Tom and fellow Drake enthusiasts,
Tom has made some good points about servicing the power supply. As long as you have it open, might as well replace everything that is reasonable for a supply that is 30+ years old. When servicing the supply for the L4 (which is slightly different than the B model or L7PS), it is a good idea to replace all resistors, diodes, and capacitors. In my repairs/upgrades, I have found approximately a 75% failure rate of the 5K 7 watt resistor on the underside of the supply that is part of the ALC network. When it fails, it typically takes out the ALC pot through the high voltage that is applied across it. Replacing that pot on the L7 is easier than the L4B due to parts availability. As to the 50K resistors, I've seen about a 30-50% failure rate on these and when one fails, it does not take out the ALC pot, but still needs to be replaced. As to "bleeders", there are two parts of the supply that serve this purpose to varying degrees. One is the string of two 50 K 50 watt Dale resistors and the 5 k 7 watt resistor (I always use two 10K 10 watt resistors in parallel for increased reliability) on the underside of the supply. Note that there are articles out on the internet that instruct you to disconnect the 50K resistors to reduce heat buildup in the supply. While the heat is reduced, you get poorer supply regulation and you have introduced a potential safety hazard. Proceed at your own risk. The other part of the bleeder function is via the equalizing resistors across each of the electrolytic capacitors. When turning the amp off, if you watch the plate voltage meter you can observe the bleed-down of the electrolytics which typically can occur within 10 seconds. If it takes longer, it suggests one or both strings of resistors in the two bleeder systems might be faulty. Failure of any of the resistors should be corrected for improved voltage regulation and safety. But be aware that a fast bleed down can also be indicative of leaky capacitors. There are several cost effective ways of replacing the electrolytic capacitors including a method described on the WB4HFN website. I always use 330 mfd styles, instead of the 200 mfd originals, for improved regulation and easier parts availability. And when replacing the diode strings, it is always a good idea to replace the mounting boards as well. My recommendation for the diodes is to use 1 Kv 3 A styles. Note that some of the diodes that I've seen were only 600 piv 1 A styles, so spending a few pennies extra for the higher ratings is a good idea. As always, if you are unfamiliar with high voltage power supplies, are tired, annoyed, or whatever, stay clear of servicing high voltage power supplies. You can be killed in a heartbeat and we don't want to see the ranks of Drake enthusiasts reduced prematurely. Enjoy those Drakes. 73, Evan, K9SQG -----Original Message----- From: Tom Delano <[email protected]> To: Mail List - Drake Gear Drake Gear <[email protected]> Sent: Sun, Aug 1, 2010 6:36 am Subject: [Drakelist] A Drake L-4B/L-4PS Cautionary Tale Greetings Drake Aficionados, I have been working on refurbishing and updating a 1972 Drake L-4B since late March as time permits. This L-4B was a "closet find" and had not been in use for many years and has an unknown history. The L-4PS was updated in April with the Heathkit Shop L-4PSR power supply board and a new R11 bias resistor was also installed. At that time, I decided NOT to replace the two 50K 50W bleeder resistors (R9 & R10)as they checked and looked OK and figured I could replace them later on. About three weeks ago I noticed the ALC stopped working and the Plate Amperes meter was "sticking" on transmit and of course the components for both of these circuits was the bleeder resistors and bias resistor. Yesterday I finally got around to replacing the 50K bleeder resistors and in the process of replacing them, I found that R9 had opened up entirely! Replacing both R9 & R10, and also the fuse resistor (R12) for good measure, fixed both problems. If you are interested, the replacement of the 50K bleeder resistors can bee seen on my web site, www.w1cc.net, under the Project menu under L-4B. So, I was very lucky indeed that there was no damage done to the L-4B when R9 opened and wish I had taken an extra 20 minutes to replace the 50K bleeder resistors three months ago. Just as you shouldn't trust 38 year old filter capacitors and diodes, you shouldn't trust 38 year old bleeder resistors! 73, Tom, W1CC _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
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