Harris 20K Kaboom after cleaning

[ChiefEngineer]

As big bangs are concerned, we used to change the tubes on a regular basis in Evansville on a 20K. ANY activity in the rf cavity would result, when power up took place, a 60 second period of operation followed by a large kaboom.

When we cleaned the PA cavity the result of moving all the electrons around into a new jetstream configuration different from the self generated stream was believed the culprit.

I never ever saw this on a 20H which is the same cavity.

Now sometimes the kaboom would just require engineers to change their underwear. Other times it would kaboom, fault, and take the transmiter down.

We ended up blowing the cavity out and not using spray cleaners on the surface parts. And yes it was always dry when we cleaned with cleaner. Even without the cleaner it kaboomed.

Harris would always say they had never heard of this. I watched this thing for most of a decade and got used to the quirk. Anyone else see this?

 

[fm-engineer]

Back in 94 I cleaned a Harris 10K that was filthy. After cleaning, kaboom, it would not stay on the air. After messing with the thing for a while I found the HV rectifier circuit had been wired with strands of solid #12 romex wire. When cleaning I must have bumped them closer to the cabinet. Replaced the wire with triple jacket turbo cable and it ran fine after that. Whatever kaboom you are getting is HV DC going to ground, probably in the plate voltage circuit. Cleaning may have pushed a wire too close to something. Those old HV wires can get pin holes in them, especially when in bundles. You have to cut the wire ties and sepearte them out to trace them. The old Rockwell Power Rocks were famous for bundling HV cables developing arc pin holes. For smaller kabooms, I've found pin holes in the bottom of old filter caps that harris mounts directly to the chassis. 8)

 

[chrish]

Had several FM-20's both K's and H's of various ages and had this problem several times......scared the .... out of me the first time it happened and at times would blow the main 240 volt breaker for the
plate supply.
It always turned out to be a severly dried out and cracked RG-11
cable used to feed the 9.3 KV plate supply voltage from the power supply cabinet to the PA tube in transmitter. The problem was always
found along the rear of the PA cavity accessed through the small
removable panel on the front of the transmitter. As the
series FM-25-K's and HT-20-25-30-35's age, I suspect this same problem may show up.
Let me know if I put you on the right track

 

[Tom Wells]

Harris would always say they had never heard of this.

When I worked for Harris Graphics, I occaisionally was in the office when the service manager was on the phone with a customer with a press electrical issue.

When I heard them say those words, I always made faces at them and rolled my eyes.

Better to say nothing and LISTEN to the customer than pretend "issues" don't exist.

 

[ChiefEngineer]

My "last" case oif Harris bull was the Gates 1.

4 stations installed them at the same time and I did all their work. Day by day i was replacing a thermal component at the transmitter rear. I became quickly adept at bypassing the part.

I got a newbie on the phone and when I asked about all the other stations having problems he indicated no one else was having this problem. I told him I had 4 that had the problem and he better find a new (verb or noun you decide) answer. Silence. Red face. "Let me doi some checking".

In the next month all 4 got new components that didn't fail after 2 weeks. This is on new transmitters.

With the 20k we never found a problem. AM noise was fine and no supply issues. It just ran fiune after the first explosion....

 

[BobOnTheJob]

If the transmitter is not running at full power, tap the plate voltage as low as you can & still maintain full TPO. From experience, less HV = less arcing. Had a CCA that popped plate blockers once a year at 8KV. At 7.1 KV, the current blocker has been in there for 25 years now. A little less HV makes for a happier transmitter.

 

[chrish]

Not a good idea to drop the plate voltage lower than 9200 or so, it is the sweet spot for this transmitter and tube, in some cases audio
fidelity will suffer especially if you are unfortunate enough to be on a harmonic laden "wye" connection rather than the superior closed delta
feed. If you look at you original Harris test data all FM-20 series
transmitters went throught the test bay with the power supply tested
at 240 volts Delta though the transmitter cabinet was tested at 208 wye. I brought this to Harris's attention when complaining that the
performance results were much different in the field on real world
urban AC with severe harmonic content.......Remember 3 phase transmitter power supplies are filtered only at 360 hertz.......what happens to all the other harmonic trash......thats why these days
using a wye is asking for trouble, a closed delta circulates the harmonics keepinging them from entering the DC.

 

[littlejohn]

Anyone who never heard one of the 'trombone line' 20KW boxes BANG simply never listened to many for long. There are some fierce voltage gradients in that design; and some nooks and crannies where dirt cen build up close to some of the higher voltages.
My fave response? "Never hard of that happening before. Service Kit #5 ought to take care of it though." On a 25K. Same transmitter, 3 in the morning, and the tech gerbil has >finally< got one of the engineers on the phone. "It can't do that." "Hey, buddy, I'm standing in front of it and it is doing that. And I didn't shake me out at 3 ayme to shake you out at 3 ayem to tell me somethiong blatantly incorrect." They have improved some over the years - as has the product.

 

[RadeoEngineer]

I always loved the "never heard of that one before" response. I would usually say something like "Wow! I must really have a lemon box here!"