Electronics > Repair

Power Supply Repair Help

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smd75jr:
Hello Everyone!!!!

For my first thread on the EEVBlog Forums, I request your assistance!

Yesterday i managed to pick up a Fluke 8000A multimeter, an General Radio 1340 Pulse Generator, and a Power Designs Inc. 5015T Variable power supply, for a total of 20USD.

The Fluke 8000A and the GenRad 1340 both work beautifully, however the power supply does not. after rigging up a fuse (the fuse holder is broken) it will only output a maximum of about 5.8VDC when its stated voltage range is 0-50VDC at 0-1.5A.

So i found the manual online and have started testing voltages starting from the line voltage switch and lamp (old style 115VAC lamp) and working my way in.

The first issue i have encountered is a transistor marked Q9 on the schematic (http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Power_Designs/Power_Designs_5015T_Power_Supply_Service_Manual.pdf). The transistor is a 2N4888 PNP can type. The manual says that it should have 0.6V between the base and the emitter which i have measured as 0.66V, and 0.9V between the emitter and collector which I measured as 0.03V.

I plan to continue my tests on the unit and update this post when I find a problem. If there is anyone who knows an equivalent to the problem components I would be eternally grateful!!



And now, for those of you who love old tech, pictures!! (pardon the imgur, the files were too big)

http://imgur.com/a/r0rLm



Problem Parts:

Q9 - PNP - 2N4888

Q3 - PNP - TI1028A

alm:
0.66V sounds fine, but a collector-emitter voltage of 0.03 V suggests that they may be shorted. I would desolder Q9 and measure resistance from collector to emitter. The other option would be that I2 is shorted, but that doesn't sound likely.

smd75jr:

--- Quote from: alm on September 29, 2013, 08:30:18 pm ---0.66V sounds fine, but a collector-emitter voltage of 0.03 V suggests that they may be shorted. I would desolder Q9 and measure resistance from collector to emitter. The other option would be that I2 is shorted, but that doesn't sound likely.

--- End quote ---

I2 comes up clean, no short there, and I desoldered the 2N4888 and it reads open circuit between the collector and emitter.

With the negative on the base and positive on the emitter it reads 0.68 ohm and with the positive on the collector it reads 0.66 ohm

Accordion to this helpful guide (http://www.learnabout-electronics.org/transistor_faults_06.php) the transistor is dead. :(

Is there an equivalent part that is actually still made?

Mr Simpleton:
Not fully convinced the 2N4888 is gone... you say high impedance betwen collector and emitter, fine and then you se a low impedance with positive on collector and negative on base or positve on emitter negative on base. This sounds perfect for a PNP transistor. You should reverse and measure the impedance with negative on collector/emitter and positive on base to verify high impedance.

Usually one diode do break down and shows short in both directions.

Andy Watson:

--- Quote from: smd75jr on September 29, 2013, 09:07:11 pm ---Accordion to this helpful guide (http://www.learnabout-electronics.org/transistor_faults_06.php) the transistor is dead. :(

--- End quote ---
The page you linked to shows a NPN being tested. I believe the 2N4888 are PNP - so reverse the polarity of the tests.

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