[ARC5] Reviving squealing "command" receivers (re-capping)

[Brian Clarke]

Hello Les,

The squealing is feedback via the HT line, where dc by-pass capacitors have 
effectively left the planet.

I have produced a re-potting process, that is currently ... dunno where. In 
essence the process for the mica terminal plate ones is:
1. identify which flower pots to refurb
2. put the dead flowerpot in a lathe chuck and, with a very thin parting off 
tool, cut through just below the rolled mica-holding flange; lightly polish 
around the parted off edge in anticipation of re-soldering the terminal 
flange back in place
3. unsolder the terminals; remove the terminal plate and clear the solder 
terminals to take the new leads
4. unsolder the ground lead on the other end of the flowerpot; remove the 
dead capacitor guts - may need heat, or turn out with a boring tool, holding 
flowerpot in lathe chuck; clear the ground lead hole to take the new leads
6. find suitable low-delta, 400 V replacement capacitors
7. fit the caps in the flowerpot - feed the ground leads through the bottom 
and the other leads through the 'live' terminals
8. offer the terminal flange up to the top of the flowerpot; tack solder
9. holding the flowerpot in the lathe chuck and holding the terminal plate 
in place with a live centre, turning at a very slow rate and using a propane 
gas torch, flow-solder the flange back in place - a quick plumber's wipe 
will remove any sign of your work
10. solder the ground and terminal leads; cut off excess wire.

For the Sprague metal terminal plate capacitors, step 2 becomes 'use a 
propane gas torch to unsolder the terminal plate'; no other differences. If 
you are quick, removal of the terminal plate will have heated the capacitor 
guts so much that the guts will fall out when you pull the unsoldered plate 
away.

Throughout, I have suggested a propane gas torch rather than an 
oxy-acetylene torch to limit the temperature and the possibility of 
scorching the tin plating.

The whole repotting process takes at least an hour per capacitor - allow up 
to three hours for the first one. At $100 per hour, plus depreciation on the 
lathe and gas equipment, is it worth it?

So, pros and cons? Do you want cosmetics or performance? Repot if you want 
cosmetics - the cost will never be recovered; otherwise, remove faulty 
capacitors and solder in new, better quality capacitors directly to the 
valve bases. This latter refurb will give a more reliable long-term 
solution, because even with refurbished flowerpots there is a high 
likelihood of corrosion between the brass screws, tinned-brass flowerpot 
capacitor cases and the aluminium chassis - I have seen such bad corrosion 
that the screws have failed or do so under the slightest attempt to remove 
them. When I refit flowerpot capacitors, I use a zinc-rich grease to provide 
high conductivity and low risk of corrosion - this is the same kind of 
grease the Americans use in reticulating mains wiring in aluminium conduit, 
eg, Almanox.

In my experience, there are no particular capacitors that fail - for the 
earlier ARC-5 and SCR-274-N contracts, they all fail, some more 
catastrophically than others. If the Rx is dead, I have found that the dc 
bypass capacitor servicing the 12K8 is often the main culprit. If you can 
get ARC series 12 and series 15 capacitors, these are rated at 400 Vdc, 
generally do not fail, and won't cost $100 each!

73 de Brian, VK2GCE.

----- Original Message ----- 
From: "Leslie Smith" <vk2bcu at operamail.com>
To: <arc5 at mailman.qth.net>
Sent: Friday, March 04, 2011 7:03 PM
Subject: [ARC5] Reviving squealing "command" receivers (re-capping)


> Hello List-members,
>
> I have a number of "command" (ARC-5) receivers running from 60V B+ line.
>
> When bought about 1/2 the sets were working.  The rest were repaired.
> After repairing each set it was found to work according to expectation.
>
> Generally I have noticed several sets (eg R-23, R-24) now squeal.  I have 
> never re-capped these sets, and suspect (from what I have read) that I 
> must re-cap all the sets.  I run the sets from a 60V B+ line, and 
> capacitor leakage does not seem a problem at this voltage.  I have the AN 
> 16-30 ARC-5 manual; I know the location of the circuit diagram and under 
> chassis component layout photographs.
>
>
> I would like to hear from other members on the list about
> (1) which caps are "sus" and need to be replaced
> (2) which caps are un-likely to give trouble
> (3) the pros and cons of re-stuffing the "flower-pots".
> (4) a web-page that describes how to re-stuff a "flower-pot".
>
> Thanks,
>
> Les
> ex vk2BCU