Kevin, With the rf decks and supplies I've rebuilt I've found some common problems.? The bleeders have a failure rate between 60-75% in the supplies I've rebuilt.? In the older supplies, the diodes had marginal voltage and current ratings; I replace them with 1 KV 3 A as a better alternative.? Also, the 5K 7 watt resistor on the underside of the supply is problematic.? When it fails, it puts HV on the ALC pot which, for the L4B, hasn't been available for over 20 years.? I use a replacement that is 5K at 20 watts.? When I rebuild the supplies, I replace all resistors, capacitors, and diodes, plus the diode boards themselves.? The capacitors I use have more capacity and thus better regulation; please see the article I wrote on WB4HFN's Drake website for the technique that I use.
I also add a cooling fan to the supply to draw heat out the top.? I typically use 220 v fans that will start and run reliably on 110v.? They are so quiet that you can't hear them running and they really lower the case/component temperature and thus you have higher reliability. As to the RF deck.? A tube flashover, parasitic oscillation, and other problems can and do occur.? Typically, one or more of the RF chokes that grounds the grids will melt, or even open, due to the flashover.? These chokes are no longer available unless you order over 200 of them and are willing to wait 6 months for the production run; those are the details you'll find out when trying to order them from Mouser, Newark, etc. or even Bourns directly (they bought out Miller who made the chokes originally).? Replacing each rf choke with a 33 ohm 1/2 watt carbon (not wirewound, not carbon film, not...) resistor is an alternative.? Beyond that, one needs to look at the various components in the RF deck and power supply to see what might have failed, flashed over, etc. Please keep the Drake enthusiasts informed of the progress so we can all learn. 73, Evan -----Original Message----- From: Kevin LaHaie <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Sent: Wed, 10 Oct 2007 1:50 am Subject: [drakelist] L4B Power Supply Burning Smell Kevin LaHaie <[EMAIL PROTECTED]> made an utterance to the drakelist gang? ----------------------------------------------------------------------? Hello Group? ? A good friend had his L4B experience a tube short, and she went with a BANG! ? 1 tube went, but he replaced both tubes, and brought the amp back up. It is putting out power, but he is experiencing a burning smell from the power supply and there is a noticable distortion on his transmitted signal. I convinced him to stop using the amp, and to open up the power supply (urging he use extra caution to bleed EACH capacitor in case a bleeder had failed)? ? He has performed a careful exam of the power supply internally, and he could see no evidence of a charred or burned component.? ? In looking at the schematic, there aren't too many candidates that would fail and still allow the supply to still function to a degree. Diodes, capacitors, resistors, and not many at that! (Of course, there is the transformer - yikes!)? ? I am sure someone has had a similar experience with the L4B supply, anyone care to share it and what you found?? ? Thanks and 73? ? Kevin K7ZS? ? ----------------------------------------------------------------------? Submissions: [EMAIL PROTECTED] Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body? Hopelessly Lost: [EMAIL PROTECTED] - help in body of message? Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net? ----------------------------------------------------------------------? ________________________________________________________________________ Email and AIM finally together. You've gotta check out free AOL Mail! - http://mail.aol.com
