Echoplex Runaway Sustain Oscillation Problem

[chinrest]

Greetings,

I've been cleaning up an EP-3 (Harris Teller model), and everything seems to be in tip-top shape, except the echoes will not ramp up into "runaway" oscillation. After studying the schematic and checking voltages, I still can't figure out what would prevent runaway, if the circuit and transport are operating as expected. I have given the unit a thourough cleaning, alignment and demag.

It seems to me that if the signal is coming off the playback head with a strong signal (it is), and the Sustain pot is functioning (500k to ground), then it should runaway. Many people will suggest upgrading the tape and rebiasing, but I don't see how a "better" tape would help. I'm clearly lacking something in my understanding of this.

Can anyone help shed some light on this?

[g1]

Not so much a "better" tape, but one that is guaranteed up to snuff. As far as I know there are no tapes of new construction available. As tapes age they lose signal strength. Without proper signal strength I don't think you can get runaway.

[52 Bill]

Not so much a "better" tape, but one that is guaranteed up to snuff. As far as I know there are no tapes of new construction available. As tapes age they lose signal strength. Without proper signal strength I don't think you can get runaway.

Yes, signal strength is very important, so check everything to be certain that you are recording a strong a signal as possible without distortion and that the feedback circuitry is all correct. That includes the SOS switch, make sure that the contacts are clean and have low resistance.

[Enzo]

Agree the tape is suspect. Not about some superior type of tape, but the old tape wears away the oxide, and over time it also loses some of its ability to store magnetic information. That is a lot of words to mean it gets weak.

[Kazooman]

Best modern tapes?

Along these lines can anyone suggest the best modern tape to use with the tape echoes from the 60's. I currently have four Echolettes and one Dynacord. The latter came to me in terrible condition, but, ironically is the one that currently performs the best.

I am home brewing my own tape loops, but I do not know what is the best tape for the job. I have a reel of tape that came with one of my Echolettes, but it is pretty fragile. I can make loops, but the are prone to breaking. I have another reel of tape (can't recall the pedigree right now, but I can dig it out of the stuff if need be. I bought it as the best equivilent I could find). The old one is oxide coat to the heads and the modern version is the opposite. At any rate, they both work, but I would like to eliminate at least one more variable from the crazy equations of getting a 50 year old tape echo to sound like new again. BTW, the Dynacord Echocord had been repainted bright red. I fixed that and it is currently the best sounding echo inmy collection.

[chinrest]

This circuit has a variable capacitor as part of the bias trap. I've read that adjustment of this cap has an effect on the feedback of the sustain circuit. Any thoughts? Of course I will twiddle the trimmer, but I always welcome a little theory whilst I'm twiddling.

[g1]

Forum member Regis (where you been?) has a nice echoplex page with that kind of info here:
Regis's Echoplex Page

[52 Bill]

This circuit has a variable capacitor as part of the bias trap. I've read that adjustment of this cap has an effect on the feedback of the sustain circuit. Any thoughts? Of course I will twiddle the trimmer, but I always welcome a little theory whilst I'm twiddling.

Twiddle away, it may help if it is mis-adjusted.

There are so many things that you should be checking, and we have no way to know what you have done, so here's a list.

1-Replace the tape
2-Clean tape heads
3-Demagnetize the heads and the transport path
4-Align the heads
5-Adjust the record level

Download the service manual and follow the service instructions if you have the equipment to do so.

[km6xz]

Tape systems require a balancing act between the flux density that the tape formulation can retain, and bias level plus mechanical alignment between the record head and repro heads, plus the repro sensitivity and eq. Oh, yeah the tuning of the bias trap. Head alignment for height i not so important on those because they are full track. Azimuth and tilt are important contributors to optimum recorded level and repro recovery level. A complication is that the tape that deck was designed for has not been made for decades and newer tape stock tended to be high bias type with higher flux density record level that accounted for the better signal to noise ratio of newer tape. Loop tape is different than reel tape, since it has lubricant on the back instead if impregnated into the binder of the oxide layer which always is the head contact side. A good source can be a local old radio station that used carts for spots, sounders, themes. If they have not cleared out the closets they probably had lots of old carts that can yield enough tape to make a number of loops.
The bias level has a very big impact on recoverable signal on the tape. As you adjust it and monitoring off the closest repro head to the record head, you can see that the signal peaks then abruptly falls off. The peak is the max signal but not the best sound, often has more distortion so adjust just past the peak. That adjustment is done after mechanical head alignment. The adjust the record level for the maximum undistorted repro level, again, maximum is not the best sound but a bit lower in level is. Maximum record level is based on at what level odd order harmonic distortion reaches 3-5%. Then adjust the repro gain so the output level is the same as you put in with your 1khz signal generator. Mechanical head adjustment should be done at 1khz for rough and some higher frequency for fine adjustment. Normally it is 10khz but that unit is not intended to reproduce 10khz because most of the signal content at 10khz would be noise.