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QEI 10k arcing tube cavity

C

ChiefEngineer

QEI 10k. Station got it used and it had an unusual looking thing on the back of the tube cavity.

After a problem I looked closer. It was an Avery sheet protector taped in the area behind the tube in between the tube and the back door. Yes, a piece of plastic.

In the spare parts I found many nylon materials with an arc to ground. I replaced it but the arcing continued. RF was arcing from the output to the back panel to the tube ground. Placed teflon material in between the sheet protector and the tube cavity and it stopped.

Ideas?
 
Hmmm... is this an FMQ 10K? Did you get a VSWR overload?
 
If I remember correctly, the 10kw QEI had a lot of stuff crammed into one little red cabinet. Made for a lot of heat and arcing issues especially in high humidity areas.

RFB
 
Bill- SWR Overload did take place. Also had a plate overload.

It seemed the rf output was shorting to ground. Output section has teflon bolts that isolate the rf portion from ground. One of these was black as if it were part of the arc. Also found pieces of the sheet protector burned to a crisp.

It looked as if someone had spent time on the placement of the sheet protector.

RF - A friend worked for JAM who did many of the WJR Jingles. They incorporated the original WJR Logo from early (PAMS?) Jingles into their Jingles. Continuity. We would listen in Southern Indiana for the new cuts. Was Berkowitz there , still there? This is his transmitter.

The humidity may be a factor. This all reared as the humidity approached levels we haven't seen in some time. I hate to note lack of current activity in the arc regard as , well, you know what might hapen.

I found spare parts with this used unit. In addition to a bag full of teflon bolts, I also found rf portions (large teflon sections). Then there was the largter equally impressive burned parts.

My mythbusters test : Keep turning it on until you found the arcing. When I found it I looked to amke sure the plastic was equally replaced as it had been previously in the rf cavity. This didn't stop the arcing.

My fix was I placed a large teflon piece in between the sheet protector and the rf components and the hellachious arcing stopped.
 
Had a similar issue on the 107.1 backup transmitter in Danville a few years ago. The Teflon arced inside of the internal harmonic filter & as you described it, the RF out was shorted. When a point shorts, shove twice as much teflon in where it arced & eventually you'll solve all the most likely to fail spots. Poor design, but it's nice to not have to hang an external harmonic filter.
 
If the transmitter was originally on a frequency which was quite different from the current frequency, it's probable that the plate choke is the wrong value.
I had a similar problem with a CCA FM transmitter some years ago.
Also check the VSWR between the IPA and the PA. A bad (or incorrect value) doorknob cap can cause instability of the PA stage.
 
Transmitter is on an adjacent channel to current frequency. Now on 100.5. Originally on 100.3. No vswr on IPA.
 
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