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Power Supply Repairs

posted Nov 4, 2020, 11:37 PM by Charles Boling   [ updated Nov 4, 2020, 11:38 PM ]
Something over a decade ago, Ray bought VHF radios for all of the units in the stake and put them in the church buildings with antennas & other things needed to set up a field-expedient radio station. All of the power supplies are the same model, Samlex SEC-1223. In 2014, one of the supplies started making noise, and I determined that the cause was a set of failed capacitors. As I wrote at the time:
for several years around the turn of the century several Taiwanese
manufacturers used some bad formulas for the electrolyte in their caps
that rotted them, and millions were put on the market with severely
shortened life -- in some cases as little as a few months.  It was a
huge deal, w/ computer manufacturers like Dell taking a lot of heat and
losing a ton of money having to replace failed motherboards (I know one
guy who had a board w/ over 30 failed caps on it), and "re-capping"
high-end boards became a lively business.  That pretty much all blew
over after the 1st decade, but I suspect that these caps were part of
that group.
More testing:
Power supply open w/ arrow indicating problem caps

I tested the supply, and the output is still pretty good -- 13.8V drops
to 12.7V under 70W load w/ <1V ripple -- so I don't think any other
components are damaged yet.

I put it on a scope and confirmed that without a load there's a lot of
pseudo-random noise on it; with a slight load it stabilizes to a 2-tone
waveform w/ the primary frequency around 50kHz (I didn't bother to
measure it carefully -- just going from memory of approximate size on
screen and dial settings) and the 2nd an octave up and almost as strong
(any higher harmonics weren't obvious) with ~20% jitter.  Putting a
~60-70W load on it (my radio transmitting on high) made it go bananas,
with a large variance in amplitude and frequency, peaking at over 1V.  I
listened on another radio while transmitting and verified that no lower
frequency components were making it through unfiltered by my rig (an
IC-208H).

These 3 caps just sit across the outputs to stabilize it and work w/ the
chokes to get rid of the last of the ripples, so they don't directly
affect most of the circuit and unless they short instead of open they
shouldn't do catastrophic damage unless the inductors don't choke enough
and allow higher-voltage spikes somewhere that take out something more
important.  Judging from what I saw on the scope, that probably won't
happen, but it's possible that it would throw something upstream off
just enough to destabilize the PWM chip or something so it shouldn't be
used until they're replaced.

I found someone else who had the same problem
and learned from his mistake to measure the physical size of my caps
instead of believing the 16x25mm specs.  Sure enough, with the change in
board layout (ours is 3 in a straight line, but all the official pics
show one offset w/ inductors in the corner where it might otherwise be)
they tightened the spacing, and the 16mm won't fit!

Equivalent caps are about $1 each:

It was a good education for Alexander as I led him through the process of testing and fixing it.

Apparently, that radio was used more than the others.  Fast forward 6 years to last Sunday...
It was pretty funny today. Remember during the exercise when the Castle
Alexander &amp; Steven working on the PS

Rock radio wasn't working right?  Bp Morkert brought it to me at the
stake ctr today so I could inspect & test it, trading parts (e.g.
microphone) w/ another radio if needed. I pulled the stake radio out to
use as a receiver, turned on its power supply, and poof! Smoke began
pouring out. "Not worries -- I have another one!" I pulled out the
Longview 3rd radio, turned it on, and a few seconds later *it* began
producing smoke!  All 3 supplies were emitting a complex sound that,
well, sounded like a classic failing switching supply.

CR's supply isn't smoking yet, but it's putting out bad power. The radio
can transmit @ 5W w/o noticeable artifacts on the received audio, but as
the power draw goes up it gets worse. Needless to say, Bro. Morkert
didn't return with a radio...

I'm going to open them up and inspect them, and assuming that it's what
I think it is, I'll order enough caps to do all the radios in the stake.
 Once these are back in operation, I'll start pulling the remaining
working radios and replace the caps *before* the supplies fail....
My middle boys are going to get a lot of soldering
practice, and Alexander's going to be doing a lot of teaching.

I ordered 30 more capacitors, and Alexander & Steven fixed those 3 radios this evening. That's a total of 4 down and 6 to go! A good education for Steven; I think I'll have William help with the next few that come in.

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