Just like most of us...Age :-)
73,
Lee
-----Original Message-----
From: GALE STEWARD <k...@yahoo.com>
To: Dino Papas <k...@cox.net>; Mail List - Drake Gear Drake Gear
<drakelist@zerobeat.net>
Sent: Mon, Feb 20, 2012 10:30 am
Subject: Re: [Drakelist] L-4B/L7 Power Supply Rebuilds
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
------------------------------------------------------------
From: Dino Papas <k...@cox.net>
To: Mail List - Drake Gear Drake Gear <drakelist@zerobeat.net>
Sent: Saturday, February 18, 2012 2:22 PM
Subject: [Drakelist] L-4B/L7 Power Supply Rebuilds
I finally bit the bullet yesterday and got around to rebuilding my
L-4BPS and L7PS supplies, one with a Heathkit Shop board and the other
with the Harbach version. Both procedures went well, with the second
going a LOT faster than the first! I will be rebuilding a friend's
L-4BPS next week and figured I could minimize the time to do his by
working out the bugs doing mine.
As my L-4B is my main amp, and based on Evan's note to me a few years
ago, I went ahead and changed out the 5K 7W resistor with the 13W
version, replaced the 0.82 ohm HV line resistor and the 50K 50W
bleeders as a precautionary measure. In the other amp I guess I was
just lazy and simply installed only the Harbach board.
Now sitting here thinking about it should I go back and just get it
over with and replace the resistors in the L7PS? Has anyone ever had a
bleeder go bad? Or the others for that matter. Sure, age will wear
anything down.
Also, the Harbach board doesn't include the diode balancing resistors
that Mike's board does...as the original Drake diode strings used 'em
what exactly is the difference? Do modern (read that "better") diodes
still need that protection? I'm just curious.
I was pretty impressed that my 32 year old L-4BPS kept going like the
Energizer Bunny all these years and I didn't notice any deterioration
at all on the bodies of the orange electrolytics....but, better safe
than sorry and I'm looking forward to many more years of reliable
operation with both amps.
Thoughts?
Dino KL0S
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