Electro-Harmonix Electric Mistress Repair

[clarisso11]

Hi everyone,

I recently got an Electric Mistress for repair, and had a question regarding voltages:
When I first got the pedal, one of the battery wires was detached (see photo). I re-attached it to the DC jack, but got no output signal at all.

When I checked the circuit for voltages, none of the ICs received power. I realized that there was no voltage at the DC jack, but -18V at the input jack (the ground?). Shouldn't it be the other way around?

I'm quite puzzled, it sounded like the pedal was working before the wire detached, I simply reconnected it - how can the voltages be switched? ITo clarify, I'm powering this using two fresh 9V batteries.


Many thanks in advance!

[clarisso11]

Mystery solved! I used a stereo cable when first testing the pedal, so the power wasn't carried over to ground!

[Helmholtz]

Did you have a plug inserted to the input jack when trying to measure voltages?

The power jack wiring in the schematic looks wrong/incomplete. There should be some switching contacts which short the red to white wire when no ext. power supply is used.

[clarisso11]

Did you have a plug inserted to the input jack when trying to measure voltages?

The power jack wiring in the schematic looks wrong/incomplete. There should be some switching contacts which short the red to white wire when no ext. power supply is used.

Thanks for your reply. I did have a jack inserted, but a stereo jack, hence the problem.

I just tested the pedal, using a regular guitar cable, and both batteries are getting very hot quickly. Short circuit somewhere?

[Helmholtz]

I just tested the pedal, using a regular guitar cable, and both batteries are getting very hot quickly. Short circuit somewhere?

What do you mean by "tested"? Did you get sound?
Def. looks like short. Are you sure you connected the red battery wire to the correct lug?

[clarisso11]

What do you mean by "tested"? Did you get sound?
Def. looks like short. Are you sure you connected the red battery wire to the correct lug?

Thanks again, really appreciate your input!
Instead of feeding in an audio signal through the stereo cable, I plugged in my guitar to test it, with a mono cable.

I checked the wiring using reference photos I found online, see attached (left one is mine).

The fact that this was working before the battery cable came off tells me that something about the power supply isn't working; really hope it's not one of the ICs.


Any idea what I could try next?

[Helmholtz]

I plugged in my guitar to test it, with a mono cable.

Again, did you get sound?

Measure the resistance between the red battery wire and ground.

[clarisso11]

Again, did you get sound?

Measure the resistance between the red battery wire and ground.

I got no sound at all once the effect was engaged. Resistance between battery red at DC jack and ground at input jack is zero Ohm.

[Helmholtz]

To exclude the DC jack, remove the white wire from DC jack and measure resistance of red wire to ground again. Then measure resistance from white wire to ground.

[clarisso11]

To exclude the DC jack, remove the white wire from DC jack and measure resistance of red wire to ground again. Then measure resistance from white wire to ground.

Thanks, I'll do that next. I have checked all battery wires, one had almost come off. Soldered back on, still no sound, and I measure only 8V at ground.

[Helmholtz]

. Soldered back on, still no sound, and I measure only 8V at ground.

No wonder. Batteries getting hot means excessive current. That will pull voltage down and rapidly discharge them.
Do not reconnect until problem is solved.

[clarisso11]

No wonder. Batteries getting hot means excessive current. That will pull voltage down and rapidly discharge them.
Do not reconnect until problem is solved.

Got it. Just to make sure I measure the right thing next, do I remove the white from DC jack, then measure resistance between red at the DC jack, and ground (i.e. the enclosure)? And then the detached white cable to ground (enclosure)?

Thank you again, I'm really lost right now!

[clarisso11]

To exclude the DC jack, remove the white wire from DC jack and measure resistance of red wire to ground again. Then measure resistance from white wire to ground.

Okay, I've inserted a mono plug to complete the circuit, and done some measurements, all without batteries connected, and white wire removed from DC jack. Ground is enclosure:

• Red battery wire (1) at DC jack to ground: Zero Ohm
• Red battery wire (2) at PCB to ground: OL
• White wire, detached form DC jack, to ground: 15+ MOhm, continuously rising

[Helmholtz]

Okay, I've inserted a mono plug to complete the circuit, and done some measurements, all without batteries connected, and white wire removed from DC jack. Ground is enclosure:

• Red battery wire (1) at DC jack to ground: Zero Ohm
• Red battery wire (2) at PCB to ground: OL
• White wire, detached form DC jack, to ground: 15+ MOhm, continuously rising

That's good news, as it means that the short is not in the circuit but at the DC jack.

Now remove also the red wire from the DC jack and connect it to the white wire. If this connection measures high resistance to ground reconnect batteries and see if it works.

[clarisso11]

That's good news, as it means that the short is not in the circuit but at the DC jack.

Now remove also the red wire from the DC jack and connect it to the white wire. If this connection measures high resistance to ground reconnect batteries and see if it works.

Great news (sort of) - I followed your steps, and get no more overheating batteries, plus output when the effects is engaged! So far, so good..

However, there is no flanging. Some observations:

• I can here the clock, changing speed with 'Rate'
• Turning 'Range' all the way down changes the sound, to almost like a steady pitch shift, hard to describe. It "swooshes" back to normal when I turn the 'Range' back up.
• Voltage at the new white wire - red wire connection is 16.2V

Any ideas what to do next? Could I have fried any of the IC chips?