A somewhat 'crude' but simple thing you can
try is to listen to the
audiotape sound of the tape you created which you say was
successful. Then listen audibly to the tapes you're trying to
recover. If the pitch seems to match for the most part it isn't a
speed problem. If you have an oscilloscope, look at the amplitude
of the signal out of the cassette drive of the new 'working' tape
and compare to the one you're trying to recover.
As others have suggested, use the simplest tape player you can find.
Noise reduction circuits and other fancy technology will only cause
problems.
The 'fancy technology' I suggested was an oscilloscope. I don't see how
I think you've misunderstood the message. I think Sellam was refering to
'fancy technology' inside the tape player -- things like noise reduction
circuits. You want a player without anything like that.
just using the cheapest garage sale tape player you
can find is a better
technical approach than actually looking at the waveforms coming out of
the various cheap-garage-sale tape players you've rounded up. Looking
for clipping and comparing the pulsewidths of the signal on an accurate
time base should clear up a lot.
I would agree that looking at the signal (and looking at the signal 'as
far into the computer as possible' on machines where it's not all hidden
inside a ULA -- for example the output of the comparator that produces a
TTL-level signal) is well worth doing. It's much easier to put things
right when you have evidence as to what's wrong!
-tony