On Mon, 12 Jan 2026 10:34:12 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
Cursitor Doom <cd@notformail.com> 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?
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.
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
wouldn't normally be used for music. 3+3/4 ips is the bare minimum for
domestic music and 7+1/2 ips is the minimum for professional recording.
I recorded a few folk music items at 7+1/2 ips for local radio and they
were happy enough with that - but for orchestral music nothing less than
15 ips was considered good enough for mainstream radio.
The speed variation could be caused by dirt on the surface of the
capstan or pinch wheel, or their bearings could be getting a bit tight
(unlikely for the capstan). The pinch wheel or the tyre of the flywheel
could be a bit dirty or oily - or it could have an indentation where the
machine has been left 'in gear' with the motor not running for long
period of storage. Other casues include dragging friction clutches,
unevenly-wound spools and binding in the guides.
The frequency of the speed variations and their 'waveform' can often
give a clue to their cause. Does the speed vary smoothly and
sinusoidally (unven pinch wheel) or does it 'hiccup' slightly (dirt or
indentations)? Watch the rotating components and try to work out which
one's rotation corresponds to the speed of variation.
--
~ Liz Tuddenham ~
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