Electro-Harmonix Electric Mistress Repair

[Helmholtz]

Also measure resistance between the DC jack's lug where the red wire was connected and enclosure.

[clarisso11]

Also measure resistance between the DC jack's lug where the red wire was connected and enclosure.

That resistance measures as close to 0 Ohm

[Helmholtz]

That resistance measures as close to 0 Ohm

So it's either a wrong lug or a bad/wrong DC jack.

[clarisso11]

So it's either a wrong lug or a bad/wrong DC jack.

Good to know, thank you so much!
It looks like the remaining issues are related to something else, so I will open a new threat for that. Just a few final questions:

• Could this short, caused by the DC jack, have damaged any components, like transistors or ICs?
• If it damaged, say, the SAD1024 chip, would it mean I would get no effect at all?

Playing around a bit more, I get flanging coming through, but only at low Range settings. It either cuts out from there, or distorts.

[Helmholtz]

Good to know, thank you so much!
It looks like the remaining issues are related to something else, so I will open a new threat for that. Just a few final questions:

• Could this short, caused by the DC jack, have damaged any components, like transistors or ICs?
• If it damaged, say, the SAD1024 chip, would it mean I would get no effect at all?

Playing around a bit more, I get flanging coming through, but only at low Range settings. It either cuts out from there, or distorts.

Both batteries still good? What is supply voltage when engaged? Should be 16 to 18V.

Why a new thread?

I don't think the jack problem could have damaged components. Typical reason is too high voltage or wrong polarity power supply.

[clarisso11]

Why a new thread?

I thought I've taken up enough of your time already, haha

Both batteries still good? What is supply voltage when engaged? Should be 16 to 18V.

Will get back on this tomorrow, and measure a bunch of voltages. Thank you for helping me through so far, I've made more progress than I could have done on my own!

[clarisso11]

Both batteries still good? What is supply voltage when engaged? Should be 16 to 18V.

Why a new thread?

I don't think the jack problem could have damaged components. Typical reason is too high voltage or wrong polarity power supply.

Okay, supply voltage is 17.09V, so that should be okay. I noticed that the voltages at SAD1024 should be 12V for pin 5, 7, 9, 11 (according to this: http://www.metzgerralf.de/elekt/stom...pair.shtml#ts2).
I measure 14.98V at those pins.

Happy to do any additional measurements that will help, I do also have an oscilloscope (although not a great one).

[Helmholtz]

Check if the Zener voltage is 6.8V.

[clarisso11]

Check if the Zener voltage is 6.8V.

I measured 6.3V at the Zener, is that close enough?

[Helmholtz]

I measured 6.3V at the Zener, is that close enough?

Do voltages change when you lift one leg of the tantalum cap across the zener?

[clarisso11]

Do voltages change when you lift one leg of the tantalum cap across the zener?

Just lifted one leg of the tantalum cap (next to the cathode side of the zener), and the voltage doesn't change across the zener.
Not sure if this is relevant, but I measure the ~6.2V only at the cathode of the zener, and 0V at the anode side.

[clarisso11]

I have done a bit more testing, here are the results:

• Disconnected 10uf Tantalum cap, tested: measures fine. Soldered back in place
• Pin 8 4558: 14.85V
• pin 7 LM741: 16.93V
• pin 14 4013: 14.37V
• pin 3 LM339: 14.3V
• Sweep at pin 2 LM339: Max ~6.8V when Rate fully clockwise, Max ~6.4V when fully counter-clockwise
• SAD1024 pin 5, 7, 9, 11: 14.4V
• 2n5087: E = 13.87V, B = 13.23V, C = 0.476V
• 2n4354: E = 14.53V, B = 16.26V, C = 16.93V

[clarisso11]

I have also checked the resistance across the sweep of all three potentiometers, everything fine. Visually inspected the board, especially capacitors, all looking good.

[Helmholtz]

I have done a bit more testing, here are the results:

• Disconnected 10uf Tantalum cap, tested: measures fine. Soldered back in place
• Pin 8 4558: 14.85V
• pin 7 LM741: 16.93V
• pin 14 4013: 14.37V
• pin 3 LM339: 14.3V
• Sweep at pin 2 LM339: Max ~6.8V when Rate fully clockwise, Max ~6.4V when fully counter-clockwise
• SAD1024 pin 5, 7, 9, 11: 14.4V
• 2n5087: E = 13.87V, B = 13.23V, C = 0.476V
• 2n4354: E = 14.53V, B = 16.26V, C = 16.93V

I think you mixed up emitter and collector of the 2N4354. If so, both transistors should be good.

Look at the schematic, everything connected to V+ must be at the same voltage, say 14.5V.

The circuit with the zener diode, the LM741 and the 2N435 is a voltage regulator, that supplies the rest of the circuit with a constant voltage (V+).
Acc. to the schematic this voltage should be around 13V. The zener voltage being low by around 10% might cause the regulated supply voltage to be increased by 10%.
Typical zener voltage tolerance is 5% or less. You might consider replacing the zener.
Acc. to the site you linked, V+ can also be adjusted by varying the 22k resistor.

Generally that site provides excellent info and I recommend to follow the adjustment and troubleshooting instructions, including verifying signals with the scope.

[clarisso11]

I think you mixed up emitter and collector of the 2N4354. If so, both transistors should be good.

Look at the schematic, everything connected to V+ must be at the same voltage, say 14.5V.

The circuit with the zener diode, the LM741 and the 2N435 is a voltage regulator, that supplies the rest of the circuit with a constant voltage (V+).
Acc. to the schematic this voltage should be around 13V. The zener voltage being low by around 10% might cause the regulated supply voltage to be increased by 10%.
Typical zener voltage tolerance is 5% or less. You might consider replacing the zener.
Acc. to the site you linked, V+ can also be adjusted by varying the 22k resistor.

Generally that site provides excellent info and I recommend to follow the adjustment and troubleshooting instructions, including verifying signals with the scope.

Excellent advice, thank you! I probably did mix up 2N4354, it was hard to see which lead is which.
Based on those voltages and the symptoms, can we conclude that the SAD1024 is undamaged, although acting funny because the voltages are wrong?

I will start by replacing the zener, and report back.