Subject: Corning Laboratory Hotplate/Stirrer
From: Adam
[email protected]
Date: 1/6/2005 2:09 PM Central Standard Time
Message-id: <
[email protected]>
A friend of mine picked up a surplus Corning model PC-620 (product info:
http://tinyurl.com/4xmmy) laboratory hotplate/stirrer for a couple
bucks. It turns out that the stirrer works but it doesn't get hot. I
took the unit apart but didn't see anything glaringly obvious wrong with
it. The element shows 14.7 ohms but I don't know what else to look at
so I figured that even it's a long shot I'd ask here hoping for a
service manual or someone that has some experience with these units.
You might wonder why I am "wasting" my time on a hot plate. Well, these
things aren't exactly cheap at nearly US$500. Hotplates capable of a
very consistent/stable 1000 deg F (550 deg C) don't come cheap.
--adam
Hi, Adam. The technical literature download doesn't have a schematic, so
you're going to have to do a little detective work. It's definitely worth a
bit of time -- you've got a good lab hotplate. I'd spend some time trying to
get it going. And the 14.7 ohms for the heater itself is also a hopeful sign
-- that comes close enough to the 1100 watts rated for 120VAC, which means you
don't have a burned-out heater.
You didn't mention whether the amber heater light was on. If not, I would look
at fusing first. Also check around for a thermal cutoff, probably present
somewhere in the enclosure, which will fuse in the event of catastrophic
overheating.
Another beginning point would be the thermal feedback to the controller. If
it's a thermocouple (likely) and the thermocouple is open, all heaters are made
to shut down to prevent thermal runaway. If you can find the thermocouple,
disconnect one end and ohm it out. If it's more than an ohm or so, you might
want to see what needs to be done to replace it. Another possibility is that
the thermocouple came loose off the surface it was measuring, leading to
thermal runaway and the cutout of the thermal cutoff. Also, I'm not sure which
model this applies to, but there's supposed to be an external plug which is
supposed to be plugged in if you're going to provide an external temp control
(I believe this is for an external RTD or T/C). Make sure that's plugged in if
you've got the socket. The lit says that always has to be plugged in if you're
not using it (it's a spare part if you don't have it).
Once you've looked at these first cut checks, you'll have to get a meter and
look at where the power is going. If the heater ohms out OK, and there's no
busted fuses, you're probably looking at a problem with the temperature
controller board or one of the connections.
The connections should be easy to check with an AC voltmeter. The voltage drop
will be where the resistance is. If you've got a bad electrical connection,
there will be a voltage drop. Usually these will occur over time, which means
you will see heat damage at the joint, splice, or connection. Play safe with
the cover off and line voltage applied.
Other than that, it's probably a controller board issue. Since it's a
resistive heater and made for continuous use, you'll probably see a triac
control, possibly with a series relay. If the problem isn't there, you'll have
to troubleshoot the controller board itself. It would help to know what you're
getting on the temperature display (nothing at all might indicate a power
supply problem, a dead microcontroller or a shorted component on the controller
board, a nonsense display might mean a problem with the microcontroller, and
all lines on the display (- - - -), OT or OL might mean a problem with the
thermocouple.
I haven't worked with that specific model, so I don't have any shortcuts there.
But the controller board is also a spare part, available from the
manufacturer.
I hope this general information is useful.
Good luck
Chris
