Stew
Based on your description my expectation is that you had a voltage
breakdown of the bleeder resistor. While we normally think of resistors
in terms of the power they can dissipate, there is a voltage limit that
can be sustained across the resistor as well as from the body to
ground. I learned this the hard way when I had a transformer fail and
ended up replacing it with a P. Dahl transformer with a 10% higher
voltage. (3100 V no load, 2950 V load). In that case removed the 2 50K
bleeders and installed 10 20k 20W resistors mounted on standoffs, lost a
couple of those due to voltage punch through from resistor to chassis
before understanding the issue and properly insulating the resistor
bodies from the chassis. These particular resistors had about a 2KV
breakdown limit through the ceramic insulator to the resistor surface.
I don't remember what the actual voltage limit across the resistor was
but is the reason that I used the 10 devices in order to keep the drop
in the 300V range per resistor as well as keeping the power dissipation
in the 5W range, I think around 1K volts was the voltage breakdown limit
for the devices I had on hand, but not sure (voltage limit due to power
is 630V) . The original Drake 50K 50W bleeders obviously could sustain
in the 1500V range across the device, not sure what their rating to
ground was, but the power limit is at 1.581KV which I think is pushing
them more than needed.
The L4PS chassis is not at ground until you hook to the amplifier. So
if you have a resistor body that can flash over to ground it will not be
apparent until you hook up to the amp. My initial mounting arrangement
had the resistors heat sunk to the chassis which turned out to be a dumb
idea and was corrected by mounting the resistors on standoffs. I would
test the supply out on the bench, worked great then hook up to the amp
and experienced quite a light show as the resistors arced to chassis.
The above transformer change also required adjusting the bias level due
to operating point shift which was done by placing 2 5.6 V zeners in
series with filament center tap. This has been working fine now for 8
years.
73 de K5MWR
Date: Mon, 20 Feb 2012 07:26:18 -0800 (PST)
From: GALE STEWARD<k...@yahoo.com>
To: Dino Papas<k...@cox.net>, Mail List - Drake Gear Drake Gear
<drakelist@zerobeat.net>
Subject: Re: [Drakelist] L-4B/L7 Power Supply Rebuilds
Message-ID:
<1329751578.83763.yahoomail...@web110514.mail.gq1.yahoo.com>
Content-Type: text/plain; charset="iso-8859-1"
A timely question about the L4/L7 bleeder resistors. I recently had an issue
with one of my L4PS supplies. It turned out to be a bad cap which I replaced.
I'm going to install a Heathkit Shop board eventually but I wanted to get the
amp back on line for use in the ARRL CW DX contest. While the supply was open I
checked the 50W bleeder resistors and found them both open. I replaced both
with new Ohmite resistors.
Sunday morning at about 1230Z, one of the new bleeders failed in dramatic
fashion by lighting up like a 100W bulb. I quickly swapped in another L4PS and
continued on. Last night after the contest I opened up the failed supply to
find that the resistors had failed but all other components were all OK (no bad
caps, etc.).
I replaced the failed bleeder (again) and all was again normal.
I'm still quite puzzled as to why this resistor failed in the first place.
GL with the project.
73, Stew K3ND
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