To: | amps@contesting.com |
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Subject: | Re: [Amps] Drake L7 in need of TLC? Advice please! |
From: | "Ian White, G3SEK" <G3SEK@ifwtech.co.uk> |
Reply-to: | "Ian White, G3SEK" <g3sek@ifwtech.co.uk> |
Date: | Sat, 24 Jan 2004 21:18:39 +0000 |
List-post: | <mailto:amps@contesting.com> |
M1AQV wrote:
Hi all, Iâm new here so please forgive me if these questions have been asked before!Don't know about that particular capacitor mod - the US guys will be able to help you with specifics about the L7. But if some of the capacitors are suspect... and they're all old... you'd be well advised to replace them all with a new set. What diodes and resistors should I use? I assume it is best to replace them all as a matter of course?Since you're in the UK, the following suggestions about power supply components may be useful. If you have to work on the rectifier/bleeder board at all, it would be worthwhile to strip it and replace all the diodes and all the resistors with modern and well matched components. All the components are easily available from Farnell (www.farnell.com/uk and they take credit cards). Since I have the catalogue open, numbers in [square brackets] are Farnell's order codes. Replace the 1N5407s with 1N5408s [3525405] which have a higher voltage rating and are the same price. Should the diodes be rated higher than standard due to the bigger capacitors?No... but you should seriously consider adding a step-start circuit if the amp doesn't already have one. Equalizing resistors need to be equal, and stable against long-term drift. Replace the 150K resistors with metal film power types, like [550310]. Note that these small resistors will run hot, so space them a few mm off the board. I have read about replacing the 0.82 ohm fuse resistor with a 10 â 15 ohm glitch resistor, what is the best one to use?10-15 ohms is a bit low, so try two of [106931] in series. These are 14W long-bodied, glass-coated wirewound resistors and are very good for 'glitch' service where they may have high voltages from end to end. I was told by the previous owner that one of the 50 k ohm 50 watt bias resistors shorted to ground (to one of the fixing screws?) and went open circuit â this was replaced along with the method of mounting.Not sure about these. Another worthwhile mod is to add a solid-state T/R switching interface, and speed up the antenna relay. There's a circuit on my 'In Practice' pages.
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