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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 8th May 2022, 1:56 pm   #1
Linnovice
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Default A77 to revert or not?

A few years ago I bought a Revox A77 MkIII machine. It was an estate purchase and from what I've been told, the chap who had it originally was an experienced electronics engineer. Experimental radar and guidance systems for the military so I'm told.
I've not used it for a while, probably two years. The machine is badged as a standard 7.5/3.75ips machine but in reality it has a 7.5/15ips capstan motor installed. I have started to service and recalibrate it but have hit a problems in as much as I can't get any of the recording settings to match anything listed in the service manual. It also came with a Revox manual and on looking at it I've noticed that on some of the channel cards alterations have been made to the circuitry of the drawings. I'm not terribly experienced in electronics so for me to work out what has been done and why, I'm hitting a brick wall.
The machine overall is in very good mechanical condition with extremely light wear to the heads. I also have a spare 7.5/3.75ips capstan motor also in very good condition. I'm seriously considering reverting it back to the standard version and not worrying about the 15ips capability (I do not need it as I already own other Revox/Studer machines with that).
My questions are these, what existing cards would I need to revert to standard (or buy replacements for) and would the considered opinion be that it is worth making the effort?
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Old 8th May 2022, 5:22 pm   #2
Ted Kendall
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Default Re: A77 to revert or not?

First thing to check is the position of the capstan motor. The bearing housing projects through a circular hole in the headblock. If the larger gap is towards the front of the machine, then you probably have a proper conversion, achieved by replaciing the low speed motor with a high speed. If the larger gap is towards the rear, as it should be on low speed machines, then the conversion is a bodge, where the standard capstan shaft is replaced with a larger one, without offsetting the motor mounting.

If you have a proper conversion, then it is a simple matter to change the motor back to standard and, because the driving face of the capstan appears in the correct position, no adjustment of the head position is needed.

Reversion of a bodge is more difficult - because the capstan driving face is offset forward by about 3mm, the holes in the r/p head mounting plate are generally hacked about with a file to bring the play head forward. Depending on how this has been done, it can be a right pain to put things back the way they should be, as the reference points can be lost. The wrap is never quite right on bodges, and this may be the cause of your line-up troubles.

There was a factory-approved modification of the rec and rep cards for high speed, and there were...others. Best thing to do is to revert to standard layout if you are changing speeds and line up more or less from scratch.

Last edited by Ted Kendall; 8th May 2022 at 5:38 pm.
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Old 8th May 2022, 6:48 pm   #3
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Default Re: A77 to revert or not?

Hi Ted, thanks for replying. I'm not sure what you mean by 'gap'. With the machine standing upright the bearing housing appears lower and slightly to the left in the hole of the headblock. I've fitted a 7.5/3.75ips capstan this afternoon and it works ok. All I've had to adjust was the pinch roller pressure by adjusting the solenoid position. ie. there is a bigger gap at the top. Would this be correct?
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Old 8th May 2022, 8:36 pm   #4
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Default Re: A77 to revert or not?

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Old 8th May 2022, 8:37 pm   #5
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Default Re: A77 to revert or not?

This machine evidently was converted by merely changing the capstan shaft. To make the head wrap work, the steel plate which holds the rec and rep heads will have had to be moved forward. You need to move it back where it was, or the wrap, particularly on the play head, will be excessive. If you remove the screws holding the plate to the block (you don't need to touch the actual head mountings) I expect you will see that the holes in the plate have been filed oval. This being so, you will get somewhere near the right position by setting the plate back as far as it will go. With luck, the forward edge of the holes will not have been filed.
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Old 8th May 2022, 11:07 pm   #6
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Default Re: A77 to revert or not?

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Yeees, well, I would if I could but it appears not to be as easy as that.
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Old 8th May 2022, 11:15 pm   #7
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Default Re: A77 to revert or not?

The head wrap doesn't seem to be a problem, it is governed by the fixed guide after the playback head and before the right hand guide 'roller' which is fixed. Ted, I think you are right, the capstan motor was just swapped for a 15ips variant. ie. the centre of the shaft is the same whether 3.75/7.5ips or 7.5/15ips. The R H guide determines the tracking line of the tape over the head. How high or low the capstan is doesn't seem to be a problem.
I do have a problem now though, with the changing and 'fettling' it seems to have lost the 3.75ips speed. When selected the capstan just stops.
Oh boy, I think the single malt will have to come out . . .
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Old 8th May 2022, 11:32 pm   #8
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Default Re: A77 to revert or not?

The guide before the capstan should not determine wrap, only height. The barrel is soft brass and is not designed to be in contact with the tape surface. This guide and/or the head mounting plate is out of position and should be moved back.
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Old 9th May 2022, 8:20 am   #9
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Default Re: A77 to revert or not?

This is how the capstan is positioned with a standard motor. Am I right in thinking Ted that the head mounting plate needs lifting to straighten the tape path. ie. Move the tape away from the guide?
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Old 9th May 2022, 9:25 am   #10
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Default Re: A77 to revert or not?

That guide looks forward to me. I'd loosen the securing screw, push it back towards the counter and retighten, also slacken the three screws holding the head mounting plate and push that back and retighten. The result should be close to standard, as the edge of the holes you will be bearing on should not have been filed.
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Old 9th May 2022, 12:49 pm   #11
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Default Re: A77 to revert or not?

As a comparison, I checked my A77. With the machine switched off, a tape loaded and the pinch roller pressed by hand against the shaft, the tape clears the brass post by 0.5mm to 1mm.

If the tape wrap has been uneven against the repro head, the head could have worn unevenly. Once you have the shaft and guide heights sorted it might be worth carefully inspecting the repro head's face.

This isnt easy with the A77 because the panels block a direct view of the heads. It helps to temporarily remove the A77's control fascia panel (and the swing down aluminium panel - watch the two springs!) to gain an uninterrupted view of the tape path, heads, guides etc when checking or setting up the heads. Even cleaning the heads is a lot easier with a clear view of what we're doing.
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Old 9th May 2022, 2:39 pm   #12
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Default Re: A77 to revert or not?

Hi Timtape, as you can see in the photos, the machine is stripped of all covers so its easy to see the heads. They are in very good condition, hardly any wear at all.

I've checked my other A77, PR99 and Studer A807, all of which have the same brass tape guide. In all cases the tape bears against the brass, not heavily but definitely touching. I agree with Ted when he comments that the head plate is too low so will have a look later this evening to see if I can move it up a bit.

It never rains . . . during the swapping of the capstan motors I have now lost the 3.75ips (or 7.5ips on the HS capstan). No idea why though.
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Old 9th May 2022, 4:31 pm   #13
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Default Re: A77 to revert or not?

Quote:
Originally Posted by Linnovice View Post
It never rains . . . during the swapping of the capstan motors I have now lost the 3.75ips (or 7.5ips on the HS capstan). No idea why though.
That's more likely to be a problem with the drive card (switching, maybe?) than the motor itself.
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Old 10th May 2022, 11:12 am   #14
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Default Re: A77 to revert or not?

Quote:
during the swapping of the capstan motors I have now lost the 3.75ips (or 7.5ips on the HS capstan). No idea why though.
It's no substitute for the expert advice you get on here I know but I have a troubleshooting guide for speed issues that may help...
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Old 10th May 2022, 1:02 pm   #15
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Default Re: A77 to revert or not?

Quite right Ted, the screw holes in the head mounting plate had been elongated. I've now loosened of the screws and pushed the plate up. Tape path is much smoother now.

Now for the motor problem.
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Old 11th May 2022, 6:37 am   #16
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Default Re: A77 to revert or not?

Quote:
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Quote:
during the swapping of the capstan motors I have now lost the 3.75ips (or 7.5ips on the HS capstan). No idea why though.
It's no substitute for the expert advice you get on here I know but I have a troubleshooting guide for speed issues that may help...
Thank you for the info but I believe this to be for a later version A77, MkIV or B77. My MkIII does not have ic’s on the board.
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Old 11th May 2022, 8:27 am   #17
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Default Re: A77 to revert or not?

What exactly is not happening at the lower speed? Is the capstan not turning at all, is it "hunting", is it running fast? I'd guess that either the motor isn't getting power or it has no signal to lock on to.
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Old 11th May 2022, 9:02 pm   #18
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Default Re: A77 to revert or not?

It might also be that the 21V which is used as a switch is not missing?

Have you checked that the speed selector slide switch is moving when you change speed?
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Old 11th May 2022, 11:54 pm   #19
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Default Re: A77 to revert or not?

The output from the variable reluctance tacho will be proportionately less at the lower speed which makes it more likely to have the servo loop drop out of lock, than at the higher speed.

The clues needed are in that the capstan motor is doing when low speed is selected, as Ted said, and then a look with a scope at the tacho waveform as it passes onto and through the amplifier stages on the capstan control board.

If someone's taken the bodged approach of filing holes etc to get a bigger capstan in, then what horrors have they perpetrated on the capstan control board?

It's probably also worth looking for anything done to the spool drives.

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Old 12th May 2022, 8:01 am   #20
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If someone's taken the bodged approach of filing holes etc to get a bigger capstan in, then what horrors have they perpetrated on the capstan control board?
The whole point of the "shove a bigger shaft in and hack it" approach was that it was cheaper than doing it properly by simply fitting a high speed motor. No modification to the capstan card was needed in either case, but some fairly "creative" stuff went on in the audio electronics, depending on who did the conversion. It could be, of course, that some bright spark decided to have a fiddle at the same time...

Without prejudice, as the legalists say, the motor has to be disassembled for the shaft to be replaced. Could the tacho head have been knocked out of position in this process? It evidently still works, as the higher speed is OK, but, as David points out, the output will be lower at low speed. This was brought home to me when I modified one of my A80s to run at 3 3/4 ips - halving the tacho frequency was easy enough, but the thing wouldn't lock until the tacho head was about a fag paper away from the rotor, when order was restored.

Last edited by Ted Kendall; 12th May 2022 at 8:11 am.
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