Weird problem & repair of DER EE DE-5000 LCR meter

[dfmischler]

I bought one of the DER EE DE-5000 LCR meters from a Japanese ebay seller for about $115 US.  When I received it, it would not turn on with the power button.  I discovered that if I removed the battery and put it back in it powered up immediately, and seemed to work properly.  When I pushed the power button to turn it off it said it was switching off, but would not turn on again.  I felt that after waiting a month for delivery that sending it back to Japan for a problem like this was out of the question, so I decided to fix it.  I found a fragment of the power circuit schematic (I think it was a jpg here on EEVblog, but I can no longer find it).  Anyway, Q14 was shorted so the power was never actually getting switched off.  I assume the processor was just sitting in a tight loop waiting for death so it could be resurrected when the power came back up.  I replaced Q14 with an SI2301-TP MOSFET, and it seems to be working fine now.

Does anybody know where the power circuit schematic was so it can be linked to this thread?

[Zucca]

I would also love to have the schematics. Congratulation for your repair! Mine is working fine, it could be you got a bad apple.

[dfmischler]

Found it!  The circuit attached to this post matched my meter very closely, if not exactly.  And it was very helpful.  My thanks to jarvis.

[Co6aka]

Aren't these essentially the same as the IET (and Extech, and others?) that Dave tore down...

[dfmischler]

As far as I know the IET folks buy the meter from DER EE.

[Fsck]

pretty sure DER EE is the OEM of the meter, but I could be wrong.

[PA4TIM]

The difference is the IET does not come untested 

[dfmischler]

The difference is the IET does not come untested 

I can picture a production tester missing this kind of problem simply by ignoring the fact that the unit comes on as soon as the battery is installed, if the test procedure does not check that the unit can be turned off and back on again.

[adriten]

after lurking this meter for years (have a BK Precision 878 and did'nt really need it) I finally took the plunge and bought it
I received my unit 4 days ago and yesterday it failed showing exactly the same symptoms as dfmischler's (thanks for your post and for the schematics)
And yes , it looks like the culprit is Q14, a smd P-channel Mosfet
Turned out is not shorted, it is rather leaking, preventing its Drain to go to zero volt, so that the system is not getting the correct Power ON Reset and the meter does not turn on
Since I was not seing any short between pins and the diode inside tested good, initially I was not convinced the culprit was Q14
So, while the meter was supplied with 9V from Lab Power Supply, I forced Q14 Drain to go down ( a 10 ohm resistor between Q14 Drain and ground), and  bingo: pressing the power button the meter came alive !
Now waiting for a spare Q14  (SI2301) to arrive
adri