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Under temp Gas board VSM


Mar 9, 2014
Mar 9, 2014
Hi guys I'm after a bit of assistance with the attached schematic. It's for a gas board for a 40 year old furnace bank. We are trying to change the use of the tube from Phosphorous to wet Anneal and are needing to introduce oxygen and hydrogen into the process. For this to happen we are calibrating the board, we have successfully set the o2/h2 ratio setpoint, so as not blow the place up. The sticking point we have is with the undertemp setup.

The instructions ask us to set the setpoint with R4 to 31mv at a temperature of 650c. We are using a calibrated temp simulator to produce this.
We then measure the recorded voltage at chip U4 pin 4, record this.
Go to U4 pin 5 and using R10 set the voltage on pin 5 to what pin 4 was reading.
We then adjust the temp down and the voltage on U4 pin 5 changes and goes below what pin 4 reads. We move the temperature up and it reads above what pin 4 is reading. On both ways the undertemp light remains lit.

There is another pot beside R10 on the board that has something to do with the U4 chip but we aren't too sure what it does. As I've said this is a 40 year old board and I'm not the first one at it so who knows what adjustments have been made previously to any of these pots.

When we started to look at the board and calibration instructions it states that jumper 8 should be on 1-2 and if it is jumper 9 should be on 2-3 to avoid damaging the PCB. This was not done and it may have damaged the PCB. We have two of these boards with the same problem, I'm a bit skeptical both are damaged the same way.

We do have a working board in process but don't want to tinker with it incase we lose the settings on it and then have no Anneal tube and are worse off for it. When measuring the setpoint for R4 it's reading - 15v, so reckon its been poked and prodded at until they've got it to work eventually.

We will be sending the board to semitronic in Germany to test for defects but just feel we are missing something. Any help or advice would be amazing.


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