Hi Guys,
I'm in the midst of repairing a power supply in an Emtron DX3.
I'm after some other opinions and advice on what is going on with this
device.
The Amp was in use at the time of the Fault, and had been operating for
some hours on 160m in CW Mode, then was switched to standby and let idle
for some hours. The Operator then returned, keyed the Amp...and was
greeted by a loud band and that horrible smell that we all know.
I was given the task of repairing it...and after popping the Lid off,
everything looked ok. Further investigation and removal of the two EHT
Modules ( These have the Rectifiers, Caps and bleeders all on one
board...fairly tightly arranged ) came up with the source of the bang
and smell...ONE 400uf/450v Cap had blown its safety seal...about half
way down one Cap stack.
The power supply in this thing is simply two DX2 Modules in
parallel...eight 400uf/450v 85 deg C Caps with 75k equalizing resistors,
and a string of 6 330k circa 2W resistors and a 10k at the earthy end as
the voltage divider chain for EHT Volts measurement...all expected
stuff, excepting I would have used 10 Caps, and 100k equalizers had I
made the thing....and probably more voltage divider resistors.
Now...no blown diodes, and only the one 400uf Cap gone to God...all the
equalizers measure around 75k ( lowest is 69k and highest 74k...none
measure at the labeled value of 75k ). All the voltage divider resistors
measure around 290k, and the 10k at the bottom is about 9.8k.
Each B- ( or the bottom end of the Cap stack ) has a 1 Ohm 25w Resistor
to earth with three meaty type diodes connected across the Resistor,
Cathode to the B- Rail. This setup is in parallel with the same thing on
the other module...so effectively 0.5 Ohm to earth from the B- Rail with
two strings of three diodes in parallel.
What is worrying me, is that both boards have been very hot at some
stage...the solder around the voltage divider string looks as if it has
been molten at some time, and the tracks that join each voltage divider
sting resistor are burnt black...but the resistors themselves don't
appear overheated. It appears that the heat has been coming from the 75k
Voltage Equalizers, and has burnt the board that is in close proximity
to the Resistor.
But...it doesn't make sense to me...the dissipation in these Resistors
is not that great...75k times 8 is 600k across circa 2000v EHT...3mA
current flow, so there is not that much power being dissipated in the
resistor stack...so why is the board burnt ??
I am about to replace all the Caps, and put the thing back
together...but I thought I would run this past the Group...am I missing
something here...I just can't see why the board is so burnt.
Oh...I did try to just simply oder two more complete modules and just
replace the lot...no go, as no Stock ! I was advised to ring back in
about a month...glad it's not my Amp and its not under warranty.
So...I'm faced with cleaning up the boards and firing it up.
Any clues ?
73
Glenn
VK4BG
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