Miller 350D - panel gets hot around FINE tuner

Now for how long it's been doing this I've no clue since it didn't seem to impede the welder's function in the slightest. Before I try to describe what's going on I'll elaborate (briefly) on what I have.

Aforementioned 350D Miller w/ the 2 dials. Left-hand is range. Right is fine tuning. The rig used to see duty at oil rigs in New Mexico and dates from about 1981. I got it about 3 years ago.

It is in the vicinity of the FINE tune rheostat that it will get abnormally warm to hot enough that holding a bare hand on the panel in the upper corner...or on top directly above...will get uncomfortable. It was being used in the 65-130 range (though this is probably unrelated) with FINE setting about midway...about 100A. Wasn't being used on anything particularly major or thick (only 5/16") and for only about 30min total.
It also did so this morning when it was fired up to use it as a generator to keep the all important fridge//freezers from getting too warm after storms knocked out power last night. Ran it in 1hr intervals to permit cool down even though the draw of the 3 appliances was nowhere NEAR what welding pulls.

After the 1st time (after shutdown of course) I lifted the sides to look and tentatively feel. The rheostat was HOT...warm I'd expect but not singing hot.
This second time I took a closer look during interim downtime and did note that the sliding contact looks quite worn down just from PO (ab)use. If it's making poor contact then yes that would do it...BUT I'd like to seek other opinions.

Also, behind the rheostat are 2 cylindrical bars...almost resemble corncobs in appearance except they have zigzag raised lips with leads clamped to different distances along them. One of them is connected to the minimum side of the FINE range.

Images forthcoming.

My guess would be that it's time to replace the rheostat. You can probably find a replacement at Allied or
Mouser.

Might also try adding a cooling fan to move some air through.

The cylindrical bars are adjustable resistors. It's possible they could be set wrong, but without some
procedure to follow, I wouldn't move them.

"My guess would be that it's time to replace the rheostat. "

Steve, by definition rheostats/ variable resistors turn CURRENT into HEAT.

If the welder is working properly (and welding) the rheostat is doing it's job. WHY would you recommend replacing it???

Since I still have yet to get out there to take pictures...and it appears its already known what I was referring to...I'll forgo them unless really necessary.

The contact pad between the rheostat and the spring-loaded armature is what looks really worn down. It's maybe 1/16" thick at this point.
My main concern is that it's getting TOO hot. Reminds somewhat of when welding with a bad ground...the ground gets crazy hot after a short time while a good ground is little more than ambient air temp.
It's enough that the handle itself is as well meaning the parts that the heat must cross to reach it...etc.

You can find the owner's manual, including schematic, at the link below. You'll need to know your serial number to get the correct schematic.

I pulled up a couple of the Wildcat 350D manuals, and they were similar. It's not real clear, but I think the fine adjustment rheostat (R1) controls the alternator field current through a an adjustable resistor (R2) and rectifier bridge (SR2). I think the "corncob" device you see is resistor R2, and it's probably set at the factory and should not need to be messed with.

Note that R1 is rated at 300 watts and R2 at 375. So between them they can safely dissipate 675 watts, which is a lot. Maybe you have no problem at all. If you're concerned about the heat, switching to the next higher range on the coarse adjustment switch will reduce the required field current and the amount of heat generated by the rheostat.

You could try adjusting R2 so it has more resistance, which will cause it to take some of the load off R1. Since the manual doesn't give an adjustment procedue, I'd be inclinded to leave it alone. My guess is they set it at the factor so there aren't any gaps between the different ranges, and you're not going to have the test equipment required to do that.

If there's something actually bad, it's probably rectifier bridge SR2. A shorted diode in the bridge could cause an increase in field current. To check the bridge for open or shorted diodes, you'll need to disconnect it and check each diode individually. Note that you have to use the diode check range on your multimeter, as most DMMs won't check diode continuity on their ohms scales.
Miller manuals

I'd already picked up the manual from there to have on hand...but thanks for the reference anyway. And yes it's none too clear. I also agree that what I'm experiencing might be normal or merely just on the side of abnormal given its age. Never occurred to me to feel the panel when the rig was seeing actual work when having to weld up the tractor to take the new rams I had to put on it.
Though I think I will do one of the things it recommends by taking the compressor to it and (carefully) blowing it out since fine dust can do all kinds of unwanted things to electronics. And if there's anything I have out my way in abundance is dust. Perhaps even carefully check connections to be sure they are still all tight in case vibration by simply running has done just enough to cause heat buildup w/o otherwise affecting function. Will also look into checking the diodes and replacements (just in case). But with my tendency it seems to be getting old equipment//vehicles that works fine yet parts hard (and harder) to come by I'm not holding out hope.

Thanks