Dino et. al.
Well, let me briefly relate what I have observed and encountered when rebuilding L4, L4B, and L7 power supplies. Your mileage might vary. My experience has been that the supplies I have worked on have had in excess of a 50% failure rate, and maybe another 25% had whitish or light gray regions, typically toward one end, where there has been excessive heating suggesting and impending failure. Having a stock of the resistors on hand, and having the supply apart, it was a quick job to replace the bleederrs for safety and better regulation. Many people have failed bleeders and are totally unaware of it. There is generally better quality control with diodes nowadays compared to decades ago. Still, for as cheap as resistors are, it is a small price to pay for an added measure of protection, can't hurt. There are two schools of thoughts on this topic so one goes with the one that is most appealing. Final note on that 5K 7wt resistor on the underside of the supply, I too replace it during a standard rebuild and use two 10K 10wt resistors in parallel. Unknown to many, when the original opens, it takes out the ALC pot which, for the L4B I haven't seen in 30 except the ones I have in stock. Enjoy those Drakes, the Drake Nets, and our hobby. 73, Evan, K9SQG -----Original Message----- From: Dino Papas <[email protected]> To: Mail List - Drake Gear Drake Gear <[email protected]> Sent: Sat, Feb 18, 2012 2:24 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
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