CopiCat Tape Echo - Oscillation

[guitarrock04]

Seeking help to resolve an oscillation issue

Background: I have a Guild CopiCat tube tape echo, made by WEM. I re-capped it and the unit was functional, but two of three read heads were inoperative. I recently replaced all heads with new from Amp-Fix. Now, all heads are functional and the unit functions perfectly for a few moments (Anywhere from 30-120 seconds) until a loud, tonal oscillation occurs. Time seems shorter when the unit is switched off, then on again, leading me to suspect a thermally driven concern..

I recognize that the circuit has a built in oscillator, but I believe it is designed to be outside of audible range. Because the noise corresponds to head replacement, wondering if there's something peculiar about a different characteristic of the heads or something stupid i've done with lead dress that could result. Though, correlation does not necessarily represent causation..

I've swapped the 12AX7's thinking microphonics may be a concern - no change. The noise is present with or without the motor on, anytime Swell (VR4) is up. The noise goes away if the footswitch is bypassed or all of the play head switches are unlatched and reappears if any is latched. Would a grid stopper on V3B be of use?

 

[JohnRoberts]

look for a positive feedback path

JR

 

[CJ]

Did you change the tape? There might be something on the tape that was previously recorded, when you put on the new heads, the alignment might have been off such that the noise is not getting erased by the erase head. I hope you fix it, that is the best tape echo ever made.

 

[abbey road d enfer]

The noise is present with or without the motor on, anytime Swell (VR4) is up. The noise goes away if the footswitch is bypassed or all of the play head switches are unlatched and reappears if any is latched. Would a grid stopper on V3B be of use?

VR4 is the regeneration control, which can easily result in oscillation if the loop gain is too high. First I would suspect the new heads have a higher output level. Have you noticed an increase of the echoes level? Maybe the solution is to tame the playback gain.
Does the oscillation level change with the number of heads engaged? Do teh A, B and C registers affect the oscillation?

 

[guitarrock04]

Did you change the tape?

Yes, but based on motor on/off not affecting the issue, I had more or less ruled this out.

Have you noticed an increase of the echoes level?

Tough to say without a back to back, but at least not appreciably - from memory

Does the oscillation level change with the number of heads engaged?

No noticeable change in freq or magnitude with one or all of the play head switches engaged.
I could try reducing the value of R15 (perhaps closer to R17) to reduce gain in V3a?

 

[abbey road d enfer]

I could try reducing the value of R15 (perhaps closer to R17) to reduce gain in V3a?

That would be a first step to a solution.

 

[guitarrock04]

I swapped out V3 for a 12at7. Figured that was a less permanent means to trial reducing gain. And to no avail. The screech still appears

 

[abbey road d enfer]

I swapped out V3 for a 12at7. Figured that was a less permanent means to trial reducing gain. And to no avail. The screech still appears

Are you positive the condition for oscillation to appear is having one or more head engaged and VR4 in any position except fully CCW?
Or does it happen only with VR4 fully cranked up?

 

[guitarrock04]

It occurs with vr4 in any position. The knob does control the volume, with the tone low but still audible in the fully ccw position

 

[industrialarts]

It's possible the problem existed before the head replacement but without all the heads didn't exhibit. Also, V4 is echo volume (swell), V3 is regeneration (sustain).

Did you replace all the caps? Did you replace the filter caps with the same value of capacitance? C17 is a possible issue - also check for good grounds everywhere. You say 'screech' - can you give an approx freq of the sound?

 

[Tubetec]

You could try replacing the unsheilded cable from the sustain and swell pots back to the input stage , On my MKII I was able to cut out a lot of induced noise this way .

I never met a Copicat that wasnt noisey mainly 50/60hz drone , it tends to osscilate at a low frequency .

These are a quirky unit at the best of times , proximity of one high z unscreened wire to another could mess everything up .

 

[Tubetec]

Definately check the all the higher value resistors around the mixer stage . R26 and R29 especially

 

[guitarrock04]

So I've made a change - Let's call that progress. Thanks all for the input thus far..

I found a few things:
-My hardware doesn't match the schematic. Not surprised.. This actually more closely matches my circuit, FWIW:
http://wem-owners.com/wp-content/uploads/2015/10/Copicat-Schematic-iss5.png-I found R29 to be damaged. One of the leads separated during desolder
-C12 (new linked schematic, only) [.0047uf] had a crack in the body

After replacing these components and replacing leads for VR3 and the pushbutton array with shielded cable, the oscillation still comes on after a bit, but seems lesser in magnitude. The most significant difference noted is that it ceases when the tape is stopped. The tone is on the order of 3.5 to 4 khz, best guess.
I suspect there were a few things amiss, but wondering now if there isn't something going on with the oscillator. If it's creating a harmonic that is lower than intended, such that the low pass filter isn't cutting it out. Thoughts?

 

[guitarrock04]

Turns out, it was a tube.
I clipped a lead on c7 and echo was all but nonexistent, but it didn't oscillate.
Since the 6br8 was about the only thing I hadn't replaced, i gave it a shot. A NOS replacement has this thing back makin' music.
Not sure if the triode portion was microphonic or some interelectrode characteristic was out of whack, but the Brimar tube was likely original to the unit.