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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.