Would agree with you about the input AC power, but if you look at
the high voltage the high voltage return side is not tied to the PS
chassis but runs to Pin 6 of the power connector and then in the
amplifier where it is only tied directly to chassis when the
interlock switch is engaged. The interlock switch also shorts the
HV to amplifier chassis ground.
Under normal operation the high voltage return feeds through the
plate current meter and to the center-tap of the filament
transformer with a 1ohm 10W resistor to ground, this 1 ohm resistor
forms the voltage necessary for the grid current measurement as the
grids are tied to chassis and the voltage across this resistor is
only due to grid current. This metering arrangement for both plate
current and grid require that the HV return not be tied to chassis
in either the PS or amplifier.
Pin 7 of the power connector is used to tie the PS and Amplifier
chassis together. So when you are using just the supply the chassis
is at AC power ground but the high voltage return is floating with
respect to chassis. With the cable connected to the amplifier there
is the meter resistance in series with 1 ohm resistor to ground
which is the change that does result in the PS chassis referenced to
HV and therefore you can arc the bleeders if you do something stupid
like I did of not providing a good HV isolation mount for the
bleeders.
Playing with these units is not something to be done without a lot
of prior thought and making sure you really understand the entire
circuit and safety precautions needed. I was probably lucky in not
destroying the meter and other parts.
73 de K5MWR
On 2/20/2012 11:08 AM, K9sqg wrote:
As a side note, a properly wired L4, L4B, or L7
power supply chassis is ALWAYS at ground potential for safety
reasons. The safety ground from the outlet goes directly to
the chassis of the power supply, whether or not the RF deck is
plugged into it or not.
A number of years ago, there was a
production flaw in the Dale resistors (pitting, gaps, etc. )
in the ceramic that led to flashover on the ends of the
resistor.
Nit: the L4 power supply used two
100K 50 watt resistors while L4B/L7 supplies typically used
50K 50 watt resistors.
-----Original Message-----
From: David Box <[email protected]>
To: k3nd <[email protected]>
Cc: drakelist <[email protected]>
Sent: Mon, Feb 20, 2012 12:30 pm
Subject: Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
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<[email protected]>
To: Dino Papas<[email protected]>, Mail List - Drake Gear Drake Gear
<[email protected]>
Subject: Re: [Drakelist] L-4B/L7 Power Supply Rebuilds
Message-ID:
<[email protected]>
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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