[Amps] Drake L7 in need of TLC? Advice please!

[M1AQV <M1AQV@qsl.me.uk>]

Hi all, I’m new here so please forgive me if these questions have been 
asked before!

I have recently purchased a Drake L7 amp, which I was told was in good 
working order but after some examination I believe requires attention. 
Apparently it has not been used for 18 months and the seller was unable 
to demonstrate. I have not attempted to power it up at all, opting 
instead for a close examination to hopefully correct any weaknesses 
before they cause any further problems.

My findings so far:

Nine of the PSU diodes have been replaced with new 1N5407’s. I can 
clearly see where at least one of the previous diodes had blown leaving 
black marks on the adjacent filter caps. The remaining 5 diodes look to 
be originals. Also, three of the 150K ohm resistors are replacements

Some quick checks with a multimeter revealed one suspect shorted 
(original) diode – it measures 4K ohm across it rather than about 160K 
ohm I found across the others (all still in circuit).

I’m also concerned about the filter caps – 4 of them have what looks 
like small ruptures on the paper covered positive ends. Is this a 
problem? If they measure OK on a capacitance meter can I retain them or 
are they ready for the bin? If so I wonder if replacing them with 330 
mfd, 450 v Nichicon’s as per instructions on the following webpage is 
safe and/or recommended? –

www.wb4hfn.com/DrakeArticles/TechTips/ElectrolyticRelacements.htm


What diodes and resistors should I use? I assume it is best to replace 
them all as a matter of course? Should the diodes be rated higher than 
standard due to the bigger capacitors?

I have read about replacing the 0.82 ohm fuse resistor with a 10 – 15 
ohm glitch resistor, what is the best one to use?

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 – this was replaced along with the method of mounting.

Hopefully with the help and wisdom from some of you chaps I will end up 
with a reliable and correctly operating amplifier, so I’d like to thank 
you all in advance and anticipation! I expect that I'll have more 
questions as I discover more problems, but for now, 73!

Tyron, M1AQV



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