Amp - people.
I have been working on a TL922 at my Barbados QTH for some time and have
some observations and some questions for this esteemed group.
A while back, I was using the amp and it was working fine on several bands.
Upon a QSY to 21 MHz, there was a violent commotion during tune up. A fuse
blew and the grid choke at the base of the tube had clearly melted and had
opened up.
I returned to the US, got some spare parts and replaced the grid choke and
tried again. It worked well on 40 and 10 as I got on the bands and worked a
bunch of guys. When I tried to tune up on 15, there we a big bang again and
the grid choke on the other tube had apparently failed.
Once I again, I got some parts and attempted a repair. This time I decided
to fix another problem that I was neglecting. There is a binocular ferrite
bead on the output of the filament choke between the tube sockets. Due to
lots of shipping, this part had been broken for years but I decided to get
the right part and fix it.
After putting in the new choke and ferrite bead, the amp seems to work
perfectly on all bands. To be fair, I did not do any long term testing but
it clearly is better than it was.
My question pertains to what really was the root cause (or is it possible
that I really did not fix anything). Was the missing ferrite bead that
important to the stability of the amp.
As another piece of data, the failed grid choke I took out was not open. It
consisted of two windings held in place by glue on some type of ferrite
form. It was clear that the part had gotten hot enough to melt the glue
causing the neat windings to slide together and unravel. This amp sits in
very high humidity year round and was I curious whether water could be
absorbed into the windings or the ferrite material making them lossy. It
just seems odd that one would fail then the other, and then everything would
be OK.
Any thoughts would be welcomed.
73, Tom W2SC 8P1A
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