From: cd@notformail.com   
      
   On Wed, 14 Jan 2026 11:30:29 +0000, liz@poppyrecords.invalid.invalid   
   (Liz Tuddenham) wrote:   
      
   >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'll get the probe I improvised re-jigged, but won't have time to do   
   any more to the Uher til Sunday, unfortunately.   
      
   >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.   
      
      
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