Well I was all set to go to a remote transmitter site where I have been cleaning things up, documenting the setup and so on, then I got a phone call and I was diverted to my own transmitter. Seems it went down, we were still on cable as we have a direct feed but we were off the air.
When I walked in I could tell high voltage was not up as the sound was wrong. It had crowbarred and cycled three times and was down for the count. I went through the history on the transmitter’s LCD screen and found the first problem was a simple one, Forward Power too high and Drive Power too high. I think this happens when there is a power glitch. Anyway, most times you can just put it back 0n the air but when I tried this time if just crowbarred again, real quick.
Now when an IOT has been off for a while it can crowbar after high voltage is applied, it can do it a few times, don’t really mean much. But this one did not wait at all, just up and down, like it was a real short.
So I isolated the tube from the HV and tried again, and once again it came up and went right back down with a crowbar.Thats not good, well it sorta is, it means its not the tube ($$$$). Now I go to the high voltage power supply (HVPS) and open it up and look at the high voltage section, everything looks fine, no burns or arc marks. The transformer has taps that can be changed with a rotary switch, normally we sit on tap 4 so I lower it to tap 1. This will reduce the amount of high voltage and maybe it wont arc. But it does.
Now I go into the high voltage compartment in the transmitter, making sure I ground the heck out of everything, and put the load wire out of the Crowbar assembly, this isolates the crowbar completely. After the crowbar the high voltage goes to a current limiting resistor and the isolation switch, those resistors have been known to develop arc paths. If it fires now it means the crowbar is defective. I wrap the end of the plug in electrical tape as precaution. I apply high voltage and theres no crowbar, the circuit is good, but why was it firing?
I had this kind of thing happen before so I do what I did then. I killed the AC to the transmitter and ground all the high voltage,even in the HVPS and get out the cleaning rags. It gets pretty dirty in there, the high voltage ionizes the surfaces and the missing electron gives it a positive charge, this attracts dust. Dust can provide paths for arcing. So I use alcohol to clean all the HV wires and all the gear in
the HV compartment and a regular cleaner to do the walls inside, doesn’t take too long. Then I apply AC and let it warm up.
I apply HV with the tube isolated, it holds, no crowbar. I connect the tube and still no crowbar, we are still on tap 1 with the lowest high voltage, about 30,000 volts. So I let it run for a few minutes to make sure it will continue to hold. Then I shut down the HV and move to tap 2, run for a while and go to tap 3 and it’s okay. Finally I move it to tap 4, where it was and bring up the HV and after a minute or so it crowbars. Thats okay, it could happen. I reset the fault and put HV back on, and it holds just fine.
adjust bias and bring it back within it normal range.