An FM-30T I maintained used to occasionally arc from the large cylinder around and above the tube up to the top of the PA cavity. I found arc burn marks at the top of the cylinder and the top surface of the cavity. Also found holes in the bellows, but not sure if these were caused by the same type of arcs. The plate breaker would trip when it arced, requiring a trip out to reset it.
BE support said the chance of arcs can be minimized by setting the plate tuning slightly to one side of peak, but don’t remember which side.
From: John Stortz [mailto:email@example.com]
Sent: Sunday, November 18, 2018 10:26 PM
Subject: Re: [CRTech] Used Bellows for BE FM-30T
I worked on an FM-30T before retiring. The bellows used to get what used to appear to be random holes burned thru the thin bellows copper. BE was aware of the problem but said they didn't know why it happened.
It might be related to a problem connection in the output load coupling. The Load coupling includes a pivot, which must make a solid electrical connection between ground and the output coax.I discovered they had a small silver-plated ring of pressed wire as a spacer in each end of the loading coil. The grounded end was easy to replace. IIRC, it was nearly impossible to replace the spacer on the "high" end. BE's solution was to buy a new assembly but they didn't have any in stock. I took some copper braid from a short length of small coax, greased it with graphite grease & wrapped a couple turns of the braid TIGHTLY into the upper pivot space. Both ends were tied together to prevent the braid from loosening.
If this connection becomes intermittent, the transmitter will suddenly have the load removed & internal resonant voltages probably go to some extreme value when you have to send 25 or 30KW somewhere other than out the antenna.
Not much FUN in "FUN Size" chocolate candy
Does anyone know of a source for used transmitter parts for a BE FM-30T?
I need to replace the bellows and BE doesn't have them in stock. I am
waiting on information on delivery. But in the meantime, I might like
to install a used one if they are out there and not too expensive. Let
me know if anyone knows of good junk yards out there.
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