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

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

M1AQV wrote:
> Hi all, Iâm 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 1N5407âs. 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 â it measures 4K ohm across it rather than about 
> 160K ohm I found across the others (all still in circuit).
>
> Iâm also concerned about the filter caps â 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 Nichiconâs as per instructions on the following webpage is 
> safe and/or recommended? â
>
> www.wb4hfn.com/DrakeArticles/TechTips/ElectrolyticRelacements.htm
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.


--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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