From: cd@notformail.com   
      
   On Sun, 11 Jan 2026 17:50:44 +0000, liz@poppyrecords.invalid.invalid   
   (Liz Tuddenham) wrote:   
      
   >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.   
      
   Good point; I had kind of forgotten about the sound quality of those   
   things over the years since I last used one (or should I say decades).   
      
   >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.   
      
   Yeah, I'm going to forget that approach altogether. Shame, but you're   
   quite right of course.   
      
   >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).   
      
   No, it's not coming in bursts. As I said in my original post, it   
   really does sound for all the world like an old 45 vinyl record that's   
   been played with a worn stylus. Then a few minutes later it might be   
   fine again - for a while.   
   I made up a test tape of a 5khz sine wave using my Ferrograph deck and   
   played it on the Uher. I could hear the distinct difference in quality   
   cutting in and out periodically when I played it back. I then tried to   
   make another test tape of the same tone on the Uher to play back on   
   the F'graph, but didn't have sufficiently low source impedance from   
   the generator in this instance so I had to use a radio broadcast   
   instead. Not ideal, but the distortion is still readily noticeable on   
   high quality audio recordings. Unfortunately, however, even on the   
   lowest speed of 3.5ips (IIRC) to was still whizzing past way too fast   
   to be of any use. I'd recorded it using the second lowest of the   
   Uher's four speeds and there's no point trying again on the lowest   
   15/16ips as of course the slow speed alone will introduce distortion   
   sufficient enough to render the test of no value. So all in all, not a   
   very productive afternoon!   
      
   >[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   
      
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