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Ian Bell Ian Bell is offline
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Default 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
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TimPerry TimPerry is offline
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Default 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?


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[email protected] dpierce@cartchunk.org is offline
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Default 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.

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Ken Ken is offline
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Default 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.



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Ian Bell Ian Bell is offline
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Default 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
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Ken Ken is offline
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Default 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.

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Ken Ken is offline
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Default 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.

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Ian Bell Ian Bell is offline
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Default 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
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dave dave is offline
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Default 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




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Kees-Schoen Kees-Schoen is offline
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Quote:
Originally Posted by dave View Post
"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
I had the same problem yesterday. I repaired the A77 myself as I have the service manual. I bought the manual together with the machine in the early 70's.

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
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