From: liz@poppyrecords.invalid.invalid
Cursitor Doom wrote:
> On Sat, 10 Jan 2026 22:25:54 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
>
> >Cursitor Doom wrote:
> >
> >> On Sat, 10 Jan 2026 20:17:23 +0000, liz@poppyrecords.invalid.invalid
> >> (Liz Tuddenham) wrote:
> >>
> >> >Cursitor Doom wrote:
> >> >
> >> >> On Sat, 10 Jan 2026 18:12:35 +0000, liz@poppyrecords.invalid.invalid
> >> >> (Liz Tuddenham) wrote:
> >> >>
> >> >> >Cursitor Doom wrote:
> >> >> >
> >> >> >> Hi all,
> >> >> >>
> >> >> >> I've identified an issue with my Uher tape recorder and I'd like
> >> >> >> your opinion. There's a problem with the audio amplifier chain.
> >> >> >> I've eliminated all other possible causes like dirty heads or
> >> >> >> faulty tape etc etc. So when I'm listening to playback, the
> >> >> >> quality is frequently varying from 'pretty good' to 'really quite
> >> >> >> crappy'
> >> >> >
> >> >> >Does it become good and bad at the same places each time you play
> >> >> >the same tape? If it does, the problen is in the recording.
> >> >>
> >> >> Yes it does. But that recording still had to be processed through
> >> >> the amp chain.
> >> >
> >> >I can't follow your logic. If the tape sounds good in some places and
> >> >distorted in others - and these are the the same places each time you
> >> >play it - the fault is clearly something to do with the tape and not
> >> >the amplifier.
> >>
> >> Makes no sense to me, that observation. The tape is brand new and from
> >> a well-respected manufacturer. Think about it, the signal on the tape
> >> didn't get 'impressed' there direct from a microphone or whatever, it
> >> had to be amplified. It had to pass through the amplifier chain and be
> >> mixed with the output from the bias oscillator, so whatever defects
> >> were present at that moment from moment to moment from the amp as the
> >> tape passed, will be faithfully recorded and re-playable. I assume
> >> we're not on the same page, so will not address your follow-up until
> >> we're straight on this one.
> >
> >My apologies, I was assuming you were playng back a tape that had been
> >recorded some time ago and it had recently begun sounding distorted. I
> >hadn't realised you were looking for the cause of distortion in the
> >*recording* amplifier.
> >
> >In that case you need to monitor the signal going to the recording head
> >(preferably just before the point where the high-frequency bias is
> >added). If that is clean, suspect there is poor contact between the
> >head and the tape. The head-tape contact is far more critical on record
> >than on playback. If the bias is varying, that can cause all sorts of
> >problems but that is much less likely to be happening. Frequency
> >modulation of the sound by tape head squeal is another possibility,
> >especially with new tape that might have a manufacturing defect, or very
> >old tape that has begun to deteriorate.
> >
> >Dirty contacts in the record/playback switch can manifest themselves as
> >variations in signal level and distortion and are more likley to occur
> >in a machine that hasn't been used for recording for a long period..
> >....But, the head-tape contact should be your first suspect.
> >
> >My previous remarks about distortion being much easier to detect by
> >listening, rather than with an oscilloscope, still apply. If you have a
> >sinewave signal generator, that will make it much easier to hear
> >distortion than trying to spot subtle changes by listening to programme
> >material.
>
> Thinking about your very valid suggestion about using the sound rather
> than looking at a waveform, I'm guessing a crystal earpiece would work
> just as well rather than making up a hi-z probe+ amp would it not?
You really need something that gives good sound quality so that you can
hear the difference when the distortion occurs. My experience of most
high-sensitivity earpieces siggests that they could give more distortion
on undistorted signals than the distortion you are listening for - so
you may not hear much difference.
If your distortion is really gross and isn't likely to be caused by the
loading of the amplifier by the earpiece itself, you might be able to
hear it and track it down by that method. Although crystal earpieces
may read open-circuit on a DC test, they have a fair bit of capacitance
and long leads, which could upset the circuit you are trying to monitor.
You haven't really described the distortion but if it sounds like harsh
bursts of sound on loud notes with almost silence in between, that is
usually an indication of a weak or failed bias oscillator. Overloading
the recording gives very sudden onset of bad distortion on loud peaks,
so watch your recording level meter closely (assuming it is correctly
calibrated).
[OT]
Overloading would have much worse consequences if you were disc
recording; it may cause groove inter-cutting, which renders the disc
useless, or it may set fire to the cutterhead which will cost a few
weeks wages to repair (if the nitrate swarf doesn't catch fire too). As
most professional tape recording engineers either began their careers
with disc cutting or worked with disc-cutting engineers, watching the
meter becomes second nature. Turning down the volume control as soon as
the recording has finished is another useful habit, to prevent loud
studio noises from blowing up the cutterhead when not actually
recording.
--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk
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