EP-2 Echoplex Thumping From Playback Head???

[goldstache]

I recapped and serviced my early serial EP-2.  Was sounding great for a couple years, then on a gig the other day a LOUD oscillating thump appeared on the wet mix.  It's a lowish frequency thump.  I removed the playback head signal to the grid of V1A and grounded the grid.  No Oscillation.  But no delay obviously.  So I attached a 100k to grid to ground and the oscillation was not present in the V1's line noise. 


So do I have a bad Playback head.  It's not measuring resistance either.  Open???????


Anybody have any thoughts on this?  Also, as compared to the published schematic below, my unit is wildly different.  Especially in the RC filters and voltage department. 
I rebuilt it with the values that were in there that were original.  Wondering if there are their are other things to consider. 


Thanks in advance.

[sluckey]

Playback head is just a coil of wire. Should have some measurable resistance. Compare to the resistance of the record head.

[goldstache]

Thanks for the reply sluckey!
Over load when measured. Would an open playback cause it to oscillate? 
Should I order one ya think?

[sluckey]

Don't know. Probably depends on the circuitry. But for sure, an open head ain't gonna play anything. Did you measure the record head?

[goldstache]

Apologies, just got back at it today.  I seemed to get a faulty reading across the playback head first time round (out of circuit).  Measured it today and it is reading around 400ohms. I would think that means it's still operational.  This makes sense as I was probing upon initially opening it up and the worked, then it started to work as intended.  Then it just stops passing audio through the wet return mixer and oscillates at a low frequency.  (Thumps). No wet audio with this oscillation. I can manipulate the grid where the playback head is located, with a probe, and the wobble seems to slow down.  Almost like a lead dress issue.  The cable to the playback head is shielded up to about an inch away from the grid and the shield is connected to the chassis ground that shares the cathode decoupler for that stage.  All associated passive and filters are replaced. 


Any ideas?

[goldstache]

sorry here is the attachment for the schematic, though my part values/voltages are wildly different. Operating correctly, just differences in bias points. 

[goldstache]

Resolved:


The lead going to the grid of the V1a was stressed and had a intermittent connection in its insulator.  Stripped it back and re-terminated.  Seems to be going pretty good.
It must have just been the grid floating without the coil as a source load and was oscillating?????


Anyhoo, THANKS.......THAnks.........thanks........than.......tha................! DELAY

[goldstache]

Spoke too soon.


Same issue with oscillation persists. So I unsoldered the playback head and have been gator clipping it into the circuit.  I noticed that when the playback head was still in circuit,  soldered up and I measured, I would get an overload (OL) reading when it was exhibiting failure mode. When it is working the playback head reads 400 ohms.  Which is what it calls for in the schematic.


Once I was gator clipped.  I stuck the DMM across the leads of the Playback head to monitor Resistance.
Sure enough, as I play it starts creeping up after a couple minutes.  When it gets to around 750 ohms it just gets swamped and oscillates.
If you leave it in circuit eventually it's creep back down and biases in and there's the delay again. 


So what causes that to happen???  Would a degauss help, can's see magnetism reflecting as resistance (Impedance) but I don't have a clue. 


You can also reset this "scaling" DC resistance by lifting the ground to the playback head and reattaching it to ground. 


What gives?  Any tips?  Worked on a bunch of these and have never seen this Phenomenon.


Joe

[Ritchie200]

Joe,

I think it is S2, the double pole double throw switch - is it making good contact in each position?  That is your only path to ground on the "bottom" side of the head. If that switch is mildly intermittent could it be picking up some of the other head's resistance? Maybe jumper that out and see what you get.

Jim

[goldstache]

Thanks, I ended up replacing the playback head.  New one measures 750 ohms.  There was a noticeable output difference from the stock one measuring 400 ohms (when it would give a stable reading.
Tweaked the bias resistors and mixers to allow a good mix.


Resolved, I guess.