[R-390] OT - Lambda power supply repair

[Paul H. Anderson]

For whatever reason, I like the mid-80's lambda lab power supplies with 
the LED volt/amp front meter.  I had over the years picked up a few from 
various places, some NOS, some beat up.  I use these for powering various 
projects, including R-392s, R-391 autotune, and my T-195.

A few years ago, I wrote to this list asking about ideas for repair, and 
really got no useful feedback.  I assumed for a long while that repair was 
not feasible due to internal complexity of the switching design.

Here is an example of the type I'm referring to: 
<http://login.pdq.com/boatanchors/lambda/lambda_lq-204.jpg>

I recently had some time to spare, so started looking into several common 
problems they have, such as:

    unlit LED display
    wildly flashing voltage/amp levels
    osciollating voltage output

and so on, not to mention one with a variety of dead shorts.  I feared 
fried major components or ICs.

It turns out that besides the primary and secondary electrolytics failing 
short, that the other main problem is electrolytic axial caps failing in 
large numbers.

I've started rebuilding several units like this, and am finding it to be 
easier simply to replace all the caps with equal or slightly higher values 
(and higher voltage ratings) from Mouser, et al.  Only after doing that is 
it worth doing any real in depth testing or repair.

So far, I'm 2 for 3, with #3 being a 45 amp 28V unit that I haven't yet 
replaced the axials in (I plan to do this shortly).

In the two unit I tested and repaired fully, more than half of the axial 
electrolytic caps tested bad on my ESR meter or ohmmeter, and replacement 
of all circuit board mounted electrolytic caps (aside from the filtering 
caps) returned it to normal operation.  My approach ended up being 
wholesale replacement of axials on the circuit boards, and selective 
replacement of the large power filtering caps, which are much more 
expensive.

My suspicion is that wholesale replacement of electrolytic caps in this 
type of units will tend to return them to as-new condition, or at least to 
ham hobbyist bench usable condition.

I'll try and get some useful inside pictures of original and replacement 
caps for folks to look at.  I think most are failing due to long term heat 
exposure.

I have not had these types of problems with the modular brick style 
lambdas - I don't know why - possibly better thermal controls or a 
different style of usage for those.

Anyway, definitely worth a shot if you've got one and been wondering what 
on earth is wrong with it.

Please remember to be extremely careful with these when opened.  The 
primary side is over 200V, and should be treated with extreme caution. 
Even when powered off, either normal or failing power supplies may have 
high voltages present.

Paul