MEP-803a voltage drop during monthly test

Fired up my 803a for its monthly test run, noticed my UPS hooked to my computer was showing an input voltage of 111-113V.
Last month it was around 119-121V.
I got my Fluke 17B out to confirm and L1-L3 was reading 231V. (When I last set it earlier this year I put it at 244V)

I was able to adjust it back up to where L1-L3 was at 241V but I noticed 2 things.
1. The voltage adjust potentiometer is almost maxed out.
2. When I was adjusting it it didn't seem to be acting right. The first thing I did was turn the knob down and the voltage went up by 0.5V before it started going down.
I also noticed the voltage seems to be ranging a bit more than usual (+-2V)

Going to figure out what TM the regulator test procedure is in and run that test sometime tonight, but I have a feeling its the probably the potentiometer going bad.
I should note I've sprayed it with Deoxit in the past as a preventative measure.

To be honest I'm probably going to just replace the entire VR with one of @kloppk 's units since I've been considering doing that anyway.

I did a quick range test for Min/Max Voltage.
First the Knob Position for 240V.


Min Voltage


Max Voltage (I should note here the voltage stops going up with the knob at around the 4 o'clock position)


I found if I wiggled the knob at minimum value the voltage would fluctuate


According to the TM the voltage adjustment range in 120/240V mode should be 228-252V

I see the Hz gage is pegged up on 65 Hz, is it malfunctioning or is the gen running excessively fast?
It does sound like the external potentiometer is either dirty or going bad.
At first that non-standard AC volt gage along with the L3-N AM/VM switch setting and 240V reading on your DMM really threw me for a loop until I looked closer and saw the 100-300V scale on the bottom of the gage!

The Hz gauge is off by about 1.5.
At the time I took the pic it was probably around 62Hz.

This spring I did a governor droop adjustment.
It should be 61.8 at 0 and 60.0 at 100%

Right now at 75% it was running 60.5hz

Have you tried to adjust the gauge with the small panel fitting at the center, with a screw driver? You might be able to bring the hertz gauge back to reading correctly.

One thing I have done to pots when they act funny, with the set off crank the pot all the way up and back down several times. sometimes the wiper gets corroded and needs a degunking. If its an open pot (you can see the wiper) a little deoxit goes a long way

Light in the Dark said:

Have you tried to adjust the gauge with the small panel fitting at the center, with a screw driver? You might be able to bring the hertz gauge back to reading correctly.

Click to expand...

Yea I tried that, the closest I could get was 0.6Hz above what it was running.
So I decided to adjust it so it was exactly 1Hz above what it was reading.

Any suggestions on a replacement unit? that doesn't cost me a kidney.
I would prefer not getting one of those super cheap Chinese ones but at this rate I might as well. While I don't like them I do believe they are fairly accurate which would be good enough for my use.

Scoobyshep said:

One thing I have done to pots when they act funny, with the set off crank the pot all the way up and back down several times. sometimes the wiper gets corroded and needs a degunking. If its an open pot (you can see the wiper) a little deoxit goes a long way

Click to expand...

I did try going back and forth yesterday, all that accomplished was I figured out if I wiggled it at minimum the voltage would fluctuate.
Like I said I sprayed it with Deoxit sometime last year? maybe the year before.
Since COVID everything has mushed together in my head.

I have one here to be tested out of a machine. They aren't cheap, thats for certain. Don't actually see a mil spec one for sale anywhere at the moment.

Quick question, been reading the TM on the potentiometer check procedure and it wants me to desolder the 3 connections on the back.
For just checking it couldn't I just disconnect the wires from the VR?

It looks like terminals 1 and 2 on the VR and a wire is looped around from R to C on the potentiometer itself.

So I could probably disconnect wires 1 and 2 from the VR and desolder the center wire on the potentiometer?

I believe that would work just fine.

No need to unsolder any wires.
Just remove the wires attached to VR terminals 1 & 2.
Then measure the resistance between the two wires as you rotate the voltage adjust. Should go from about 0 ohms to 5,000 ohms smoothly.

kloppk said:

No need to unsolder any wires.
Just remove the wires attached to VR terminals 1 & 2.
Then measure the resistance between the two wires as you rotate the voltage adjust. Should go from about 0 ohms to 5,000 ohms smoothly.

Click to expand...

I kind of figured that would be enough, but the TM wants to check between the 2 outer terminals to see if it has 5k ohms.
If I wanted to follow the procedure to the letter I'd have to desolder the center terminal to check that.

But to be honest I'm pretty terrible at soldering so I may just go with Kurts plan here, also I would only have to bring a screwdriver and multimeter down to the generator instead of my whole soldering kit.

I just ran the test and got 40-4.5k ohms with some very minor fluctuations on the way up.
I also noticed I could go past 4.5k ohms on the knob but the reading wouldn't change.

because a video is worth a thousand words here you go

nextalcupfan said:

I just ran the test and got 40-4.5k ohms with some very minor fluctuations on the way up.
I also noticed I could go past 4.5k ohms on the knob but the reading wouldn't change.

because a video is worth a thousand words here you go

Click to expand...

the rheostat is worn out and past it’s prime. I would get a new one.

Agreed. Readings are jumping around a bit in the video.
If you need a new one with wiring let me know. I have a new potentiometer with wiring ready to ship.

Sent kloppk a PM on a new VR.
I also noticed my S14 is looking pretty brown on the inside so I'm getting one of those too. As a preventative measure.

Got my new VR from Kurt in the mail today.

Decided not to waste any time on replacing the old one.
Out with the Old.

In with the new.

Voltage Testing (Generator is in 120/240V Single Phase)
Frequency (no load)

Voltage knob at center (as instructed for first startup in the instructions)

Voltage knob at min

Voltage knob at max

WOO now that's alot of angry pixies.

@kloppk Is it normal to see that high of a voltage?
I figure its to allow people to push 240V in 3-Phase 120/208V mode.

Yes, the higher than voltage range is there so that users can use the generator to power applications that require higher voltage than the nominal 120/240/208.
Had received several requests early on for that specific capability.
This can be done as long as one doesn't exceed the overvoltage threshold of the set.