Benignium
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Corning PC-420D issue: Near-total loss of control
I've had my Corning hotplate stirrer for nearly three years now and have loved every second of using it. However, it has now developed an issue where
the stirring and heating only initiate at full power. The stirring function completely ignores the adjustment dial, initiating as soon as the unit is
plugged in. The heating element remains disengaged as long as its respective dial is in the OFF position. The equipment manual[1] seems to
imply that an error like this is known. Losing the use of the hotplate would be quite devastating and so I intend to fix the issue if at all possible.
I figured that I might as well make a thread where I can document the project and ask for help.
First, some photos.
(Fig. 1) Stirring control dial with the knob removed. Prior to this I have never removed the knob (a no-brainer, now that I know
I can simply pull it off) and have opted to use tweezers and a moist paper towel to wipe the surface underneath. The results are evidently poor and,
while there is a rubber gasket underneath the 13 mm nut, I suspect that the inner shaft lacks such protection and can potentially wick moisture into
the potentiometer.
(Fig. 2) Overall view of the unit with the bottom cover removed. This is the 230V model. To my untrained eye, nothing seems to be
obviously fried, disconnected or otherwise out of order.
(Fig. 3) Top of the control PCB. This also passes my visual inspection.
I must emphasize that my knowledge and skills are quite lacking as they relate to working with electronics. However, I feel like I have arrived at a
reasonable guess at where the issue might be. Apparently, inside a potentiometer there is a moving contact that turning the knob causes to move along
a resistive track. The contact between these two parts is supposed to remain clean and lubricated. Otherwise, there can be disruptions which can cause
the device to malfunction. There is an article which goes through the process of servicing two old potentiometers in detail[2]. Upon
reading it I suddenly recalled that indeed, the motion of the knobs on the brand new unit used to be incredibly smooth and realized that I must have
gotten used to its gradual change into what I still consider decent but clearly a little bit frictional. As to if whether this actually is the culprit
I seem to have no way of finding out other than to disassemble the potentiometers and clean and lubricate the contacts.
If you're reading this and have a different suggestion, I'd love to hear it.
Now, in the heat control potentiometer (fig. 4) there seems to be an abnormal (yet seemingly inconsequential) gap between the black plastic part and
the metallic part of the assembly. This gives me some hope that the two portions are simply pressed against each other and that separating them might
expose the contacts that I need to get to. (Optimistically) assuming this is the case, I would only need to detach the "top" metallic portion from the
circuit board. For this, there are seemingly two different approaches:
1: De-solder and wick the three prongs from the board (fig. 5), or
2: Pry open the ends of the three prongs that are clamped onto the potentiometer. (fig. 6)
Either approach seems to have the potential for failure with disastrous consequences. I'm terribly inexperienced with the soldering iron (and circuit
boards) so the second approach seems more attractive. However, I believe that the clamped connections are supposed to be permanent.
(Fig. 4)
(Fig. 5)
(Fig. 6)
I'd be grateful for any thoughts as well as advice for deciding on my strategy for tackling the issue!
References:
1: Equipment manual (Corning), section 7, page 9
2: Newgate Simms: Potentiometer Maintenance and Lubrication-Case Study
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Texium
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Thread Moved
5-7-2022 at 12:47 |
B(a)P
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If you have a multimeter probe the resistance of the potentiometer over its full range you will then be able to confirm or rule out your suspicions.
In my experience potentiometers are not very serviceable. I would be looking for replacement potentiometers, if the probing indicates they are the
culprit. Once you take the potentiometers out, test them again. Testing insitu can give false results.
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PirateDocBrown
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Absolutely desolder. With the right equipment, is actually very easy. If you need to replace the pot, you would have to do this anyway.
It definitely looks like corrosion has gotten into the pots. Whether or not this is the source of the problem is unclear, but replacement is cheap and
straightforward. You should probably do it, anyway.
Phlogiston manufacturer/supplier.
For all your phlogiston needs.
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monolithic
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A soldering iron and a solder sucker should be all you need to remove those potentiometers. https://www.youtube.com/watch?v=Aw4lZGk90i4
If you have difficulty with the solder sucker (solder isn't melting very easily, some residual solder on the potentiometer leads that won't get sucked
up, etc.) it helps to sometimes add more solder to the existing solder joint and make sure your soldering tip has melted solder on it. This allows you
to more effectively transfer heat and melt all of the solder, to be sucked into the sucker.
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B(a)P
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I have always found I get better results with solder wick for de-soldering, but I am by no means an expert.
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