On Wed, 29 Dec 2004 22:45:38 -0500 Andy Cuffe
wrote:
On Wed, 29 Dec 2004 19:28:14 -0600, Jim Adney
wrote:
I have a question about my Sony SL-2000 Betamax VCR; that's the little
1982 portable that is the mate to the TT-2000. It's not clear to me
where the best place to ask this might be, so I'm going to start by
just asking here. Just in case, does anyone know if there is a Beta
VCR newsgroup?
My SL-2000 works just fine except in Rewind. While rewinding, the tape
gets loose and flops around until the RH reel slows down and then the
tape goes briefly tight speeding up the reel. This repeats over and
over again. It didn't do this when the deck was new, and I'm sure this
isn't good for the tapes.
I have an SL-2500 which is the non-portable version of your VCR. It
also has the same problem you're describing. I think the problem is
caused by a worn upper head drum (the part that doesn't spin). The
extra friction seems to cause an oscillation in the servo system
that's supposed to keep the tape tension constant in rewind.
The fact that you have an SL-2500 which did the same thing is somewhat
reassuring. At least it suggests that this might be a well known
problem (at least to someone) and that there might be a known cause
and solution out there.
Forgive me, but I don't understand how extra friction in the upper
head drum could cause this symptom. [Doesn't the upper part spin? I
thought the lower part was stationary....]
The cyclic operation I see has a period of a little more than a
second. Surely any feedback servo would be much faster than this.
My assumption has always been that I'm not getting any back tension at
all. Since this is not in play mode, I was willing to guess that the
Rewind backtension was less important and that it was probably much
smaller, just to reduce wear on everything.
I keep thinking of the small AC motors we used to use at work. Braking
on those could be produced just by sending them some DC. If Sony did
that, then it MIGHT be that there is simply a resistor somewhere that
has gone open, setting the braking current to zero. For Rewind, and
FF, I'd expect just some small fixed amount of electronic braking. So
I'm thinking that the problem might be an open resistor, or a dead
transistor which switches it.
If I'm dead wrong here, tell me quickly. ;-)
Is there really a servo in play here in Rewind?
Rethinking this, I'm wondering if the feedback you're suggesting is
feedback to the LH reel motor, for drive speed. I had been assuming
that you meant feedback to the RH reel motor, for backtension.
One of the things I'd been toying with trying, was to see if the deck
will play while upside down. (Will it?) Then I could monitor that
board, which is on the bottom of the VCR, while it was doing various
operations. I figured that I could monitor the LH motor during FF just
to see what should be happening to the RH motor during Rewind,
assuming symmetry in the design. Is that reasonable? A comparison of
the voltages seen on each might help point out the problem.
The problem with this is that these are not just simple DC motors;
they are brushless (stepper?) motors, so I think it will take some
careful scope measurements to compare their inputs.
I'll check out the head drums anyway and see what they look like.
Thanks for the suggestions.
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Jim Adney
Madison, WI 53711 USA
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