[MRCA] PRC-25 battery adapter

[Gene Smar]

Gents:

     I spent a couple of hours this afternoon home-brewing a battery adapter
for a PRC-25 I acquired a few months ago.  Let me pass along a few tips or
observations based on the experience.

     I used an almost-dead BA4386/PRC25 battery for the case (outer
covering) and the connector assembly.  (The battery worked long enough for
me to test out the 25 and make sure it wasn't DOA.)  I carefully opened the
end of the case near the connector and slipped out the guts.  Like all the
mil batts I've seen, this one looks hand-assembled into a cardboard frame.
The original cells were approximately C-sized, just a bit smaller in OD and
longer.  There were 18 of them arranged in series-parallel to yield a hefty
3 VDC source for the filament of the 2DF4 final tube and 14.5 nominal VDC
for everything else.  They were held into the folded cardboard frame with
some type of sticky, black substance (probably a bituminous material of some
kind.)  I slit the tape that held the cardboard frame together and unfolded
the cardboard.  I removed and discarded all the old cells and scraped off
the bituminous material from the cardboard.  I wanted to reuse the frame
with my new batteries and holders.

     I found instructions (one of many available) at www.armyradio.com .
However, I intended to re-use the entire original cardboard frame for the
batts, rather than try something else.  Unfortunately the Radio Shack
battery holders (PN 270-402), recommended in the article, and the normal
C-sized batteries were a bit thicker than the mil batts (more on this
later).  So the cardboard wouldn't fold around the new holders and C-cells.
Also, the finished assembly wouldn't fit into the battery case.  Rather than
bore you with the details, take my word - follow the www instructions for a
better-fitting assembly.

     I reused one of the larger pieces of the original cardboard frame for
my new holder.  I roughed up the slick surface of the cardboard and the batt
holders with coarse sandpaper and holt-glued the holders to both sides of
the cardboard.  BTW - I used 14 batteries instead of the recommended 12.  I
paralleled two additional C-cells with the bottom two in the chain of cells
to provide more A+ (filament) current, just like in the mil batts.

     Instead of running the original #30 AWG enamel wire (which provides a
bit of resistance to inrush current for the cold 2DF4 filament) directly to
the battery holders, I wrapped it into a small coil and soldered a new
length of #20 wire to the end.  I then soldered the #30 wire to the A+
(filament) terminal on the connector and the free end of the #20 wire to the
+3 VDC point in the battery chain.  I then used small wire ties to tie the
A+ wire to the neg (black) wire for additional support to the #30 wire.  In
hindsight it might have been better to reverse this arrangement - connect
the larger wire to the connector and hide the #30 wire inside the battery
frame.

     I placed the finished assembly into the original outer case along with
plastic support piece for the contact assembly from the original battery.  I
placed the battery onto the bottom of the PRC-25, engaging the connector
pins, and turned on the radio.  However, when I pushed the bottom of the
PRC-25's battery case onto the battery the radio stopped hissing.  I tried
this a few more times and each time I applied pressure to the battery case,
the radio stopped working.  I traced the source of this intermittent
operation to the weak contact springs on the RS C-cell holders.  When I
pushed the 25's battery case against the new battery assembly, the tight
space inside (thick holders and C-cells, remember) bent the plastic of the
holders a bit.  This moved the + contacts away from the ends of the C cells,
killing the radio.  I cured this by stretching each contact spring quite a
bit to ensure a snug fit of the C-cells against both contacts in the holder.
I might have to remove the one rubber pad in the bottom of the 25's battery
case to provide some room inside the steel enclosure.

     Now the entire radio works reliably, including the panel light (that
piece didn't work previously, meaning the +3 VDC source in the original batt
was weak.)  Next I'll work on a more permanent holder for the batts in my
PRC-10.  Right now the 10's battery case contains several packs of D-cells
and 9-volt batteries just thrown in, not mounted onto a frame of any kind.
After that, the same for my PRC-6 and CPRC-26 adapters.

     CU at Timonium.

73 de
Gene Smar  AD3F