Hello Jim,
This is something that should be easy enough to do and does make some sense.
Will wait till next week to deal with it as I will be very involved in
the WPX CW contest this weekend.
Thanks.
73 de Steve, NR4M
On 5/28/2015 14:50 , Jim Hargrave wrote:
Steve,
It sounds like a bad connection on the variable capacitor. It
could be external or inside the bottle.
Depending on the construction of the capacitor, you may or may not
be able to fix it. In some construction they use finger stock on the
variable shaft. I have had some success in positioning the shaft
facing up and spray a little Tri-flow lubricant on the shaft and
rotate it back and forth until it goes In a little.
73, Jim - w5ifp@gvtc.com
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of
Steve Bookout
Sent: Wednesday, May 27, 2015 6:47 PM
To: amps@contesting.com
Subject: [Amps] Strange behavior (at least to me)
Hello all,
Since it's gotten quiet here, i thought I would get the
collective 'gray
matter' working again.
I am having a strange problem with a homebrew 8877 amp. It has
worked almost flawlessly for 30 years with the same tube and
still
easily puts out legal power. 4000 volts at a load of about 550
ma.,
driving with about 22 watts.
During the night shift in CQWW (I believe) the amp started acting
up
and was replace with a spare. It is a 40 meter mono band amp.
I was told that there was no output but 'the meters still were
'jumping around''. I opened it up expecting to find an obvious
issue
with the
vacuum relay on the output. Power has to go SOMEWHERE, right?
All
looked as good as it ever did.
Just before Dayton, myself and two budddies, fired up the amp,
after once again looking inside.
This is what I am seeing:
We tuned it up with 10 watts drive into a oil filled dummy load
and
got
700-750 watts out with a few hundred mils of plate current and
about 10 mils of grid current. Unkeyed it and then rekeyed it.
Upon
rekeying, the 100 mil grid meter slammed and the grid trip
circuit did
it's job.
I have it set for 75 mils of grid current. Wound up reducing the
grid
capacitance so that the max grid current would be more
reasonable.
After trying it on/off again and again, when it goes 'nuts', the
grid
current stays on the scale at about 60 mils and about 'just a
little bit'
when it seems to conduct normally. When it acts up, if I turn
the
TUNE vacuum variable just a wee bit, I get my plate current as I
should and the grid current goes down to normal. My multi turn
counter on the TUNE cap is marked '0 to 100' on the knob skirt
and a
'wee bit' is from
88 to 96, just 8 marks, or 8% of one turn. This is on a 375 pf
vac
variable cap.
Taping on the chassis would make the problem come and go, but
not every
time and not easily. Try it again and I would have to turn the
cap
back the other way to resonate it (seemingly.)
In my mind, it seems like something in the RF side of things,
since
changing the cap brings it back from the 'dark side'. Keying
the amp
with no drive and swinging the caps back and forth showed no
instability. I also took all the connections in the tank circuit
apart
looking for loose joints and corrosion. None found. Reseated the
tube.
Works fine every time till it doesn't.
It's doing the same thing as I think I would expect to see if I
manually de-tuned the tank circuit with the input cap. It also
showed plate current during the 'event', although lower than
normal, so where is the
power going? Quiescent plate current is about 120 mils.
Any ideas out there? I can supply pics if needed. Maybe I
should
record the event and put it on you tube with a link.
Look for some of you this weekend in the WPX CW contest.
73 de Steve, NR4M
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