Re: PET 2001 internal tape drive

Den Fri, 9 Feb 2018 22:21:10 +0100 skrev Francesco Messineo
<francesco.messineo@gmail.com>:
> On Fri, Feb 9, 2018 at 3:39 PM, Bill Degnan <billdegnan@gmail.com>
> wrote:
> 
> >>
> >>
> >
> > I assume not necessary, but did you verify that the tape drive belt
> > is OK? I had to replace mine by using an external tape drive's belt.
> 
> 
> The belt looks ok. It's a bit weak, but it doesn't slip. I might try
> to substitute it out of desperation.

If you have any skill for hearing wow/flutter you could play a music
tape with either a pure sine wave tone or piano music and hook up an
amplifier early on in the amp of the datasette (or even hook up it's
rec/play head to the electronics of an audio tape recorder) and just
listen.

You can also use software with a modern pc to analyze a tone for
wow/flutter. I'd assume that the rec/pb head can be hooked up directly
to the mic input even though the signal level might be a bit off.

> So far I've replaced the LS14 with absolutely no difference. The write
> circuit, according to the schematics and what I could find in the
> actual board
> is just part (4/6) of the LS14 and three resistors.
> The erase head works, if I press record and play without sending any
> data, the tape is erased very cleanly.
> The first part of a program (I assume it's a sync record) is very
> clean, but as soon as normal data is recorded, the recorded tape
> become quite
> too noisy (Yes, I should try maybe with another cassette, but I need
> to find another one I can erase...).

If you follow my idea of hooking up the electronics of an audio cassette
player to the rec/pb head of the datasette, you should easily be able
to judge how it sounds.

Do you have an oscilloscope?

Noise could be from bad decoupling capacitors on the 5V DC line, or
maybe noise from the motor (bad decoupling inside the motor).

Is the recording level about right? You can check that by recording a
bit with the problematic datasette and also a bit with a known good
datasette, and check the level indicators while playing back the tape
on an audio cassette deck. If you don't have any audio cassette deck
with meters you could hook up an AC volt meter or oscilloscope to the
speaker output of a simple audio cassette recorder. (But if you have an
oscilloscope, you might aswell measure the signal to and from the
rec/pb head).

Have you tried cleaning the rec/play switch? That is by far the most
common problem in general on old (two head) tape recorders. Usually it
helps at least temporarily pressing rec-stop-rec-stop many times in a
row, but in the long run some contact cleaner that also lubricates is
the way to go. (Without lubrication it usually gets better for a short
while but the switch might wear out faster).

> I would very much like to try a head azimuth adjustment. All the other
> tape drives I have, needed a bit of tweak to read/write reliably after
> all these years. I think the hole could be under the label, but I'm
> afraid it would bend if lifted...

In general, on cassette recorders for audio usage, in some cases you
have to remove the lid completely, usually by bending it in a certain
way. A safe work-around is to remove the mechanics from the cover and
run it naked. That works as long as the tape ends up in the correct
place even without the case - not always the case. (Pun intended ;) ).


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