Selmer Model 200 Tape Echo Bias Frequency

[Ted]

Got a Selmer 200 tape echo from the early 60s here. The echo is good and strong, but it sounds sort of gritty - almost like it was being clipped.


I measured the bias oscillator at about 40kHz (there's tuning slug in the oscillator inductor which lets you vary it between 37 and 43).


It seems a bit low. I believe WEM Copicats run at between 75 and 100KHz which would be similar to domestic tape recorders of the era


http://www.vintagehofner.co.uk/selme...c/echo200.html

Any thoughts?

[Ted]

Right. Just got back to this.

I measured the inductor in the bias oscillator circuit (3.8mH) and with the cap (4.4nF) gives a bias frequency of 38KHz. The inductor is slug tunable between 2.8 and 4.2 mH.

Looking at the signal on the record head , it looks like the audio is impressed on a bias signal of about 50V pk-pk (You can peak the bias level by tweaking the oscillator slug) (The schematic rather unhelpfully says 30VAC at this point on the schematic - doesnt say whether its referring to bias or audio or both or whether its rms or peak) (30V rms would be 85V pk-pk by my reckoning. Which might explain something. Or might not.)

So: Looking at the signal on the play heads a sine wave comes back looking like it has the worst crossover distortion you've ever seen. Which apparently is a sign of inadequate or absent bias.
Tried disconnecting the bias altogether and the audio on the playback heads looks exactly the same.

Tried tape loops from different types of tape I had lying around. No change.

Anybody got any tape wisdom?

[Jazz P Bass]

You may be confusing the ac bias voltage with bias of a class ab amplifier.
It should not affect the playback signal. (other than cleaning it up)

"The characteristics of the recording system change quite markedly as the level of the bias current is changed. There is a level at which the system gives the minimal distortion (which is the highest bias). There is also a level at which the high-frequency response is at maximum (lowest bias). These conditions unfortunately do not occur at the same bias level."

The distortion that the above quote refers to is, as far as I know, not crossover distortion.

Note: attachment on bias is from Ampex.

[Ted]

No I'm not mistaking the two kinds of bias. It was just an observation that it looks a lot like crossover distortion. I guess they are both caused by a dead band at low signal levels, so its not surprising they look the same.

[g1]

(The schematic rather unhelpfully says 30VAC at this point on the schematic - doesnt say whether its referring to bias or audio or both or whether its rms or peak) (30V rms would be 85V pk-pk by my reckoning. Which might explain something. Or might not.)

Unless otherwise specified, AC volts refers to RMS. So I think you should have 85v p-p of bias voltage, as you guessed, and yes you could have a bias circuit problem.

[Mick Bailey]

Check the AC either side of the bias coupling cap to see if the signal is being attenuated. The last tape machine I worked on had got a faulty cap here, so was a cheap fix.

[Ted]

Check the AC either side of the bias coupling cap to see if the signal is being attenuated. The last tape machine I worked on had got a faulty cap here, so was a cheap fix.

Tried that quite early on. But no.

[Ted]

Take a look at the schematic. What on earth is going on in the cathode circuit of the oscillator?

There is apparently 6V DC across the erase coil. Why?
Also there is a 0.01 uF cap in parallel with a 0.0003 uF (i.e 300pF). The smaller cap is 3% of the larger one. Which I would imagine is probably well within the tolerance of a 1959 0.01uF. Why?

Is this some advanced circuit design techniques known only to a handful of savants in Selmer's Theobalds Road factory?

[Mick Bailey]

Some early tape echos used a permanent magnet to erase - no head, so I don't think the DC offset matters too much here. I don't have any theory other than maybe they saved on a capacitor......then added another with the redundant-looking parallel cap - who knows why?

Does this particular unit have any Hunts tubular caps? These are always suspect. If not, the caps are worth checking anyhow around the oscillator for leakage and ESR. It may also be worthwhile swapping the oscillator tube in case that's weakened.

[Ted]

Actually. I have a theory. Apparently Selmer bought a Copicat from Watkins with the intention of copying it for the Selmer 200. Charlie Watkins got wind of this and modified the one which went to Selmer. Hence the inexplicable components.