Very Strange Oven Problem on HP 6890

[eatlas]

Troubleshooting retention time problems finally led to checking the GC oven. I found it off by about 20% + 23C, eg. setpoint reading = 100, actual=144C. I tried power cycle the GC, and power off with clear button pressed on power up. Tests passed but oven remained way off calibration. Agilent suggests multiplexer on main board may be problem, and maybe main board needs replacing. Anyone have any other suggestions?? I have never seen this in many, many years of GC experience.

[Consumer Products Guy]

Oven temperature sensor? I just replaced one on a 6890, that fixed it. On mine, the oven fan wouldn't engage because of a fault, and the sensor reading was way off (much lower than actual, in my case).

[Johnny Rod]

Definitely try a new temp sensor. Diagnosing a multiplexer is virtually impossible in my experience, and is also expensive to fix, plus if you do it yourself you risk losing the spare parts warranty on an expensive thing (if you so much as take the mainboard out of its packaging the spares warranty is void, nice).

[AICMM]

Not to confuse the issue, but I am going to confuse the issue because of curiosity. If the temperature is consistent, then it should hit the same RT each time, no? Probably not consistent though, eh?

Best regards,

AICMM

[eatlas]

To AICMM,
Since finding this problem the other day, we translated our regular oven temp program to the "new" temp calibration, and the results so far have been very consistent. However, past tests have shown some big step changes in the retention times that I now assume had to do with the change in temperature control. Other small drifts were also observed in the past, so we will continue to monitor while we wait for a new oven T sensor to try.