[ARC5] ARC-5 RX Tips from Dennis.Monticelli

[David Stinson]

Dennis Monticelli and I had an interesting exchange
of emails.  His work and tips are interesting and
helpful, and I pass them along
with his permission and my thanks. D.S.

------------------

From: Dennis Monticelli <Dennis.Monticelli at nsc.com>
Sent: Jun 1, 2005 12:58 PM
To: arc5 at ix.netcom.com
Subject: BC-453 cap failure question


Hi Dave,

..I have two receivers, a BC-453-B and a BC-455.
Both were working for several minutes after initial power up
and then abruptly went silent.  The former went
south after the PS was ramped up to 140V from 80V
and the latter failed after ramping from 50V to 80V.
I figure at least one of the caps in screen or plate
bypass service suddenly developed a relatively low resistance.
Supply current seems high but far from a crowbar situation.
I haven't done any circuit tracing yet.  Can you clue me
into which bathtubs have proven to be most problematic?...

Dennis AE6C
---------------------

First thing to check is the 5 mfd 300 volt dyno filtering cap.
It's mounted on one of the side skirts in the middle-
(the middle on the other side is a 15 mfd 35 vdc audio
  cathode bypass, which is also suspect if you have low
  bias on the audio out).  That 5 mfd is bad 99% of the
time and will kill your B+.  Second biggest culprit
is the big triple .22 ufd in the back.
Concerning the .05 caps:
If you open the bottom of the radio, turn it over
and face the dyno deck toward you and
the tuning dial away, the two 3x.05 caps on
the left fail the most and the one closest
to the front on the right fails the least.
GL OM ES 73 Dave S.
---------------------

Hi Dave,

Thought you would be interested in what
turned out to be the real problem in my BC-453-B.
You will recall that the radio suddently quit
after the B+ was raised from 80V to 140V.
To make a long story short, there were multiple
low resistance paths between the plates
of the RF and Mixer main variables and
their associated air padders.
Somehow, the extra voltage "activated" these paths.
These variables looked clean to the eye,
but the invisible paths (whiskers?) could only be
cleared by running stiff paper strips
between each and every plate in
combination with blasts of compressed air.
Now it works FB.....

--------------------------

That is a pretty cool tip, Dennis.
I have some that die unexpectedly as well, and one of them
goes "alive" again if I push on the freq dial a little.
Could be some of the plating has "flaked."
It would only take a microscopic amount.
I have found in a couple of transmitter capacitors in which
stator contact is sometimes lost
because the connection point- which is "press fit"
on the end of the stator stack- goes "Hi-Z."
Probably age oxidation between the connection piece
and the stator stack end post that is pressed to it.
Have to use a big soldering iron to solder the connector
piece back in contact with the stator plate stack.
I guess these old caps are showing their age....

73 DE Dave S.

---------------------------
Flakes make sense, but my particular cap plates
looked very clean and intact.  Also, I could not see
any obvious surface failure under a low power magnifying
glass.  When I blew it out, no such conductive
pieces flew out (I did it over white paper towels
so as to catch the little fellers, but didn't see anything
other than some random dirt specs that my Q-tips missed
during the original cleaning).  I also don't understand
the mechanism behind voltage level "activation."
I can only guess that a thin oxide layer was breached
by the voltage, resulting in a conductive path(s)
from the whiskers or small flecks. FYI, the DC resistance
was rather low (11 ohms for one of the sections
and about 4 ohms for the other)....