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#1
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Revox A77 Capstan Servo
My high speed A77 is running too fast. The service manual description of the
capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Cheers Ian |
#2
Posted to rec.audio.tech
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Revox A77 Capstan Servo
Ian Bell wrote:
My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Cheers Ian have you verified that reel supply and takeup tensions are correct or at least close? |
#3
Posted to rec.audio.tech
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Revox A77 Capstan Servo
TimPerry wrote: Ian Bell wrote: My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. have you verified that reel supply and takeup tensions are correct or at least close? Simply not an issue with an A77. If there was NO reel tension, the tape would move at the right speed. There'd be a big mess, but it would run at the right speed IF the capstan was running at the right speed. To the originalm poster: check the obvious and easy things first. Oftentime, the problem is no more serious than the tacho pickup head having moved away from outer rotor of the capstan motor. This is easy to check fix. With the back off, look around the periphery of the capstan motor. You'll see what appears to be a gear cut in it. Now look for what looks a little like a tape head, this is the tacho pickup. There should be about 0.003" clearence between it and the rotor. Take a piece of 20lb laser paper and see if you can slip it in between the two. If you can, and you can VERY easily, and maybe you can fit 3, 4 or more sheets at a time, it's too far away. Leave one sheet in, loosen the mounting screws for the pickup head and GENTLY push it up against the paper. Tighten the screws and remove the paper. Try spinning the rotor by hand. It shuold spin freely with no interference. See if that fixes the problem: that was the #1 cause of too-high speed in A77s in my experience. |
#4
Posted to rec.audio.tech
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Revox A77 Capstan Servo
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#5
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Revox A77 Capstan Servo
On Sat, 26 Aug 2006 17:23:56 +0100, Ian Bell
wrote: My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Replace some capacitors. |
#6
Posted to rec.audio.tech
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Revox A77 Capstan Servo
Audix wrote:
Ian, I can't help with a fault diagnosis, but the circuit diagram you need is 1.077.725 for the discriminator version. As you have doubtless found out, the service manual available on the Studer website only lists the later version 1.077.724 which uses a 555. Yes, managed to find a circuit of it on a german web site complete with component values and tolerances. Does your machine date from around 1970? I had a high speed A77 from around this time which used the discriminator. I still have the service manual, but not the machine. It's earlier. Judging from the serial number it is a MKII made in 1968. The funny thing is, the manual I have describes the circuit operation as if it were using the resonant circuit discriminator but the circuit diagram is of the more modern version using a 555 timer. Ian |
#7
Posted to rec.audio.tech
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Revox A77 Capstan Servo
On Sat, 26 Aug 2006 17:23:56 +0100, Ian Bell
wrote: My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Cheers Ian I have an old A77 manual here. I made some images over the schematics with my pocket digital camera and put them here. http://tekniken.se/a77/ Is this what you want? (no 555) They are high resolution PNG-format. |
#8
Posted to rec.audio.tech
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Revox A77 Capstan Servo
On Sat, 26 Aug 2006 17:23:56 +0100, Ian Bell
wrote: My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Cheers Ian I have an old A77 manual here. I made some images over the schematics with my pocket digital camera and put them here. http://tekniken.se/a77/ Is this what you want? (no 555) They are high resolution PNG-format. |
#9
Posted to rec.audio.tech
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Revox A77 Capstan Servo
Ken wrote:
I have an old A77 manual here. I made some images over the schematics with my pocket digital camera and put them here. http://tekniken.se/a77/ Is this what you want? (no 555) They are high resolution PNG-format. Those are what I need. Many thanks. Could you also copy 3.9 which details how to remove the capstan servo board? My manual does not seem to cover this. Again, many thanks Ian |
#10
Posted to rec.audio.tech
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Revox A77 Capstan Servo
"Ian Bell" wrote in message news:44f0794e.0@entanet... My high speed A77 is running too fast. The service manual description of the capstan servo talks about a resonant circuit used as a discriminater but the circuit shows an NE555 instead. It's an old A77 and the capstan servo PCB looks like is has the resonant circuit - no sign of the 555. I have gone back through capstan servo circuits at the Studer web site and the only one with a resonant circuit is in the A80. So I don't have a definitely correct circuit for the capstan servo - need advice on how to diagnose & fix this problem. Cheers Ian Ian, I can send you a scan of this circuit, it's the older version of the speed control (1.077.725). Decode my email address and let me know where to send it. Dave |
#11
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Quote:
The culprit was the 220uF 6V elco which forms the damping circuit after the frequency discriminator on the capstan motor control board. The elco went very low resistance, about 400 Ohms........ Replacing the elco made the machine work OK again. Kees |
#12
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Приет.
блюда из яиц и помидор - фаршироанные яйца цыплята
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