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
_______________________________________________
Drakelist mailing list
Drakelist@zerobeat.net
http://mailman.zerobeat.net/mailman/listinfo/drakelist
_______________________________________________
Drakelist mailing list
Drakelist@zerobeat.net
http://mailman.zerobeat.net/mailman/listinfo/drakelist
