Troubleshooting and reviving a Echoplex Ep-3

[clarisso11]

Hi everyone,

many months ago, I purchased an old Echoplex EP-3, in very bad condition. I took it apart and de-rusted/cleaned everything, exchanged most electrolytic capacitors (one had bad ESR, others were leaking), and took care to solder all cables back to where they belong (although I might have made a mistake).

It now runs and produces a signal, but the echo doesn't seem to be erased, it just plays a loop of recordings. The actual echo of the guitar comes through very faintly.

To troubleshoot the issue, I went ahead and labeled and traced the circuit, using pictures of my Echoplex (most white mallory capacitors were changed later):




I measured voltages, and most check out. The only things I found while troubleshooting were:

1. Q4 measures 16.2V instead of 14.3V at the collector, and 1.2V instead of 16V at the emitter
2. More worryingly, if the capacitors aren't fully discharged, touching the metal enclosure gives me an electrical shock

What could be causing the wrong voltages at Q4? And does the electrical shock point to something like a grounding issue?

I've attached the relevant schematic, any help is greatly appreciated!

[Helmholtz]

Q4 measures 16.2V instead of 14.3V at the collector, and 1.2V instead of 16V at the emitter

I'm sure 16V at the emitter is a typo. An npn transistor can't work if collector DCV is lower than emitter DCV. OTOH, 1.6V at the emitter would make perfect sence, meaning a collector current of roughly 0.7mA and a voltage drop of around 7V across the 10k collector resistor. Do you get 22V at point 5?

[clarisso11]

I'm sure 16V at the emitter is a typo. An npn transistor can't work if collector DCV is lower than emitter DCV. OTOH, 1.6V at the emitter would make perfect sence, meaning a collector current of roughly 0.7mA and a voltage drop of around 7V across the 10k collector resistor. Do you get 22V at point 5?

Hi, thanks for your help! That makes a lot of sense, must be a typo then. I measure exactly 22V at point 5.
Leaves me to wonder, where do I go from here?

[Helmholtz]

Hi, thanks for your help! That makes a lot of sense, must be a typo then. I measure exactly 22V at point 5.
Leaves me to wonder, where do I go from here?

I'm not familiar with the Echoplex but I know there are members having lots of experience with tape echo repair. You will need a scope to verify HF oscillator signals.

[Enzo]

Make sure SOUND ON SOUND is turned off.

If it doesn't erase, start at the bias oscillator - the signal for the erase head ( and also used in record mode)

[clarisso11]

I'm not familiar with the Echoplex but I know there are members having lots of experience with tape echo repair. You will need a scope to verify HF oscillator signals.

No worries, thanks for your help ruling out that transistor!

[clarisso11]

Make sure SOUND ON SOUND is turned off.

If it doesn't erase, start at the bias oscillator - the signal for the erase head ( and also used in record mode)

Hi Enzo, thanks for chiming in. Is the bias oscillator the trim pot next to T2, marked '220k bias adj.'? I'm guessing this is the part where I feed a signal into the input jack, and measure point 15 with an oscilloscope (I have a digital one, hope it will be enough)?

Any idea why it would be dealing out electric shocks through the metal enclosure?

[g1]

Any idea why it would be dealing out electric shocks through the metal enclosure?

The schematic does not show the AC cord, and may be missing other components on the primary side of the mains transformer, such as a cap to chassis.

Check the the AC voltage from the chassis to the 3rd prong ground of a grounded AC outlet. If it is a 2 prong plug, then flip the plug over and measure again. Does one way read higher than the other way?

[Enzo]

I am not yet concerned with adjusting the record bias oscillator, I am concerned if it is operating AT ALL.

[clarisso11]

I am not yet concerned with adjusting the record bias oscillator, I am concerned if it is operating AT ALL.

Makes sense, here's the signal at point 15, hope that helps!

[clarisso11]

The schematic does not show the AC cord, and may be missing other components on the primary side of the mains transformer, such as a cap to chassis.

Check the the AC voltage from the chassis to the 3rd prong ground of a grounded AC outlet. If it is a 2 prong plug, then flip the plug over and measure again. Does one way read higher than the other way?

I've checked the voltage at what I think is ground of the AC cable (green cable on the photo), it measures 0V. I will add photos of the PCBs on the other side of the Echoplex, where the motor sits.

[Helmholtz]

I've checked the voltage at what I think is ground of the AC cable (green cable on the photo), it measures 0V.

Use your scope to check for voltage between your chassis and the safety earth contact of your mains outlet (ground probe here).

[clarisso11]

Use your scope to check for voltage between your chassis and the safety earth contact of your mains outlet (ground probe here).

Hi Helmholtz,

I just connected the ground probe of my scope to where the green cable is attached, and checked the chassis, as you suggested. It seemed to be mainly a flat signal, but occasionally the scope reading peaked (unfortunately I couldn't capture it). After running the Echoplex for a few minutes those peaks seemed to go away..

[Helmholtz]

Hi Helmholtz,

I just connected the ground probe of my scope to where the green cable is attached, and checked the chassis, as you suggested. It seemed to be mainly a flat signal, but occasionally the scope reading peaked (unfortunately I couldn't capture it). After running the Echoplex for a few minutes those peaks seemed to go away..

That's not what I meant. The ground lug where the green cable attaches should have good contact with the chassis and enclosure (please verify with Ohmmeter). And if so you can't have a voltage to chassis/enclosure.
I actually meant to connect the probe ground to the safety earth contact of the wall outlet (be careful!), will require a second wall/mains outlet.
You should do this only after you verified that you have continuity/very low resistance between the lug where the green cable is attached and the safety pin of your mains plug. And you first need to verify with your meter that there is 0 VAC between the earth contact of the wall outlet and the enclosure/chassis of the powered Echoplex.

If you feel a "shock", but can't measure a voltage between your body and the metal enclosure with a meter, it's most likely a high frequency voltage. Or your meter's internal resistance is low.

If you don't know what you're doing or don't feel safe, don't do the test with the scope!

[clarisso11]

That's not what I meant. The ground lug where the green cable attaches should have good contact with the chassis and enclosure (please verify with Ohmmeter). And if so you can't have a voltage to chassis/enclosure.
I actually meant to connect the probe ground to the safety earth contact of the wall outlet (be careful!), will require a second wall/mains outlet.
You should do this only after you verified that you have continuity/very low resistance between the lug where the green cable is attached and the safety pin of your mains plug. And you first need to verify with your meter that there is 0 VAC between the earth contact of the wall outlet and the enclosure/chassis of the powered Echoplex.

If you feel a "shock", but can't measure a voltage between your body and the metal enclosure with a meter, it's most likely a high frequency voltage. Or your meter's internal resistance is low.

If you don't know what you're doing or don't feel safe, don't do the test with the scope!

Hi Helmholtz,
thanks a ton for your continuing help. As you've probably guessed already, I don't have a ton of experience and am still learning, especially using a scope. I will check the resistance, but will leave the scope tests for now, just to be safe.