Electronics > Repair

DER EE DE-5000 turns on on its own

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barbaroja:

--- Quote from: Bud on November 27, 2024, 01:41:30 pm ---Did you try shorting Q3 base to ground as i suggested? Doing so should turn off Q3 fully. If it does not (device continue to turn on by itself), then Q3 is not good, does not matter if it is new or not, the current leaking thrpugh E-C is very small but sufficient to turn the Mosfet ON. You can use a transistor different than 2n3904, like BC817.
And clean the flux under Q3. The flux residue may be conductive.

--- End quote ---

Yes, I did it. It does not turn on with Q3 base to GND, so its not a leakage problem. Cleaned the flux. Same behavior.

Bud:
Ok so looks some voltage on Base of Q3 makes it conduct. If this is some stray RF , i'd try a small capacitor from Base to ground, like 0.1uF.
If this is a small voltage coming  from the Key lines, try is a 100kOhm resistor from Base to ground.

barbaroja:

--- Quote from: Bud on November 27, 2024, 10:46:39 pm ---Ok so looks some voltage on Base of Q3 makes it conduct. If this is some stray RF , i'd try a small capacitor from Base to ground, like 0.1uF.
If this is a small voltage coming  from the Key lines, try is a 100kOhm resistor from Base to ground.

--- End quote ---

Thanks, Bud.
Tried both the resistor and capacitor methods, didn'work.
Measured a slowly creeping voltage in Q3 BE. Went and measured again Q4, which developed a voltage as well. Assumed it would be leaky. Replaced (again) and seems to have stopped doing the weird thing for now.

Still not convinced as it has proven to be very temperamental. Had resorted to installing a switch superglued on the case. Let's cross fingers.

u666sa:
If it's not fixed, I'd remove R10 and try, and then reinstall R10 and remove R17 and try.

barbaroja:
Seems to be happily fixed. Thanks to all who helped :)

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