[Milsurplus] [ARC5] Rethinking Recapping, Redux

Fri Mar 23 23:08:20 EDT 2018

 Mica, per se, isn't dangerous - it's quite chemically stable & hard to get to react with anything.  The issue is inhaling dust.  Cutting a mica sheet out of a flowerpot cap will likely make a tiny amount of inhalable dust but most of it will be tiny flakes not easily airborne.  A whole mica sheet in flowerpot is on the order of a gram or two for perspective.

Interesting was the explanation of Recommended Exposure Limits as "a level that NIOSH believes would be protective of worker safety and health over a working lifetime."  Does that mean, for example, that the "3 mg in an 8 hour day" is for continuous daily exposures or is it applicable to the sort of infrequent, limited duration represented by tinkering with those radio parts?  Does anyone know? 

 Wayne
WB4OGM

-----Original Message-----
From: AKLDGUY . <neilb0627 at gmail.com>
To: milsurplus <milsurplus at mailman.qth.net>
Sent: Fri, Mar 23, 2018 5:34 pm
Subject: Re: [Milsurplus] [ARC5] Rethinking Recapping, Redux

Caution: Flower pot caps contain mica, and mica is not a benign substance. This article gives figures for tolerable and immediate threat to life concentrations:

https://en.wikipedia.org/wiki/Mica

Scroll down to "Health impact" and "United States". I understand the 1,500 mg per cubic meter to apply in a confined space of 1 cubic meter. It's impossible to achieve this concentration with the small quantity in the Command sets. More realistically, the tolerable level of 3 mg in an 8 hour day might be approached, so you'd be well advised to ensure adequate circulating air.

Neil ZL1ANM

On Saturday, March 24, 2018, Michael Kana via ARC5 <arc5 at mailman.qth.net> wrote:

Hi all 
FYI I have opened flower pot caps by taking a dental pick and scribing around the mica along the can lip 
Eventually you cut through the mica and pulling the innards out the top. YMMV but is an alternative to cutting metal
73 Mike 

Warning!  This is transmitted over a non secure medium

On Mar 5, 2018, at 2:39 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:

We, ll I'm "in between".
I too would rather not chase one problem after another, but restore the whole thing, then it's done.
However, there is something about the the under chassis "flower pots" or whatever you want to call them which gives it that antique military electronics look.
It's kind of unique.

I take a pipe cutter to the pots, gut the contents, and replace them with ceramic or mylar caps.
The cans can be soldered back together because they are brass and look pretty good -especially if I use hot air soldering .

 On Monday, March 5, 2018 2:06 PM, jeepp <jeepp at comcast.net> wrote:

Well, good comments, all.  In my humble experience, I have had it both ways; that is, some radios will work forever, others not..  I am not of the side of the house that likes to have things, below chassis, original.  Thus, I don't have a problem doing re-caps, especially that which I use or depend on.  Nor do I care to chase issues 

Sent from my Verizon 4G LTE smartphone

-------- Original message --------
From: J Mcvey via ARC5 <arc5 at mailman.qth.net> 
Date: 3/2/18  22:57  (GMT-05:00) 
To: David Stinson <arc5 at ix.netcom.com>, arc5 at mailman.qth.net, 'Milsurplus' <milsurplus at mailman.qth.net> 
Subject: Re: [ARC5] Rethinking Recapping, Redux 

 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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