MARK V QTBQ Card Replacement


Thread Starter

John Torres

Having a problem with our GE 7EA Mark V QTBQ. Device 39V-3A Vib sensor is alarming -.8 when the unit comes up to speed and stays in alarm. Changed out the accelerometer and swapped cables, no change. Does anyone know what I must do to change out the QTBQ with R,S,T redundancy? Don't want to power all 3 processors down. Is there a right way? Thanks.
I think you mean TBQB card, in Loc. 7 of <R>, and I think you mean velocity vibration pick-up, more commonly referred to as a "seismic" vibration pick-ups/sensors/probes.

I think -0.8 in/sec indicates an open circuit; or was it -8.0 in/sec? At any rate, I think a negative number less than approximately -0.2 in/sec indicates an open circuit. But you seem to be saying that the indication is good when the turbine is not running, and then goes bad when the unit starts and accelerates to FSNL?

Are there Diagnostic Alarms present when the pick-up is in alarm? Have you looked at the Prevote Data Display to see what the value of the input's signal is in all three processors? (I believe 39V-3A will be BB4, and I believe it will be connected to terminals 57 & 58.)

Have you tried connecting the pick-up you removed from the unit directly to the TBQB card and seeing what happens? (If I recall correctly, they are not polarity sensitive since they are little AC "generators" themselves.) If you do this at FSNL, and there are no other turbine vibration pick-ups in alarm, you can disconnect the wires of the pick-up from the TBQB and connect the "old", suspect pick-up directly to the TBQB and see what happens without any danger of tripping the unit.

If you can reconnect the old pick-up directly to the panel and the signal is "good" then you can probably say the problem is in the interconnecting wiring/terminations, not the pick-up or it's cable. Try to eliminate all other problems before resorting to replacing the card is all I'm saying.

To change the TBQB, the turbine-generator will have to be shut down. You don't have to power down any of the processors, but you will have a lot of Diagnostic Alarms while replacing the card. I recommend unplugging all the ribbon cables from the card before disconnecting the wires from the Phoenix TB (this to prevent shorting any of the wires together while they may have power on them). I also recommend making sure every wire has a wire number or some kind of marker to indicate where it came from or where it should be terminated; there can be a *LOT* of wires connected to the TBQB and you don't want to end up with one or two wires that can't be identified and no one knows where they need to be terminated. At the least, make a list of the wire number of each wire at each terminal before you disconnect the wires.

Sounds easier than it is; all those wires can be a real pain! And, don't forget to set the hardware ("Berg") jumpers on the new TBQB to the same positions as on the existing TBQB card, after writing down the as-found positions for safe keeping and future reference. I wouldn't trust any display or document to tell you the proper hardware jumper positions; if everything else was working properly before you decided to replace the card then the hardware jumpers were in the proper positions. I've seen far too many people think some document has the proper positions and changes the positions from the as-found ones and then can't figure out why they had more problems than before they replaced the card.

Once you've unplugged the ribbon cables from the existing TBQB, disconnected the wires from the TBQB, removed the existing TBQB, installed the new TBQB, and reconnected all the wires you can then carefully re-insert the ribbon cable connectors into the appropriate receptacles on the TBQB, and then resolve any Process- or Diagnostic Alarms which may be remaining after replacing the card.
You are correct, it is a LOC 7 of <R> seismic sensor. The sensor indicates fine until the engine accelerates and then blanks to -8.0 in/sec then returns to normal at shutdown. I didn't think to terminate the sensor directly to the QBTB card, I'll try that and see what it does. Thanks for your response, it did answer questions I had on replacing the QBTB card if it becomes necessary.