DER EE DE-5000 turns on on its own - Page 1
Forum section: electronics / repair

author: barbaroja
author_rank: Contributor
timestamp: November 24, 2024, 10:20:54 am
content: After struggling with this thing for days, checking out other threads with issues with the meter, I've come to a dead end. Have no idea on what to do next.This thread touches in something similar (https://www.eevblog.com/forum/repair/weird-problem-repair-of-der-ee-de-5000-lcr-meter/) but not quite exactly the problem.I have replaced the MOSFETs and the transistors. Checked every diode and every resistor and cap. All seems to be good. I can turn it on with the power button, but when I turn it off, it comes on its own, stays on until the auto shutdown timer hits and keeps on going and going in a continuous loop until it sends its battery to waste.Does the same when fed via the 9v jack. Found no shorts, regulators work ok...Tried to analyze the schematic but still can't pinpoint what could cause such behavior (restarting on its own). Any ideas on what could be happening? Maybe I am missing something evident?Any input is welcome. Thanks.

author: suntime
author_rank: Contributor
timestamp: November 24, 2024, 11:34:46 am
content: Hi,I would check if there is a problem with the power button.Regards.

author: u666sa
author_rank: Frequent Contributor
timestamp: November 24, 2024, 11:38:46 am
content: If you blow off R9 to the side so it is not in circuit. Does it do the same thing?

author: barbaroja
author_rank: Contributor
timestamp: November 24, 2024, 09:16:55 pm
content: Quote from: suntime on November 24, 2024, 11:34:46 amWorks fine. It gets 9v out of VPPTC? into PWR_KEY

author: barbaroja
author_rank: Contributor
timestamp: November 24, 2024, 09:17:57 pm
content: Quote from: u666sa on November 24, 2024, 11:38:46 amJust did. Won't turn on if taken out of circuit.

author: u666sa
author_rank: Frequent Contributor
timestamp: November 25, 2024, 04:03:45 am
content: So we isolated the circuit with trouble in it, didn't we?How about if you,Put R9 back into its place, and blow off to the side R18

author: barbaroja
author_rank: Contributor
timestamp: November 27, 2024, 12:40:51 am
content: Quote from: u666sa on November 25, 2024, 04:03:45 amJust did. When R18 is out of circuit, the meter turns on with the power button but not off.Keeps turning on on its own after timed power off. There are no attachments to the board (TL21 or any other)

author: Bud
author_rank: Super Contributor
timestamp: November 27, 2024, 01:53:48 am
content: Put a short from Q3 base to ground. If this does not help, you have a leaky Q3 and should replace with a better one.This is to just test Q3 for C-E leakage. Obviously the short must be removed for normal operation.

author: barbaroja
author_rank: Contributor
timestamp: November 27, 2024, 09:00:51 am
content: Quote from: Bud on November 27, 2024, 01:53:48 amReplaced Q3. Same weird behavior. It is like the processor is turning it on?It does work tho, measures correctly. It is very weird.

author: Bud
author_rank: Super Contributor
timestamp: November 27, 2024, 01:41:30 pm
content: 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.

author: barbaroja
author_rank: Contributor
timestamp: November 27, 2024, 10:17:09 pm
content: Quote from: Bud on November 27, 2024, 01:41:30 pmYes, I did it. It does not turn on with Q3 base to GND, so its not a leakage problem. Cleaned the flux. Same behavior.

author: Bud
author_rank: Super Contributor
timestamp: November 27, 2024, 10:46:39 pm
content: 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.

author: barbaroja
author_rank: Contributor
timestamp: November 30, 2024, 08:54:07 am
content: Quote from: Bud on November 27, 2024, 10:46:39 pmThanks, 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.

author: u666sa
author_rank: Frequent Contributor
timestamp: November 30, 2024, 02:13:35 pm
content: If it's not fixed, I'd remove R10 and try, and then reinstall R10 and remove R17 and try.

author: barbaroja
author_rank: Contributor
timestamp: December 16, 2024, 07:37:09 am
content: Seems to be happily fixed. Thanks to all who helped