Echoplex EP3 - Runaway with no input

[andrewnelles]

Hi All,

I'm currently restoring an Echoplex EP3. It sounded pretty rough when I got it. High noise floor, some general reliability issues. So far I've replaced all the electrolytics which cleared up the noise, put in a fresh tape cartridge, demagnatized the heads, cleaned and lubed all pots, replaced the pinch roller, put on a fresh belt, cleaned the motor, leveled the motor, cleaned all surfaces, and straightened the rails the moving head travels on.

It's running almost perfectly...

Right now, even with no input, it runs away when the Sustain knob hits about 7. Just goes from dead silent to runaway with nasty static noise. With a guitar signal it's got the same problem, and it seems to run away with this static sound earlier than it should, it's hard to get lengthy decay on the guitar. Just goes from medium decay of guitar sound to runaway with static.

Sounds fine with the knob below 6, which is where I use it most.

Any thoughts on what to troubleshoot next to combat this? Or is this normal, and I'm simply just not remembering how these normally behave....

Thanks!

[52 Bill]

Welcome to the place.

Right now, even with no input, it runs away when the Sustain knob hits about 7. Just goes from dead silent to runaway with nasty static noise. With a guitar signal it's got the same problem, and it seems to run away with this static sound earlier than it should, it's hard to get lengthy decay on the guitar. Just goes from medium decay of guitar sound to runaway with static.

Sounds fine with the knob below 6, which is where I use it most.

Any thoughts on what to troubleshoot next to combat this? Or is this normal, and I'm simply just not remembering how these normally behave....

Thanks!

The runaway with no input is just the tape noise being regenerated. Have you tried turning down the record level control?

If you stop the tape and turn up the repeat control, does the noise floor increase at around 7? If it does, then try checking the tape playback circuit for noisy components.

[Enzo]

Also wouldn't hurt to adjust the bias and bias trap.

[andrewnelles]

Thanks for the incredibly fast replies!

Not noticing any significant increase in the noise floor above 6 or 7.

A head alignment and biasing is next on my to do list. Time to power up the signal generator and oscilloscope I suppose.

I've been reading over the service manual. The physical head alignment, and the role of the Bias/SOS trimmers on the boards make sense to me. However, I'm not seeing how the variable capacitor in the bias circuit comes into play. What should I be doing with that?

[52 Bill]

The variable cap is the bias trap. I think that there are instructions on how to adjust it.

[Enzo]

You may find some useful information here:

Regis's Echoplex Page