[ARC5] Rethinking Recapping, Redux

[J Mcvey]

 Once upon a time I "let the caps be " on an ARC radio that was running well for several hours on a dynmotor.Then, suddenly , the radio went silent and the dynamotor slowed down and was obviously straining.Turns out that the large 0.2 uf (?) X3 cap filtering the B+ decided to catastrophically fail to a near dead short!Good thing I was there when it happened....
The other ones can do what they do , with little harm to anything other than performance, but that .2X3 cap filters raw B+ and LV. 
Bad things can happen.
 

    On Friday, March 2, 2018 3:44 PM, David Stinson <arc5 at ix.netcom.com> wrote:
 

 Some may remember a post back in December in which
I suggested a procedure for reviving sets while
doing the least amount of tampering and damage
possible.  The three RAX receivers, which were the
subject of that post, are still running fine
(knock wood), one full-voltage on the dynamotor,
two on a DC-DC converter at 90 VDC reduced B+.
Across the three RAX receivers, I changed three
capacitors, no resistors and one tube.  Since that
time, I did repair two bad solder joints in the
low freq receiver at each end of a bypass cap in
the BFO circuit.

Recently acquired an unmolested BC-454 receiver
(3-6 MC) and began resuscitation. 
https://photos.app.goo.gl/2Th2rhu9Coc7qUnR2
https://photos.app.goo.gl/NBrlsOR72iLcWOBh1

After the obligatory De-Ox-it, lube etc., opened
the connection to Ground for the Screen Dropping
resistors, connected +120 VDC to the B+ buss with
no filament voltage.  This should (with no local
control in the front) leave no path to ground for
the 120 Volts save capacitor leakage.  Current
draw was over 20 mills- a certain sign of one or
more very leaky caps.  

Going for the obvious first, disconnected the wire
to the 5 mFd B+ buss filter, which is not needed
unless you're going to run the radio on the
dynamotor.  This immediately dropped the current
draw to a couple of mills.  I boosted the B+ buss
(still no filament voltage) to 200V and left it
there for 2 hours.  Current draw was 4 mA- a minor
amount of leakage.  So the bypasses were either
"good" or open.  If open, stage oscillations and
stage gain would tell me.  Next, the 15 mFd Audio
PA cathode bypass checked "good."  Reconnected the
Screen dropping resistors, powered the filaments
and put 100V on the B+ buss.

The radio has been playing well for hours now.  B+
buss current remains stable.  That's all the work
it took to get the little soldier marching again.
Of course, those caps are very old.  They may play
till Judgment Day and they may quit tomorrow- but
even if they do, no unnecessary tampering was done
and the "baddies" will tell on themselves.
https://photos.app.goo.gl/wXUMwlrZM760F6X62

GL OM ES 73 DE Dave AB5S




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