5890 Series II - heat coil remains on when oven is cooling

Discussions about GC and other "gas phase" separation techniques.

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I have an HP5890 Series II with a 5972MSD. It appears that the oven coil remains on when the GC program is attempting to cool it to 45C. As a result the oven only gets to around 65-85C. I have also observed significant temperature fluctuation when an oven program reaches around 200C. This fluctuation ranges around -9C to +6C of the program temperature, even when being held at an isothermal state.

Actions and observations:

I tested/swapped the temperature sensor. Both appeared to be functioning correctly and were measured to be around 105 ohms at room temp.

The heating coil works fine.

The oven flaps open and close completely, oven motor appears fine.

Swapped out the power PCB when considering it may be a stuck relay, but had the same results.

Any ideas? Main PCB issue?
Have you checked that the flapper motor is working properly? Just opening is not enough, they need to open all the way. I have had these symptoms in the past and it's always been the flapper motor. If it doesn't open completely at the end of the chromatographic run, the oven won't cool fast enough. It's a little involved but you can do it yourself.
When I read this I also suspected the flappers, I've had this issue myself. I've changed out the flapper motors on 5890 and 6890 systems (different part numbers), even riveted a flapper disc back on when the weld failed.
I've got some used 5890 parts and circuit boards available from working units that were taken off line and discarded, if you're interested. For any circuit boards, please provide the HP part number.
If the flaps are not closing completely usually cycling the power fixes it. If it is the flaps slipping on the motor shaft then the set screws need to be tightened.

If the heater is really staying on when it should be off, then that is probably a problem with the main board relay not turning off.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
If the flaps are not closing completely usually cycling the power fixes it. If it is the flaps slipping on the motor shaft then the set screws need to be tightened.

If the heater is really staying on when it should be off, then that is probably a problem with the main board relay not turning off.


To resurrect an old thread, I'm having this same exact issue myself. My flappers are working fine, and I am starting to believe that my heat coil is not turning off. Is there any way you could provide some more information about the main board relay not turning off the heat coil, and how I might be able to troubleshoot ?

Thanks!
James_Ball wrote:
If the flaps are not closing completely usually cycling the power fixes it. If it is the flaps slipping on the motor shaft then the set screws need to be tightened.

If the heater is really staying on when it should be off, then that is probably a problem with the main board relay not turning off.


Now I'm going to check my set screws. Maybe that is why I hear a rotational sounding squeak when the flaps are closing.
Once on a 5890, I had the metal flapper disc fall off, and riveted it back on.
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