back inlet open sensor on 7890B GC

Question

Hello, 
 We are having an issue with our 7890B GC back inlet. The inlet temperature and flow were both set to "OFF," but the back inlet kept heating up while were were trying to run the front inlet (in GC-FID mode). This resulted in a thermal shutdown, which turned off all of the heated zones. In the back inlet configuration, I set the IGNORE READY to TRUE, and restarted the GC. However, we continued to get the same error. We have now unplugged the heater to the back inlet, which is letting us run, but obviously has not fixed our problem. I cannot find a part number or instructions to replace the back inlet heater. Can anyone point me towards this information? Or towards an alternate solution? 
 Thank you!

paulsalverda · Accepted Answer

If you are comfortable changing that heater/sensor you'll be fine as long as this is a split/splitess inlet. The MMI is different, of course. 
 From memory... Unplug the GC. Remove the column. Remove the insulation cup (always use it!). Remove the gold seal. On top, remove the insert weldment - the septum nut/liner/inlet supply and septum purge gas lines - from the top of the inlet and place it out of the way. Loosen and gently pull away the copper split vent line - if it's never been replaced, now is a good time to replace it with a new piece of copper. Remove threeT20 screws that hold the inlet to the mainframe. Follow the leads and unplug the heater/sensor cable. Remove three T20 screws that hold the insulation retainer on and carefully pull that cup and the insulation off. It's refractory ceramic so wearing a dust mask is prudent. Breathing ceramic dust is not horrible, but not smart, either. Remove the large nut on the bottom of the inlet that holds the heater/sensor and block to the inlet. The heater/sensor and block should slide off. It sometimes doesn't and is stuck. If that happens do not try to use a bigger wrench! Sticking it into the freezer for 1/2 hour often helps. There's a screw to retain the heater in the block, remove that, slide the heater and sensor out and go backwards to reinstall it. The sensor is fragile! Do not wiggle it around when sticking it into the block. 
 Before you put it back together, though, you can clean the injection port weldment. Do not use anything abrasive or with wire bristles, use a nylon brush or cotton tipped swabs to scrub the inlet body and a smaller brush to scrub the side-arm - the split vent connection. 
 Again, this level of repair is typically performed by an Agilent Field Service Engineer. Be careful and good luck! 
 I bet that if you use a voltmeter set to ohms and measure from each heater lead to the heater case, that there will be some resistance. The voltage goes to ground through the heater body and the inlet to ground and runs away. Other than an open heater that isn't heating at all, that would be the second most common heater failure.

back inlet open sensor on 7890B GC

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