Thermal Shutdowns on Agilent 7890a

[dscottltc]

I recently started receiving thermal shutdowns from my Agilent 7890 at work; the machine essentially displays "Thermal shutdown" on its screen, stops responding to console and Chemstation commands, and must be rebooted to continue. The machine tells me the thermal zone affected is the oven. Also, I've witnessed the error occur where all the other thermal zones are heating up (front, back det, etc.) and the oven isn't responding, and then it gives a thermal shutdown, so I'm almost certain that it's the oven that isn't responding. I called Agilent and they said I should purchase a new main board or a new power board, and couldn't tell me anything except what they cost (a lot!) 1000$ and 400$ respectively.

I read this thread here:
viewtopic.php?f=2&t=17205&hilit=thermal+shutdown
along with others from other forums. I checked the vents in the back of the GC, they work fine in cooling the machine down. I also mentioned the thread listed to my boss, and was wondering the following:

Has anyone else had success in fixing their GC by merely stretching out the heating coil and reconnecting it?
Is there a way to diagnose if it is the power board versus the main board? Perhaps by watching current flows via the info screen on the GC?
Is there any way Chemstation can be causing this error? We have always had problem with Chemstation from the beginning installation, and I am currently working on installing a fresh from format XP/Chemstation install to try and fix them (read it was fragmentation issues). The Thermal Shutdowns seem to happen far more frequently when Chemstation has the GC under remote control.

Agilent is of no help whatsoever, any and all info would be appreciated, thanks!

[cleh]

You said the oven flaps are opening when cooling, but make sure the oven flaps are closing completely when the oven is heating. If they aren't, the flap motor could be bad.
You can ohm out the oven heater at the connector (check it to ground, too) to avoid removing it. You may have a bad oven sensor and you can ohm that out at the connections to see if it's bad (check that to ground, also). Please turn the GC off and unplug it before you start working inside of it.

[aldehyde]

Is this happening frequently or just once every 28 days or so? There was a 7890 firmware revision that caused thermal shut downs. I would upgrade to the latest revision of firmware ANYWAY, but this may help your issue.

There are voltage test points present on the mainboard that you can use to check the voltage supply to the oven shroud.

[Yama001]

In cases where the problem is the oven heater coil has broken at the terminal, we have had some success stretching the coil on to the terminal - I recall it was very easy to break the insulators when tightening. We have gotten an extra few months to a couple of years out of an oven coil this way. This is not a guaranteed fix - a new oven shroud is best.

We had a rash of these problems this year - quite odd how many shrouds failed within a period of a few weeks.

[dscottltc]

You said the oven flaps are opening when cooling, but make sure the oven flaps are closing completely when the oven is heating. If they aren't, the flap motor could be bad.
You can ohm out the oven heater at the connector (check it to ground, too) to avoid removing it. You may have a bad oven sensor and you can ohm that out at the connections to see if it's bad (check that to ground, also). Please turn the GC off and unplug it before you start working inside of it.

I assume if the heating coil was improperly connected or fried the resistance would read as an open circuit right (infinity)? What would the detector read if it was fried, just infinity like a thermistor?

Is this happening frequently or just once every 28 days or so? There was a 7890 firmware revision that caused thermal shut downs. I would upgrade to the latest revision of firmware ANYWAY, but this may help your issue.

There are voltage test points present on the mainboard that you can use to check the voltage supply to the oven shroud.

This happens almost every day, and usually when one happens a lot of them occur. The current firmware version is A.01.09, so I will likely upgrade to A.01.14.

Thanks for all the info, I'll be checking this as soon as the machine is free for a bit.

[cleh]

I assume if the heating coil was improperly connected or fried the resistance would read as an open circuit right (infinity)? What would the detector read if it was fried, just infinity like a thermistor?

Yes, if the heating coil or heater in the detector or injector were fried or broken, the resistance would read open. The normal resistance reading of a cold oven heater coil should be 9.07–9.52 Ω (120 V instrument). The oven heating coil could also be shorting. It could be connected, but have a broken ceramic allowing it to touch the metal on the oven heater assembly. The detector and injector heating cartridges, oven sensor, detector and injector sensors would read open if they were fried or broken, but they could be shorted because sometimes the inslulation on the connectors wear and they touch the metal on the heating block or oven heater assembly. The cartridge heaters and sensors can also short internally. I can't find my list of heater cartridge or sensor resistance readings, but you should be able to get them from Agilent. Would love to know what you find.

[dscottltc]

First a thanks to all the replies here. Unfortunately I wasn't able to go through with doing a resistance check on all the components due to other considerations in the lab.

Second, we managed to get an Agilent rep out here. We decided to just replace the oven flap motor assembly for now, as it was the most likely candidate for failure. Their original suggestion was to replace the oven shroud, flapper motor, power board, and logic board all at once, since they couldn't diagnose the problem and the service call revealed nothing . My boss looked like he had a small stroke, but they offered to gratis the labor on a second visit if the motor didn't fix it, which definitely helped. No shutdowns so far (3 days), i'll keep you guys posted.

[Johnny Rod]

I've seen the flap motor/assembly jam 3 times on different on 6890s so it doesn't surprise me if yours has done the same. Which country are you in, out of interest? (re. Agilent service)

[three4tx]

Oven not cooling on my 7890a. seems power is still being supplied to shroud. set oven at 30' and will only cool to 55'. set oven to 60' and won't cool below 65'. checked resistance of shroud and got 9.5ohm. good. any ideas? power board G3430-60050. flappers are good. verified temp internally with probe.

[cleh]

Check your method to see if cryo is enabled. If it is (and you aren't using cryo to cool the oven) it will cause the flappers to close around 60 or so degrees (can't remember the exact temp) so the LN2 or CO2 can fill and cool the oven. A couple more things...what is the room temp? Have you moved the instrument to a different location with a wall or something behind the GC that could be blocking the air flow or moved something that could be blocking or impeding the air flow behind the instrument?

[three4tx]

room temp 25' . no cryo valve , no cryo plumbing, configuration says none also. instrument is been stationary.

[AZBiodiesel]

do you have a oven heat exhaust vent that is blocked? I am guessing not because you say you can see the flappers open and close...but if the unit can't dump the heat anywhere....

I have had the wire connecting the main board and the heating shroud short out and get fried black. But it was not able to heat to reach setpoint. There are a few cable connections to the main board I would make sure they are all plugged in appropriately.

[three4tx]

ok, well replaced the power board. surprised me but that was NOT the correct fix. I will start looking elsewhere .......

[three4tx]

behold, the oven shroud lines were arcing to the shroud body. BACK IN BIDNESS !

[thohry]

We have had this kind of problem on 6890. My case was bad flap motor.