Remember the Henry Hum thread(s)? Your help on that issue was greatly
appreciated. Several people chimed in and the answer ended up being in
a poorly designed filament choke. The stock filament choke was wound
on a toroid but it was NOT bifilar wound. Each lead was wound
separately on opposite sides of the toroid form, not interlaced in a
bifilar fashion. I replaced it with a conventional bifilar wound rod
type grounded grid filament choke and that took care of the hum.
Now after enjoying the amplifier in mostly AM service, it has developed
some more issues. Following a bad "squeal" that seemed to be coming
from one of the 3-500Z's I decided to investigate a bit. This problem
was intermittent but it scared the hell out of me. I wasn't sure If I
was seeing any flashing or arcing coming from the rf deck but then
again, I didn't wait that long to unkey. Reducing the drive a bit and
rekeying seemed to bring everything up normally. But it happened
another time a couple nights later and so I shut it down and proceeded
to pull the rf deck and investigate any visible signs of stress or
arcing. I should add that when previously observing the pair of
3-500Z's while keying up in what seemed to be a functioning amp, I
noticed one of the tubes showed quite a bit of color only ONE side of
the plates. Subsequently, I see that the inside glass on that one tube
has darkened on one side, the side that the plate was showing more
color.
See
http://www.flickr.com/photos/79319481@N00/4515561628/in/set-72157623715944749/
I'm afraid that tube has led to my problems, as I can hear a springy
sound when I handle it, as if something has dislodged inside. I missed
this during this initial checkout and put everything back together
after looking at the undersides of the sockets and my filament choke to
make sure there wasn't something going on with my previous filament
choke replacement but all seemed normal. So I put everything back
together at that point, re-installed the pair of tubes and zipped it
all up. When hitting the on switch the blower and panel lights turned
on, but no output when keying... hmmm... looked inside and notice the
filaments were not lit.
I then reseated the pair of tubes for grins but this time... BOOM...
overload relay tripped. Reset the overload relay and BRTTTDD
BRTTTDD... heavey vibration for split second and the 15 amp circuit
breaker (built into the amp) tripped with the overload relay.....
Following the troubleshooting diagram.... determined at this point
there was a HV short in the power supply. (The 15 amp CB keeps
tripping even without the HV plugged into the rf deck now.) Found one
"diode" in the FW Bridge rectifier to be shorted. The FW Bridge
consists of two HV rectifier blocks where each block represents two
diodes in series with 3 leads. With the rectifiers out of the
powersupply, she powers up without tripping and I do have filament
voltage at the sockets.
Looks like I'm needing a couple of replacement HV rectifier blocks and
some new 3-500Z's. I'm not understanding how a filament problem would
have blew the diodes in the bridge, since the filament voltage comes
from the filament windings on a separate secondary from the plate
txfrmer.
My line voltage is 242 volts not 220. The amp is indicating 3400 volts
on the panel meter and 3200 volts under a load. Should be fine but I'm
concerned about the filament voltage which is showing 6 vac instead of
the target 5 vac. (Checked with a VOM at the empty sockets.) Could
that have been the source of my sorrows? I sure don't want to spend
the bucks on the HV rectifiers and a new pair of tubes only for all
this to occur again after a couple months.
Any thoughts and tips would be greatly appreciated!
73, John KX5JT
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