PC-420D stirring speed

[gnd2]

Anyone familiar with these hot plate stirrers? I picked one up and wondering about the stirring. The speed fluctuates quite a bit up and down randomly, I expected it to be much more consistent (the display even starts blinking indicating it's adjusting to the set speed). Seller said he oiled the motor and it does seem to turn smoothly with just a little resistance. The sensor disk is in good shape.

So is this normal or should it be consistent? Any other ideas what I can check? I'm worried it might be electrical rather than mechanical.

[Conrad . Marvin]

I have the same model and I bought it many years ago. It has never fluctuated from the speed that I set unless I have decoupled the stirrer because I use a two inch size instead of the recommended one inch. That being said, previously I had bought two versions on eBay which didn’t work correctly. When I called Corning to ask about repairs the cost was enough that I returned the used one and sprung for new and have never been sorry for the extra cost.

[gnd2]

Thanks. Something is definitely not right. Last night it randomly went from 500 rpm to almost a complete stop. Up till now it was mostly just annoying, but now I'm worried something is approaching failure, so I'm contacting the seller.

Ya, it's one of those things that seems ridiculously expensive and frivolous until you use one, then you don't want to be without it. I can't help wondering though, if the Cornings are so robust and well made, why are there so many malfunctioning ones for sale?

[Conrad . Marvin]

I think that like everything else, they do wear out and the labs that use them probably work them pretty hard before replacing. If the unit has been dropped or banged around, Corning did tell me that the unit would suffer. To repair an old unit they put a new ceramic top which is integrated with the heating element and electronics in such a way as to make it cheaper to just replace the whole works. The repair price was very expensive and would end up costing almost as much as a new one when all was said and done, so I bought a new one and I know how it has been treated. I suspect that I will not be able to wear mine out…

[Conrad . Marvin]

PS
After using the PC-420D for almost a decade, I could never go back. No, I don’t work for Corning.

[Jody_S]

I don't have that model but I looked it up. Looks like the speed is set with a knob on the front, that looks very vulnerable to spilled liquids. Those are usually 0-5K or 0-10K pots, a bit of dust or dried liquid in that will give the processor a bad signal and could result in the problem you've described. But the processor itself and the speed control either work or don't, there's usually no in-between. Unless there's a bad contact or bad solder joint, but that usually results in intermittent working, easily detected by banging on the side when it's running slow.

[Duolab123]

I worked in analytical labs for years. The best stirrers are ones without the hotplate. For a lot of reasons, proximity of the drive magnet, and the heat accelerates the aging. Corning made great products. I use a prop shaft variable speed lab mixer for powders these days, but I still have a trusty 50 year old Magni-stir.

[koraks]

I don't have that model but I looked it up. Looks like the speed is set with a knob on the front, that looks very vulnerable to spilled liquids. Those are usually 0-5K or 0-10K pots, a bit of dust or dried liquid in that will give the processor a bad signal and could result in the problem you've described.

Should be fairly easy to track down this particular problem. Just measure the voltage between the outer pins on the pot to establish which one is negative (most likely ground) and which is positive; then measure between the center pin and the negative/GND pin and see if it's a stable readout that doesn't flipper from 0 to some positive value if you touch the dial.
However, my money's on some other issue. Specifically the optical detector circuit for the RPM speed seems like something where a number of things could easily go wrong.
Then of course there's the chance of a chemical spill resulting in corrosion of the PCB and/or parts, just like this person here experienced: https://41j.com/blog/2018/07/corning...tirrer-repair/

I'd open it up and start poking around. Odds are it's a fairly simple problem really.

[gnd2]

It's not the pot. Don't even need to put a meter on it, if that was the problem the set speed on the display would be fluctuating. But it's not, it's perfectly smooth and stable.

The first thing that seems to happen is the speed drops, then it senses the speed is off and starts flashing the display and trying to spin it back up, often overshooting.

I've thought about the optical sensor, but just don't see that leading to these symptoms, I would think it would be more all or nothing.

I was thinking triac (I'm assuming that's what drives the motor), but that would probably be an all or nothing deal also.

In my mind that leaves the motor coil or other loose connection.

I popped off the main circuit board and it looks clean and it good shape.

Didn't really want a project, but I'll do a little more poking. Agree, it's probably something simple, it's finding it that's tricky. But if it's the motor, I don't know if there's an off the shelf replacement and I don't really feel like rewinding it.

[gnd2]

I poked around a little more. The main PCB really looks pristine. However, I found that the motor coil was actually loose. It's also discolored, I suspect it has overheated at some point. I pushed it in and it seemed to work better, but it seemed like the coil could easily be knocked out again and the coil wires themselves may be damaged and susceptible to temperature fluctuations.

Anyway, it's off back to the seller now and I've ordered a fresh new one.