Stew,
The situation you experienced is disturbing. The failures you experienced are strange from several perspectives. Why? Well, first, assuming that the power supply is wired correctly, it is unusual to have both bleeders open. When one experiences a failure mode of open, the series circuit is open and the other resistor has no potential across it, thus no power is consumed and it can't over heat, over current, etc. Second, it is unusual to have new bleeders fail that soon after installation. When replacing the resistors, inspect the mounts for the resistors. The older style uses long bolts with mica insulators at each end while the newer style uses aluminum triangles. If there is any arcing to ground, that can contribute to a failed resistor/circuit. Same for an old, brittle wire that might be arcing to ground. Contrary to popular belief, the 0.82 ohm protection resistor will NOT protect the supply from itself for the most part, just excessive current in the RF deck and/or a short in the connecting cable. Though unlikely, ensure that the line voltage for the amp is not excessive which can stress all components, especially the tube filaments. Whenever I rebuild supplies, I always use one or two fans on top of the supply for cooling purposes. I use "hand selected" model numbers for 220 v that will start and run reliably on 110 vac, or two 110 vac fans in series, or two 12 vdc fans in series; ultra quiet and reduction of heat is dramatic. Note too that when one of those bleeders opens, it will sometimes arc for an extended period of time before completely opening. The light and smell are unforgettable. Please check the 5K 7 watt resistor on the underside of the supply to see if it too has failed. Please keep us posted. 73, Evan, K9SQG -----Original Message----- From: GALE STEWARD <[email protected]> To: Dino Papas <[email protected]>; Mail List - Drake Gear Drake Gear <[email protected]> Sent: Mon, Feb 20, 2012 10:29 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 <[email protected]> To: Mail List - Drake Gear Drake Gear <[email protected]> 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 [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
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