From: liz@poppyrecords.invalid.invalid   
      
   Cursitor Doom  wrote:   
      
   > On Tue, 13 Jan 2026 09:46:35 +0000, liz@poppyrecords.invalid.invalid   
   > (Liz Tuddenham) wrote:   
   >   
   > >Cursitor Doom  wrote:   
   > >   
   > >> On Mon, 12 Jan 2026 10:34:12 +0000, liz@poppyrecords.invalid.invalid   
   > >> (Liz Tuddenham) wrote:   
   > >>   
   > >> >Cursitor Doom  wrote:   
   > >> >   
   > >> >[...]   
   > >> >> 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.   
   > >> >   
   > >> >That distortion is a combination of intermodulation and   
   > >> >noise-behind-signal.  Really severe tape squeal is the only thing I can   
   > >> >think of that sounds like that on a tape recording   
   > >> >   
   > >> >   
   > >> >> 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.   
   > >> >   
   > >> >Have I understood this correctly?   
   > >> >   
   > >> >1)   A tape recorded on the Ferrograph shows intermittent distortion   
   > >> >when it is played back on the Uher.   
   > >> >   
   > >> >2)  A tape recorded on the Uher shows similar intermittent distortion   
   > >> >when played back on the Ferrograph.   
   > >> >   
   > >> >3)  The distortion occurs at the same place each time the tape is played   
   > >> >back and isn't related to signal level.   
   > >> >   
   > >> >   
   > >> >By elimination: 3) rules out any problems in either playback amplifier,   
   > >> >2 ) shows the problem isn't in the Ferrograph recording amplifier and 1)   
   > >> >shows the problem isn't in the Uher recording amplifier.  The only   
   > >> >things left are the tape itself or one of the transports.   
   > >> >   
   > >> >Years ago there were problems when one or two tape manufacturers   
   > >> >produced tape with a backing that caused squeal in pressure-pad machines   
   > >> >(which both the Uher and the Ferrograph are) but you would be able to   
   > >> >hear that by turning down the sound and listening to the tape going past   
   > >> >the heads.   
   > >> >   
   > >> >   
   > >> >I'm surprised the signal generator didn't have low enough source   
   > >> >impedance to feed a signal into the Uher; most audio signal generators   
   > >> >have an output impedance around 600 ohms and most Uhers have inputs of   
   > >> >around 600 ohms and 'high' (around 47k).   
   > >> >   
   > >> >Why were you using 5Kc/s as your test tone?  The second harmonic of   
   > >> >that, which is the first spurious frequency created by distortion,   is   
   > >> >10 Kc/s which you may not be able to hear and which won't be recorded at   
   > >> >any tape speed below 7.5 ips.  (Intermodulation distortion would be   
   > >> >audible as spurious low frequencies if there were other tones present,   
   > >> >but you wouldn't get them from a single sinewave tone.)   I would have   
   > >> >chosen something around the middle of the audio band  e.g. 630c/s  so   
   > >> >the harmonics fell in the area of the greatest hearing sensitivity.   
   > >>   
   > >> Thank you for your observations. I had hoped to devote some time to   
   > >> this issue today, but other events kind of took over, so the next slot   
   > >> will be Weds afternoon.   
   > >> It's a real nuisance not being able to record the test tone on the   
   > >> Uher. I'm surprised as well. In fact I'm wondering if there's   
   > >> something else going on there.   
   > >   
   > >Have you got the correct connections to the input socket?   
   >   
   > Yes, but there's an adaptor inline that could be the cause of it. I'll   
   > investigate that tomorrow. One of the really old Chinese things: prime   
   > suspect. Chinese stuff continues to improve, but god knows it was   
   > *crap* 20 years ago and sadly some of it still is (but far less   
   > prevalent).   
   >   
   > >> I'm going to follow up on your suggestion of cobbling together a hi-z   
   > >> amp for probing stage-by-stage. I think I've got some FET front-ended   
   > >> audio op amps somewhere in my parts stash which should be ideal for   
   > >> this purpose. Can't recall what I originally bought them for, but   
   > >> whatever it was never got finished, clearly!   
   >   
   > >Easier than that:  connect a piece of screened cable to the input of an   
   > >amplifier with reasonable gain and make a probe from a 1 megohm resistor   
   > >in series with a 10 nf capacitor at the free end.  For an earth   
   > >connection you can use a crocodile clip, either on the end of a long   
   > >piece of wire coming from the amplifier or on a shorter piece connected   
   > >to the screen of the cable.   
   >   
   > Delightfully old school. I like it! What you're describing sounds   
   > rather like an oscilloscope probe - except with much higher   
   > capacitance. Shame as a scope probe is very ergonomic and ideal for   
   > this kind of poking around.   
      
   The capacitance I am suggesting is in series with the signal to stop any   
   DC from getting into the test amplifier.  The capacitance quoted for   
   oscilloscopes and probes is the shunt capacitance from signal to earth.   
   You could use an oscillocope probe if you have one with a suitable plug,   
   but interpose a 10nf capacitor between the probe tip and the point in   
   the circuit you are measuring, so as to block any DC.   
      
   If you want an ergonomic probe, put the components into a ball-point pen   
   case (if you can find a small enough capacitor).  ...or just put the   
   resistor in the pen case and have the capacitor in the plug at the   
   amplifier end of the cable.   
      
   I have a home-built test amplifier screwed to the underside of the shelf   
   above the workshop bench, with a veriety of input connectors and probes.   
   It has two attenuators calibrated in 10 and 1 dB steps, with a meter   
   that reads directly in dB or can be switched with an electronic zero to   
   read variations of 0.1 dB.   There is an output at 0dBm level for   
   feeding external equipment, such as high-quality headphones or an   
   oscilloscope, and it has a small loudspeaker, which is often more use   
   than the oscilloscope for identifying the cause of fault.   
      
      
   > >> I did notice that for some reason on the slowest speed (the 15/16"ips)   
   > >> there's considerable variation in the tonal reproduction with music   
   > >> off the radio, as if the speed is wandering during transport. Not   
   > >> noticed any such effect at the higher speeds so not sure what that's   
   > >> all about. Fortunately, it's only music where it's noticeable; speech   
   > >> just sounds normal and at that speed, it's really only speech that one   
   > >> would use it for anyway.   
   > >   
   > >Speed variations are much more obvious at slower tape speeds, 15/16 ips   
      
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