to be towards the controls instead of the tape counter and there are some stickers on the front panel. It came out of a studio. Now it's working I've notice another issue. I've cleaned all the switches and circuit boards but there's still a loud pop in the left channel when switching from Tape to Input. It was there before cleaning and hasn't improved afterwards. It's also noticeable when rotating the channel selector (but only when monitoring the Input and not on tape playback). It doesn't affect the right channel.
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Old 14th May 2020, 11:49 pm   #11
Ted Kendall
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Default Re: Revox A77 issue

That's undoubtedly a conversion - factory high speed machines have the speeds silk screened on the front skin - stickers, even those marked Professioneller Apparat are an indication of a UK conversion. The motor is offset towards the controls - a proper high speed motor has the mountings offset 2.25mm to bring the driving face of the larger capstan to the same position as that of the normal capstan. The playback head mounting plate will have been bodged to bring the head into the tape path.

There is enough meat in the mounting webs on the capstan motor to drilll the holes out a fit offset bushes, but this requires a fair degree of mechanical engineering.
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Old 15th May 2020, 12:13 am   #12
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Default Re: Revox A77 issue

I could try and find the correct motor instead. Are the head mounting bodges reversible? I already have a decent 2 track machine so am just wondering if this conversion is worth tinkering with. If not I'll probably just sell it on as it is.
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Old 15th May 2020, 10:42 am   #13
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Default Re: Revox A77 issue

The head is on a single hole mounting, so you may be able to get the right position by pushing it back towards the counter and tightening there, assuming the PB hasn't applied the file to that side of the hole. The mounting holes on the motor are threaded, unlike those on the B77, so hacking them out with a file is really a bodge too far. It's as well to check whether you have a bead-blasted capstan shaft, in which case it is probably a factory item - some conversions had shafts made in the UK. At all events, if you change or move the motor, be careful to keep the tape path in line with the wear pattern on the head. The alternative here is a re-lap.
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Old 15th May 2020, 11:16 am   #14
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Default Re: Revox A77 issue

Thanks Ted. How can I tell if it's a factory capstan? The other thought I had is just to replace the capstan with a standard size (I have a spare) and return everything to its original spec. There are some mods to the amp and record boards which I don't understand but I have another MKIV I can use as a comparator. Given I don't know what other bodges have been done it doesn't seem worth going to the trouble of sourcing a new motor.
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Old 15th May 2020, 11:23 am   #15
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Default Re: Revox A77 issue

Factory capstans for Mk3 and on have a matt, bead-blasted finish. If the one you have is shiny, it's an aftermarket part. Reversion to standard might be a sensible thing - you'll still have to set the play head back to where it should be. There were ways and ways of doing the electronic mods, but if you have standard boards for comparison, I'd revert to those. Those silly stickers might even come off the front skin if you're careful...
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Old 15th May 2020, 11:39 am   #16
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Default Re: Revox A77 issue

The mods to high speed And vice versa also involve changing several components in the record and playback amplifiers, possibly even the oscillator. So if you do convert it back you’re in for quite a bit of work plus full alignment afterwards.

Having said that although it isn’t exactly to factory specs, it has lasted over 45 years with the conversion so could probably soldier on as it is.

As far as the clicking is concerned, you could swap the reproduce amplifiers to ascertain if the fault lies in there, otherwise you might have faulty parts in the input amplifier.
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Old 15th May 2020, 4:39 pm   #17
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Default Re: Revox A77 issue

Thanks for all the advice. It's very helpful. The capstan looks sandblasted so hopefully it's a factory one. I can't really follow the mods to the boards as so many parts have been changed. The oscillator looks untouched and is the correct on for a 2 track. I think it's been equalised for NAB. It does actually sound fine and there's no unusual wear so I may just leave it. The popping always occurs when the left record switch is pushed in or out. The right one is fine. Swapping the boards doesn't make a difference. I've given the switch a good clean. Does that mean it's likely to be the input amp?
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Old 16th May 2020, 9:35 am   #18
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Default Re: Revox A77 issue

No, my apologies, I thought you meant the monitor switch, try swapping record boards, it could be a slight DC going through the switch due to leaky capacitors.
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Old 16th May 2020, 10:39 am   #19
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Default Re: Revox A77 issue

Thanks Maurice. It was actually the input amp. I've swapped it for a good one and the pops have gone away. It was also affecting the left record level which sounded very scratchy as though the pot was dirty. Now it appears to be fault free I may make the effort to change the capstan motor. Do you know if the B77 High Speed motor will work? It looks identical.
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Old 16th May 2020, 10:40 am   #20
Ted Kendall
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Default Re: Revox A77 issue

Quote:
Originally Posted by Michael Maurice View Post
The mods to high speed...involve changing several components...possibly even the oscillator.
The oscillator only needs changing when the track format is changed.
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