Modding a Schober Reverbatape (tape delay from an organ)

[Pmelius]

Hey folks,

I have this unit, which I've already done some basic things to:
Changed jacks to 1/4"
Made in/out volume trim-pots regular pots
Converted weird 7 pin "control" plug to 3 pots instead (was missing the triple gang potentiometer)
Light dimmer for motor speed control

What I would like to do is modify the "erase" head circuitry to be able to adjust the amount that the erase head is erasing. That way I can go from the stock amount (basically one repeat with the signal 50% erased from the tape) to totally turning off the erase head and just looping signal forever.

The only things I've tried are:
Wiring a pot as a variable resistor in series with the erase head
Wiring a pot as a voltage divider and sending the voltage at the erase head to ground
Tried this with a 500k pot and a 10k pot

Neither thing really worked. I could get it to make the erase head turn off and loop forever, but the signal got distorted in an unpleasant way. The board is pretty old and fragile, and I don't wanna swap parts out unless I have some certainty that it might work. Worst case scenario, I'll just wire in a switch to disable the erase head, but I would rather have a range of "delay time" if possible!

While I'm tempted to swap out r35 or r42 for a pot, I'm worried it will affect the record head, as that whole bias oscillator section seems to work in tandem. I was looking at other tape delay schematics hoping to find a clue, but didn't really come across anything like this. Maybe it's impossible haha

This schematic doesn't perfectly match my unit, but it's very close. The erase head is represented by points E, E (top right)


Any help is much appreciated 

[anotherjim]

Is the Erase Head wired across E-E and Record Head R-R1 in that schematic?
It looks like the bias oscillator (Transistor 39) is both the erase signal and record bias signal and you can expect it will still erase to some extent with the Erase head disconnected. What might be causing distortion is that removing the load of the Erase head winding is changing the output level into the Record head. So maybe some resistance needs to be added between E-E when the Erase head is out of the circuit?
Perhaps it's possible that the tape transport will still work ok if you temporarily take the erase head out of the way - just to prove it it has to see the head in the circuit to get the right conditions for the recording head.

[duck_arse]

I dunno nuffink baht tape delays, but .....

if you have:

>>> play1 >>> play2 >>> play3 >>> erase >>> record >>>

and you want some of the "play" signal to bypass/get around the erase head, isn't that the thing you would do? as in, mix the signal from the play head/s via a volume/level pot/mixer into the "record" signal, so that you allow the erase to do its job cleaning the tape, but you have as much or as little of your delayed info being recorded again, as it were. as we do in the modern "repeats" controls on PT2399's for example.

[Pmelius]

and you want some of the "play" signal to bypass/get around the erase head, isn't that the thing you would do? as in, mix the signal from the play head/s via a volume/level pot/mixer into the "record" signal, so that you allow the erase to do its job cleaning the tape, but you have as much or as little of your delayed info being recorded again, as it were. as we do in the modern "repeats" controls on PT2399's for example.

Oh I hadn't thought about this at all. This makes way more sense and would also allow it to "self-oscillate" and go into crazy feedback sounds whereas my idea would only let me loop over the same areas of tape repeatedly. I'm gonna stare at the schematic for a while and think about it

Is the Erase Head wired across E-E and Record Head R-R1 in that schematic?
It looks like the bias oscillator (Transistor 39) is both the erase signal and record bias signal and you can expect it will still erase to some extent with the Erase head disconnected. What might be causing distortion is that removing the load of the Erase head winding is changing the output level into the Record head. So maybe some resistance needs to be added between E-E when the Erase head is out of the circuit?
Perhaps it's possible that the tape transport will still work ok if you temporarily take the erase head out of the way - just to prove it it has to see the head in the circuit to get the right conditions for the recording head.

This is basically what I assumed was happening but couldn't put into words. And yes, you're right about the R-R1 being the record head. I did some reading on watkins copicats that told me if I want to put the erase head on a switch, I would have to replace it with an indutor of similar value, so if I do go that route, it seems I wouldn't necessarily be able to make it adjustable.

[anotherjim]

You might get away with pure resistance. Measure the DC resistance of Erase coil and try nearest fixed resistor in series with x10 or higher pot. The idea is to find something close to the impedance of the coil at the bias frequency.
If the DC resistance of that head is about 30ohm, you might find an old earbud speaker that looks close enough to the circuit and maybe find a pot value that works to crossfade between it and the head.

Stephen (DuckArse) is maybe making the point that tape echo is a multi-head system with a separate record head and one or more play heads. It naturally should do sound-on-sound by allowing recording of a mix of new and old playback material even though its always erasing each time around. The playback is always being re-recorded so will diminish in quality each time around. But disabling the Erase and only having new material in the Record will produce a different effect in the way the old recordings sound.

[Pmelius]

you might find an old earbud speaker that looks close enough to the circuit and maybe find a pot value that works to crossfade between it and the head.

This is definitely an interesting idea that I'm willing to play around with.

Stephen (DuckArse) is maybe making the point that tape echo is a multi-head system with a separate record head and one or more play heads. It naturally should do sound-on-sound by allowing recording of a mix of new and old playback material even though its always erasing each time around. The playback is always being re-recorded so will diminish in quality each time around. But disabling the Erase and only having new material in the Record will produce a different effect in the way the old recordings sound.

But I think this idea might be more suited to what I'm expecting to hear from the result. I just didn't know previously how that was implemented in other echos. I'm hoping I can just do it passively like I've thrown together in this schematic. Should this work? Or would this give me problems?

[Pmelius]

Ended up realizing there is already a fixed feedback control (r105, 220k). Changed that to a 47k with a 250k pot in series. Feedback galore.