Echoplex Help

[jbgron]

Hi guys,

I'm a mad Echoplex collector, I have 16 of them.  I know, I'm insane   I've been able to fix any problem with them so far except my latest EP-3 which I just cannot get to do runaway oscillations.  The echo sounds great but its really weak. 

I've performed the following surgery so far;

* Demagnetized and cleaned tape heads.
* Installed new drive belt
* Replaced filter capacitors and zener diode
* Realigned heads using oscilloscope as per the service manual
* Adjusted echo sustain trimcap
* Adjusted bias oscillator trimpot
* Checked voltages of transistors and other components, all OK.

This would usually get an Echoplex working perfectly but nothing I've done so far has made any major difference.  I'm at the point now where I'm thinking it could be an issue with the tape heads even though they are the cleanest looking heads in my collection.  So after all that my question is, how can I test the tape heads to make sure they're up to spec?

Thanks in advance for any help, tearing my hair out!

jbgron

[petemoore]

So after all that my question is, how can I test the tape heads to make sure they're up to spec?
  I'd audio probe a good one, to see what 'in spec' sounds like.
  Then subject the 'off one' to the same test, comparing the sound.
  Having a working one there is nice, you can compare-test [most anything] to a non-function-er.
  Sound like a loose wire or bad feedback pot or something, that it works but doesn't do the infinite thing.

[wavley]

I'm a Space Echo guy, but I recently had to replace the cap in my bias oscillator circuit and change the trim pot to make it so I could use a wider range of tape formulations because some of them would not feed back.

Being that you own so many machines I'm sure you've already checked, but I'm gonna ask anyway because I miss obvious things all the time... How's the tape in that machine?

[~arph]

You can also measure the resistance of the heads and compare them with one of your other units. Tape heads are inductors after all.

[jbgron]

Thanks for all the responses guys.  I'm going to try every suggestion today.

I'm a Space Echo guy, but I recently had to replace the cap in my bias oscillator circuit and change the trim pot to make it so I could use a wider range of tape formulations because some of them would not feed back.

Being that you own so many machines I'm sure you've already checked, but I'm gonna ask anyway because I miss obvious things all the time... How's the tape in that machine?

Space Echos are a great machine, I have a few of those too.  That sounds promising, there is one ceramic cap on the bias oscillator circuit that could be suspect.  The tape is fine, I wind my own cartridges from a variety of different tapes, none have made any significant difference.

So after all that my question is, how can I test the tape heads to make sure they're up to spec?
  I'd audio probe a good one, to see what 'in spec' sounds like.
  Then subject the 'off one' to the same test, comparing the sound.
  Having a working one there is nice, you can compare-test [most anything] to a non-function-er.
  Sound like a loose wire or bad feedback pot or something, that it works but doesn't do the infinite thing.

Thanks, didn't think of audio probing the Echoplex, I do it all the time with my stompboxes.

You can also measure the resistance of the heads and compare them with one of your other units. Tape heads are inductors after all.

Any idea how I do this?  Do I just put the probes on either side of the head or where they connect to the board? Does the machine need to be on?

Thanks guys, I'm not out of options yet.