Revox B77 bypassing erase head

[Ali M]

Hi guys,

I have set up my B77mkII with tape loop and have physically bypassed the erase head by taping an allen-key to the front panel. So far it sounds wicked. Drones galore!
I have to remove the allen-key to erase the recording and then put it back which isn't very elegant, but doable.
To make a fully functioning tape delay would be being able to vary the amount of erasure to the erase head with a pot, and I would also be able to wipe the tape loop.

I would just try wiring up a pot and see where it got me. There are just a few things;
a) I can't find any information on this being done before
b) I don't fully understand what sort of signal's is being sent to the erasehead's. (I can read a manual but it's makes my brain melt)
c) My Revox is a beaut and I don't want to naff it up. They are tiny wires and it might not be a good idea to poke around a tape head with a antex soldering iron!
d) My idea might not even work...

I'm sure there are some helpful chaps on here that have a much greater knowlege of tape that could tell me the science of this...

Cheers
x

[JL]

Erase heads send a high-amplitude, high-frequency AC signal to the tape to more or less "reset" the information stored on the tape. Sometimes it shares the same oscillator as the record bias signal, in which case the record bias signal is just taken from the erase oscillator and attenuated before being mixed with the signal to be recorded and sent to the record head. The frequency of the signal is probably somewhere between 40khz and 150khz - it depends on the machine.

You may just be able to just splice in a potentiometer wired as a voltage divider across the erase head. By turning the pot down you could reduce the strength of the signal that gets to the erase head, in turn reducing the amount of erasure. But even with the pot set at max level, that might interfere with normal erasing if the potentiometer is always loading down the erase signal. Alternatively you could just wire up a switch to cut the signal to the erase head completely. Be aware that since this is a 2-track machine there are actually two (identical) signals being sent to the head you have to deal with.

I would advise against messing with the wiring around the tape head itself. You don't want to mess up the head alignment (which will happen if you take it off) and heads are expensive to replace if something goes really wrong. Trace back to where the erase head wires go, and see if you can find a clever way to wire up the pot/switch there. While you've got the thing open, you may as well replace those PS filter capacitors. IIRC they are notorious for going bad in those machines.

I'm just thinking out loud here, so please take this post with a grain of salt! Of course, check out the schematics first and don't do anything if you aren't 100% comfortable with it.

[Ali M]

Thanks for the reply mate. Apparently adding a pot to the erase head wouldn't work.
I'm going to wire a switch I think. Someone on another forum suggested putting things in the way of the erase head, like layers of tape.

I would prefer a less mechanical solution. But beggers can't be choosers

Thanks for your help