MEP-002A Voltage goes high after a minute

[Ray70]

I'm sure many of you have read lots of threads about machines with the output voltage going high, but this one might have a little different spin to it.
When I first got this machine I noticed an intermittent high voltage problem. After inspecting the VR board visually I noticed that at some point during it's career, someone had apparently drilled out the rivets and changed the ID plate onto the side of the control cube and proceeded to drill right through the VR board and severed the left leg of C2. I removed the board, fixed the Cap and everything seemed good.... for a while. After a recent periodic test, the voltage started pegging the gage high yet again, a few seconds to 1 minute after startup. I narrowed the problem down to the VR board by swapping control cubes, then swapping just the vR board. So... after reading lots of threads about high output voltage, I found a pair of NOS mil-spec Q3 transistors on ebay for cheap. Replace Q3, fired it up and dang... voltage goes high again. I checked for bad connections and everything but voltage still goes high after a short time. So I pulled the board out and brought it to work today. I tested the old Q3 and it appears to be good. New Q3 also appears good. I start checking for cold solder joints and bad components. Eventually I get to the R15 trimmer pot.... supposed to be a 2K 1w pot. Tested it and its reading 2K from pin 1 to 3, (ok)... reads 2K from pins 1 to 2, and 1 ohm from pin 2 to 3, both odd unless the pot was maxed out. So I gave it a few turns in and out and it bounced around a bit than jumped down to 1700 x 300. After some more measuring and turning the screw in and out it now appears to be working smoothly. Hopefully in a few days I'll have time to put it back in and test / recalibrate it. Hopefully it works correctly.
If all goes well we can add "defective R15" to the list of potential causes of high output voltage on MEP-002A, 003A machines. For the sake of <$10 I'll probably order up a new potentiometer just in case this one flakes out again. To those that I haven't put to sleep yet, thanks for reading!

 

[cuad4u]

Good info - I have a friend with a 002A that does the exact same thing. I have a spare VR board but will check his over really well before I do a swap.

 

[Isaac-1]

I have one that does the same thing (also swapped out the transitors), ended up swapping out VR boards with the intention of getting around to trouble shooting the old one and keeping it as a spare.

Ike

 

[doghead]

Could you post a picture of the potentiometer for an easy visual reference?

 

[Munchies]

Also they changed the pot setup on the two different board revisions, a good shot of the whole board would be nice if you could

 

[Chainbreaker]

Has anyone used the "Lakeland" VR retrofit board? Appears to be a quality made drop in replacement but I would like to hear from someone using it. I'm considering adding it to my spare parts inventory as a backup VR.

http://www.lakeserv.net/mep/index.html

 

[Munchies]

He is a board member. Bunch of info and threads about it. JimC is his username

 

[jamawieb]

Has anyone used the "Lakeland" VR retrofit board? Appears to be a quality made drop in replacement but I would like to hear from someone using it. I'm considering adding it to my spare parts inventory as a backup VR.

http://www.lakeserv.net/mep/index.html

Yes, I have one in a unit. It has been working perfect for over a year. High Quality.

 

[Isaac-1]

I have one too, good unit, direct drop in replacement

 

[Ray70]

I've been trying to upload the page out of the TM that shows the potentiometer I was talking about, but no luck so far.
I think it's something with the firewall we have here at work. Either that or I'm just doing something wrong....
It was R15 or item#8 in the picture on pg 30 in the Unit Direct Support manual. The other regulator design is shown on pg 34 and uses R16 as the potentiometer.

 

[steelypip]

I think this is what you're looking for:

 

[Ray70]

Yes, Sort of... One of the manuals has a drawing of the actual regulator board I was trying to post, to show the location (green rectangular pot with a screw in the end located near the bottom center of the regulator board.) It is R15 in the schematic you posted.
I need to practice posting pictures and figure out what my problem is.... (insert jokes here)