Hello Patrick.
Before you unlace all the harnesses, disconnect the plate current
meter. Then make ohm meter reading from the wires to ground. If one of the
wires is above ground then try to find the other end either with the ohm
meter or perhaps a toner (cable identifier device). Also see if the power
supply still has a path to ground. Try to determine if the meter is
actually a high ma meter with internal shunts or perhaps the shunt is
actually in the negative lead of the power supply physically located in the
PS chassis (not near the meter). I bring all of the negative wires of my HV
supply to a central above ground connection buss. Then I place a low
resistance wire wound from that buss to chassis/common. The small voltage
drop, across the wire wound low ohm resistor, provides a metering point that
can be used to measure current from the power supply. If the meter should
open up or become disconnected the power supply is still working and the
bleeders still work.
Hope this is of some help.
John, WA5BXO
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
On Behalf Of patrick jankowiak
Sent: Sunday, March 28, 2004 12:08 AM
To: amradio@mailman.qth.net
Subject: [AMRadio] KW restoration and need a bit of help on this one
Well I have about finished up the KW rig. Seeing the HV meter rise to
3700V and having nothing explode was very rewarding. The HV is
adjustable by two variacs and I will run much less, probably 2500V.
(http://208.190.133.201/tuckerkw/tucker_transmitter.html)
I have a strange issue however. I hope the wisdom of the group will
help me here.
1. I note that the modulator plate meter is in series with the B+ lead
to the mod xfmr, and has the high voltage wiring to prove it. I can
crank the HV up to 3000V and play with the modulator bias and see the
meter work. This point is made to contrast with the next point.
2. The final current plate meter is more mysterious in its operation.
It has only the regular 600V wire, and the leads disappear into a part
of the wiring harness I have not investigated as of yet. The meter
does not move, and to be honest I have not yet a safe way to observe
the plate current of the final.
3. I did reduce the bias enough to see some screen current, (bias=100V
G2=500V Ig2=50mA Ep=2500V), in an attempt to see if I could get a bit
of color on the plate by burning up maybe 300W or so there. No such
luck with the color, I do not dare to fiddle too much with this
without a working plate current meter. I know that I have HV on the
plate because I have used the final to slightly load the B+ and
demonstrate that HV current is being drawn. The final's filament CT is
grounded at all times. the unit is keyed by the final screen and the
HV. Setting the screen volts and bias to zero results in a faster
bleed-down of the HV when unkeying the rig than when bias is kept on
the final, so I know it is drawing some current. (HV filter=40uF)
4. Ok, so now we would expect that the plate meter must be wired into
the negative side of the B+ supply, and this would account for the low
voltage wiring etc. Unfortunately this would not be the case, as the
final's filament center tap is grounded, as is the B+ supply's
negative side. The design does not seem to follow any of the usual
schemes presented in various volumes of the ARRL handbook or the Radio
Handbook.
5. Both sides of the plate current meter have a very low resistance to
ground (about 1 ohm, not too easy to measure).
6. so.. any ideas how this might be wired up? It's very strange and
does not behave as expected. My only other avenue is to further
investigate (un-lace) the wiring harness and see where the leads go.
7. Another topic: Do mercury vapor rectifiers require a certain
minimum current to remain in the conducting state? I only see them
glow during the power surge when I key the unit and the filters
charge. During the test described in "point 3" above, the screen
current would suddenly jump from 0 to 40-50mA accompanied by a drop in
high voltage from 3000 to 2500, and I adjusted the final bias.
Adjusting it back to reduce the indication of current resulted in it
sudenly shutting off to 0mA. Thus is strange behavior. It is doubtful
that it is self oscillation as the plate and grid circuits are not
tuned alike, and also i moved the frequencies well away from each
other and repeat this reliably.
Any comments and wild speculations will be welcomed!
Anyway I got tired and quit for the night. I must remain alert when
working on this dangerous equipment.
Patrick
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