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Transmitter Chaos

R

Robert Bass

I experienced bizarre behavior with a transmitter, during a sports broadcast this evening. The filament remained on, but the plate kept rising and falling like someone was continuously flipping a light switch. It’s a 26,000 watt total power output unit. When it is operating normally, plate current is around 3.00 amps, and plate voltage is usually in the 10.00 +/- kV range. When the problem occurred I observed low plate current under 2.00 amps along with high plate voltage shooting up to around 15.00 kV. When it hit that range, that’s when it would shut off the plate. The problem kept repeating for about 8 to 10 minutes, before it finally settled down.

Sometimes this sort of thing happens if all three phases of AC power drop to around 113 volts, but each phase was around 118 volts on this occasion.

Any clues? Preliminary tube failure maybe?

Thanks,

R
 
Would be helpful to know make & model of transmitter. Plate voltage going high and current suddenly dropping, sounds like a loss of drive to me. Exciter/IPA working OK?

On the line voltage, did you mean 218 volts?
 
It is a Harris transmitter, but I don’t recall the model. It has been 10 years since I last visited the transmitter room. That’s a job for the engineers. ;D I'm trying to figure out how to accurately describe what I was seeing, for them. :)

R
 
I'd look at the phase loss system. If it is mis - set, it will drop when a phase goes to low voltage. once the load of the transmitter is removed, the voltage rises above the trigger point. The transmitter turns itself back on, and as the power comes up the voltage sags a bit, repeating the cycle.
We used to see this occasionally when the plant went on generator; and at a plant which shared a power feed with a huge welding shop. Their spotwelder would do Bad Things to the waveform and voltage from the power company when it was running.
 
This is not meant as a put-down or an insult, but why do you not just call your Engineer(s)? There's too little information in your post to get any real idea of what's wrong. A lot of things can cause what you are experiencing.
 
I find these interesting intellectual challenges. Especially when whomever repairs them posts what the problem actually was. You don't got to play if you don't want.
 
It sounds like you may have a problem with one of the filter chokes or with the wiring for a choke or capacitor in the plate supply. If one of the filter components gets disconnected, the voltage would go up.
It may be something as simple as a loose connection. Whatever it is be careful and use the grounding stick on all of the components before checking anything.
if it is a connection problem on a component, it would mean that it would also be disconnected from the bleeder resistors in the supply and hold a charge.
 
Harris HT-30? Sorry, I assumed from your first post that you were in front of the transmitter when all the fun was happening. My bad. From the symptoms I would guess l-j was closer to figuring out the problem.

I am getting acquainted with our three HT-35s (and just bought another one) and haven't quite learned all their tricks yet.

If you are gonna hang out with us armchair field service types, you're gonna have to visit the transmitter a lot more often ;)
 
What amazes me is that if the Ep spiked to 15KV, the doorknob cap at the PA cavity input didn't pop. I've lost count of the number of them I've seen "explode" at less than 10KV...at least until I replace them with the next higher voltage rating.
 
What everyone posting here about this needs to keep in mind, is that Mr. Bass is NOT an Engineer, based on the information posted. Remember, he said he was looking for a description to give to the Engineers. A lot of this talk is suggesting that Mr. Bass is going to find the problem himself. This is a very bad idea for someone who hasn't even looked at a transmitter in ten years. I would suggest he not even open the doors to the transmitter. Nothing to be gained, and everything to risk.

If this assessment of Mr. Bass is incorrect, my humblest apologies. I'm going by what I've read here.
 
thebeaver32 said:
What everyone posting here about this needs to keep in mind, is that Mr. Bass is NOT an Engineer, based on the information posted. Remember, he said he was looking for a description to give to the Engineers. A lot of this talk is suggesting that Mr. Bass is going to find the problem himself. This is a very bad idea for someone who hasn't even looked at a transmitter in ten years. I would suggest he not even open the doors to the transmitter. Nothing to be gained, and everything to risk.

If this assessment of Mr. Bass is incorrect, my humblest apologies. I'm going by what I've read here.
Click to expand...

You nailed it. I was looking for info to give to our station manager for him to forward to the engineering folks, as per our normal procedures. Since I've only witnessed this problem twice, the last incident occuring about six weeks ago or so, I am guessing it doesn't happen enough for the engineers to actually witness it. I should have stated that i was observing the akward readings at the studio via the AutoPilot software. My bad I guess.

Thanks,

R
 
Cheers. Now you got several good directions for the manager to pass on to the contract guy. How about making us aware of what he finds?

thanks!
 
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