[ARC5] ARC 5 AGC problem solved

[David Stinson]

WA5CAB at cs.com wrote:
>... Close the cutters and rock them 
> toward the outside of the cap can.  Work around the lip several times until 
> the lip is standing up straight.  pull the disk out.  Repair the unit.  Seat the 
> disk back down inside the lip and using a small shop hammer, work around the 
> lip tapping it back down agains the disk.

When I tried this, I found the alloy from which the cans are made
was brittle and cracked all around the perimeter when lifting it up.
What got tapped back down looked like hell; really cheesy.
Perhaps it was my tools.
I also never found a good, clean way to cut out the mica on top.
I could get it done and, with patience, get it back looking OK,
but only about half the time.

In my most recent experiments, I used a copper tubing cutter,
like the ones used for copper pluming.  It neatly cuts open
the can about 2/3rds of the way up the can.  Flipped the can open,
cleaned-out the old gunk and the solder holes, installed the new
caps and put it back together using a thin bead of steel-filled,
grey epoxy.  Those looked pretty good, but it's been awhile
now, since right after I started it I had to move and all
the cut-open cans and finished ones are buried in storage.
I'll probably try that again if we ever, ever get back
into a home (I'll be *damned* if hospitals and drug companies
get another home from me; they can take my insurance and go
*hang* for the rest).

These days, I salvage caps from "junker" sets.
The ones that pass the initial leakage tests
go on a current-limited supply at full voltage for
a minimum of 24 hours.  If they hold up, they get
to be "spares."  If not, they get a red paint dot
and a new home in my "fix'em some day" box.
I get about 60% good on the triple .05 caps (C7,C15).
Not so with the .05-.01-.05 caps (C20);
of all the non-electrolytic caps, this one
fails the most, followed by the 3x.05 (C6) on the same side.
The big 3x.22 (C16) is good about 70% of the time (surprise!).
Same with the 15 mfd cathode bypass cap (C30).
The 3 mFd AGC cap (C5) is good about half the time.

The 5 mFd dyno is nearly always bad.
It's important to note that this cap can test good
after careful reforming, but will fail, usually damaging R-13,
after running at full voltage for a few hours.
When you replace the 5 mFd, be sure to check R-13.
It's supposed to be 200 ohms, but will have
"cooked down" from leakage current.
I've found them measuring as little as 35 ohms.
I nearly always replace the 5 mFd by cutting the wire
at the terminal and tack-soldering
a small replacement into the circuit at the "inside"
terminal of R-13 to the nearest ground.
The can of the 5 mFd isn't salvageable,
unless you have a large drill press.

Back to work; the railroad calls.
73 Dave S.