Date: Tue, 2 Feb 2016 13:17:54 -0200
From: Thomas Carlsson <thomascarl@gmail.com>
To: AMP reflector ADD MESSAGE <amps@contesting.com>
Subject: [Amps] Emtron DX3sp
Any help or comment would be appreciated.
The 6 x 100 ohm HV protection resistors burned some time ago and have been
replaced by same type as well as a 3A fuse put in series with them, to
change fuse instead of resistors.
During CQWW160 the 3A FUSEr started to burn, 1st time just changed it
but burned again. Checked the tubes and one had rattling grids, changed
the tube and tuned the Bias(es) and it worked OK for long time but after
the contest burnt again. Of course, this may be a flash over in one of the
tubes, but how to find out which one?
I have another indication that may affect this. Tuning on 160m is very odd.
At full or almost full drive (indicated by Ig2) Tune and Load capacitors
have very little effect, on 80m and higher, the tuning is normal, so I kind
of suspect that some of the Padding Caps has gone by west 470 or 1500 pF.
The 1500 seems to be 2 x 750pF and on one of them the green paint is
totally gone in a diameter of about 3cm around the terminal screw, this may
have been caused by heating. This may explain the tuning problem and also
may be - the flash-over. The cap may momentarily flash over causing hi-A at
the HV.
Any comment and thoughts are welcome, please.
73,
--
Thomas, PY2ZXU/SM0CXU
Member of the PW2D Team
## Are you padding the TUNE cap with a 470 pf padding cap ? Are you
padding the LOAD cap with a pair of paralleled 750 pf caps ? They really
should be NPO types. The pair of 750 pf caps could be replaced with
3 x 500 pf caps. Typ the lower the value of cap, the more RF current it will
handle.
## I assume the 6 x 100 ohm resistors are all in parallel, and in series with
the B+ ??
100 /6 = 16.67 ohms. 16.67 ohms is not much for a ‘glitch’ resistor in the
B+ lead.
It wont limit current very well. Use 50 ohms instead...with a fuse in series
with the 50 ohms.
A 50 ohm @ 50 watt wire wound.... or a paralleled pair of 100 ohm @ 50 watt
wire wounds
will do the job.
## Your main problem is probably one or more bad tubes. You could remove
just one tube, and temp block
the hole so no air leaks, and try one tube at a time. It may be one or both
tubes, it could also be just one
socket position. The padding caps on 160m band may or may not be part of the
main problem.
Jim VE7RF
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