When listening at ‘home’ to one of the transmitters this morning, I heard it go off the air at 5:41am.  When we got to the site, Salema, the technician on call, was already there trying to get it on the air.  Earlier, some of the air piping had come apart and that was repaired, but now the Driver Bias voltage was not ‘ramping up’.

Initially, it looked like the Bias Supply circuit breaker was faulty, so that was rectified.  In the process of troubleshooting that problem, a 110 volt wire came in contact with the circuit breaker’s auxiliary switch, which created another problem that had to be traced and corrected before proceeding.  A resettable fuse for one of the 24 volt circuits had been destroyed and had to be replaced.

Back to the bias problem, we measured zero driver grid bias, but the supply had voltage going to it.  We decided removing the driver tube was the fastest way to check for a grid short.  When this was done, the bias voltage returned to normal, so a new tube was installed.  Eventually, the apparent circuit breaker problem returned and it was found that one of the wires bringing the 110 volts supply to the breaker had been inserted into a terminal block in such a way that the screw that was supposed to clamp the wire was clamping the wire insulation instead.  The connection would come and go depending on the position of the wire.  Now it should work — WRONG!

The transmitter would still not come on.  It appeared that there might not be enough RF drive, but the RF drive controller appeared to be working.  RF measurements from this transmitter were compared to one of the other transmitters.  The measurements were slightly lower.  After much searching, we found that the RF Drive controller voltage was limiting at about 3 volts.  This board was swapped out and everything returned to normal.  We found that the 15 volt regulator on the board had failed and another lower voltage supply was back feeding.  This gave the appearance of it working.  This regulator was replaced and the board was put back into service.

After all that has been done, we are not sure now whether the original driver tube was bad.  It may have been an intermittent breaker supply wire all along.  That driver tube will now be tested with a high voltage tester.  If it checks out OK, it may end up going back into the transmitter for testing.  If it passes that test, then it might be used for broadcasting again.

I spent the time after lunch locating the quarter power filament drawing and schematics.  There are a couple dozen files that I need to study before proceeding with troubleshooting the filament anomalies, which had been our original objective for today.  Maybe tomorrow!