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**cjlectronics** · 01-19-2008, 02:53 PM

Years ago, I worked on an echoplex that was doing the same thing. I pulled my hair out trying to figure out why the signal just died when voltages and signals looked like they should. The problem turned out to be a simple one. The damn ECHO OFF jack is a NC switch and the contacts were just an RCH away from making contact.
Check the ECHO OFF jack and make sure the NC contact is closed or making good contact.

I've got 6 echoplexes that need fixing. They have been on the To Do list for over a year.

CJ

**Enzo** · 01-19-2008, 03:01 PM

Good call there.

And also, scope the record bias oscilator - got strong high levels of "RF?" Without the HF bias, things won't record to tape.

**imo** · 01-19-2008, 06:24 PM

I love the echoplex. Such a unique sound, even now with all of the digital modelers, like the B3, it is difficult to get the feel. I toured with one for years without a tech, and part of my daily routine was the cleaning, which is funny in retrospect.
I will check the echo off switch. I'm hoping it is there. I think the bias oscillator is working properly. Where is the best place to scope them>?
I have a pretty healthy wave, that seems pretty quick, have to check on the frequency..it is at the bias adjust, at the recording head, at the collecter of Q4.

Dumb question really quick, as i'm not experienced with tape heads. Assuming the head measures the correct resistance, does that mean it is working, or could it measure right and still be dead?

**Enzo** · 01-19-2008, 09:28 PM

If the head is not open, it is probably OK.

**imo** · 01-19-2008, 10:43 PM

here is the schematic. the hf appears to be about the same as the working unit. the echo off connection is solid. Is there another way to check if i am recording? getting playback? I'm wondering if there is something weird happening in the echo/sos switch, and if i could isolate the signal then i could tell where it is dissapearing.
When i connect the switch at point 1 i lose signal. If i dissconnect I have signal at that point as with the other. Im confused at how this section works. I am assuming that if you were to switch it to the echo side then the SOS black is tied to Echo white at wiper which connects to sos white and ground, and then wire 1 connects to the wiper of the echo sustain. does this sound right?
Ian

**fuzzman93** · 01-20-2008, 01:14 PM

broken Echoplex

Hi Imo,

I like very much Echoplexes too so i've got three of these.One green box(old tube modelEP1)and two EP3 .I've repaired al ot of amps or effects so maybe i could help you .

First plug your gen with 1KHz in the input jack.
Scope at 23,and at 1,the sound goes to the output and a fraction goes to the recording circuit.at this state do you have the correct output signal(the same level than your other working EP3)with the pot full left(No echo),This can be help you to be sure there's no short derivating a part of the signal since it arrive in the recording block.
Well,For the recording block disconnect the record head (don't forget to turn ff the EP3) andthe bias at 3,what happens on your scope ? Signal or no signal?
If yes reconnect the record head without the biais oscillator,what are you seeing on your scope ?
If yes reconnect the oscillatoor circuit then record a10 min of tape(be sure your on off Echo is functional(maybe the sleeve of the jack is earthed on the chassis,this could disable permanently the echo fonction).
Well take your tape off and try it on your other working echoplex,it's the best way to verify if there's a signal on the tape ?no?
If yes the problem is in the playback block .If no ,the record head could be broken or the erase/record head wire or position coulb inverted,don't laugh,if you reverse the wire or the sense of the head(rotate ), the erase head is just after the record head so ,the signal disappear of the tape!
Step by step .First took the the tape of your working EP3 on the broken machine and let it play with a jack in the playback jack control ,no sound at all?check the connection,the switch(for the echo or sound on sound ),put the tip of the probe directly on the echo head ,if there's no signal the head is broken.If yes put the tip on the first transistor of the playback preamplifier circuit and check for cold solder or bad capacitor,check too the voltage on Q1 and Q2,power supply at 7 and finally the pot and the 470pf Cap.

Try this and tell me if you find the problem,i hope this help you!
Best regards
Fuzzman

**imo** · 01-20-2008, 08:19 PM

Fuzzman,
thanks so much for the help. I just had a couple of minutes, family day(sunday) but i did check the scope at points 23 and 1. 23 the signal is quite healthy, and shows up so at the output of the unit. point 1 though seems to short the signal out somehow. I have tried changing the echo-sos switch(it seems that the 3 working echoplexes I have checked have a different wiring than the schematic shows) I have checked and checked as to where this signal could be going and i am so far lost.
I will check the record head, bias oscillator. When disconnecting the record head and the blue wire( oscillator) where were you saying to put the scope to see if there was still signal?
Incidentally after messing around with it, and plugging it in to an amp (it looked like i was getting a signal at point 1) i started getting a crazy oscillation with the echo sustain up that sounded like a moog synthesizer with a pitch around 440 hz and moving as i rotated the pot. Entertaining

**imo** · 01-20-2008, 09:03 PM

i found the oscillation. In the 20th time of switching the switch around, i reversed the leads so the output pot wiper was feeding into the circuit.
I was actually able to get the echo going a bit, though i still have lots of work to do. I think it was in the heads being misalligned. I now have signal on the echo side, though nothing on the SOS side other than a low rumble.
Definitely learning a lot. I need to read up on tape heads and how they work.
ian

**fuzzman93** · 01-21-2008, 12:47 AM

Echoplex revival�

Hi Imo,

The place where you could verify the signal is exactly where the wires are desolder.If the head is dead with ashort,this one could affect the signal even in the circuity.Take out the head wires first then check the oscillator signal.Maybe you'll have to disconnect the oscillator before the biais trap. By this way you should verify the oscillator signal and separatly the signal going in the record head.I've had read before in this thread that without an oscillator signal you couldn't have any record .It's false ,the oscillator signal was a system invented for having a stronger and a cleaner than without it.The oscillator signal is modulated by the audio signal this permit more signal on the tape and finaly avoid distotion.If your record signal is small and dirty this may be a dead oscillator(Transistor,supply or self)For your hum in SOS head ,two things bad earth connection or bad solder on the switching system/head solders . If not ,try to reverse the SOS head with the playback head if the hum stops,it is a dead head.If not you could try to scope at the switch(SOS head connected ) for finding the hum source.I stay at your service!Say to me if your echo comes at live ,i will be happy for you
Best regards
Bruno

**stokes** · 01-21-2008, 03:49 AM

Hate to sound overly simple here,but have you tried raising the record level pot?I dont see any mention of it.If I missed it,I appologize.

**imo** · 01-21-2008, 06:06 AM

not at all. I appreciate any pointers. I have done that. I actually have completely rebuilt, tested every component, many repeatedly, sat with a scope for hours, adjusted every pot, checked for any possible ground errors, compared both to my working echoplex and my friends, and basically cursed all things good as i tried to understand an echo circuit. It has been pretty maddening, as though i can troubleshoot typical amps, effects, etc, the combination of the tape heads, the oscillator, and the many errors on the schematic have made in a long, laborious process....but.
I actually have echo!!!! I played with the assembly that holds the heads and after bending it down some, i was able to get more signal from the tape. I still don't have a working SOS function but i am pretty happy with this to go on. Definitely know the unit a lot better.

If anyone want to volunteer any info on how the oscillation part of the circuit works, what components play into this, and how, i would humbly bow to your glorious knowledge.. I am jumping back into it either tonight or tomorrow. Will report back soon. Thanks again for all the helpful advice

**stokes** · 01-21-2008, 03:14 PM

http://music-electronics-forum.com/s...ead.php?t=5666 Heres a link to some basic recording theory.

**fuzzman93** · 01-21-2008, 11:12 PM

Oscillator works well

Hi,

If you erase and the erase head is OK ,the oscillator is working ,try to adjust the biais to have better signal on tape and when is max ,turn just a little the pot in the other way.

**imo** · 01-27-2008, 05:52 PM

thanks. I have the bias almost maxed out, and have the echo working pretty well. Thanks so much for your input. I am now trying to go through the SOS portion of the unit and find out where that is dropping out as well.
I think ultimately, after steadying the power supply, and firming up some grounds, etc, the biggest change was made by physically moving the recording head so that it was closer to the tape. thats when i finally started to get the echo again.
Definitely learned a lot.
BTW, Stokes, that link does not work
Thanks
Ian

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vintage echoplex broken.trying to diy fix. need help

vintage echoplex broken.trying to diy fix. need help

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