[ARC5] [Milsurplus] Well I'm trying...
howard holden
holden7471 at msn.com
Sat Jul 17 10:05:39 EDT 2010
Reading Mike's tales about tuning up the ATC raises some issues I had faced with getting "bitten" while trying to tune up older rigs.
1. Put some shrink tubing over your screwdriver shank, all the way to the handle, and leaving only enough blade sticking out to grab the screw. Prevents "bites" and shorts!
2. Watch out for those "antistatic" material tuning tools. They conduct HV very well. Didn't think about that one once (thought it was just plastic), and got tickled by the 600+ volts on a Swan 350 adjusting the neutralizing cap.
Nice story Mike.
Howie WB2AWQ
----- Original Message -----
From: Mike Hanz<mailto:aaf-radio-1 at aafradio.org>
To: Discussion of AN/ARC-5 military radio equipment.<mailto:arc5 at mailman.qth.net> ; Milsurplus New mailman<mailto:milsurplus at mailman.qth.net>
Sent: Saturday, July 17, 2010 9:46 AM
Subject: Re: [Milsurplus] [ARC5] Well I'm trying...
On 7/17/2010 6:47 AM, arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com> wrote:
> Been trying to post the final chapter in the ATC story,
> but it won't reflect either here or on milsurplus list.
> No service email saying why.... worked on the
> Boatanchors list, but that's a different server.
>
Here tiz...
On 7/16/2010 3:56 PM, arc5 at ix.netcom.com<mailto:arc5 at ix.netcom.com> wrote:
I used to call this ATC ("ART-13") transmitter a "bear," as in:
"tough (to work on) as a bear." I don't call it that anymore. Bears are
tough and they can run you up a tree, but you generally see or hear a
bear coming. Now I call it a troll.... a big, black, sneeky troll that
waits under the bridge until you're allllllmost across and home-free,
then reaches up and grabs your ankle "SURPRISE!"
The more I work on this thing, the more I like my BC-375. It's the
'country mouse' to the ATC's 'city mouse;' kinda simple-minded and
humble, but honest and hard working.
The ATC has all the fancy, city-fied goo-gahs that make us go "garsh!"
but it's just more stuff to break.
Ah well.... here's some more "troll grabs:"
First off, there aren't two switches to coordinate in that
devil-possessed Frequency Multiplier chassis; there are five.
Have you seen that leaf-spring contraption they use to change the band
of the PTO? If you were going to define "kludge,"
that would be the first picture to use. There's a fiber sprocket on the
bandswitch shaft. As you rotate the bandswitch, the points on the
sprocket move this big metal leaf spring, through a hole cut in the side
of the PTO chassis, in and out to operate a switch.
If it's not "sprung" just exactly right, it doesn't work. Problem is,
in order to removed the Freq. Multi. chassis, you have to remove and
replace this thing, and it gets bent when putting it back in. It's a
huge PITA. I can't tell you how to do it "right," (I learned lots of ways
to do it "wrong") and of course it isn't mentioned in the ATC manual.
OK, ya'll: at least one of you knows the
correct way to put this thing in without hosing it.
This one almost made me weep:
There are two stacks of six variable capacitors in the Freq.Mulitplier
chassis. The first stack is used for the tank circuit of the 1st
Mulitplier, and the second stack
for the 2nd Mulitplier. The six lower bands use the first stack only,
the 2nd Multi. being bypasses and its
cathode circuit opened by one of those switches.
On the upper six bands, the 1st Mulit. feeds the grid
of the 2nd multiplier, which acts as a tripler.
These caps are very thin, fragile ceramic.
Imagine two round disk wafers about the size of a half-dollar, with a
central, hollow 2-part hub
that screws together and a " compression spider." I've made an
illustration at:
http://home.netcom.com/~arc5/ART13Caps.JPG<http://home.netcom.com/~arc5/ART13Caps.JPG>
Once you get them un-stuck with a hair dryer (thanks!),
you tune the nasty things using an insulated small screwdriver,
by pushing the end of the spider arms to rotate the plate.
Plastic sticks won't work; they just tear up and
I broke a ceramic shaft tweaker trying to use it.
Use metal and God help ya....
Yes; there will be some stray capacity issues.
After your hand slips and the 400 volts sticking out in every direction
bites the crap out of you a couple of times, you, like me, will re-think
WWII Collins engineering ;-).
That's why I mentioned gloves last time and yes,
I forgot them once and OWWWW!
The capacitor plates are thinly deposited electroplate. The "spider" on
the rotor contacts the rotor plate along the outside edge, and connects
it to the central hub,
which connects to the common side of the cap stack.
Some of the spider arms were not making good contact
to the rotor electro-plate, because of oxidation.
Error #1:
Instead of just flexing the spider arms, which would probably
have fixed them, I gently lifted the spider arms with a dental
pick and dawbed some De-Ox-It under them. Someone had written "A" to
"F' on the side of the caps with a graphite pencil (nooooo!).
Some of the De-Ox-It migrated into the caps between the plates and guess
what it carried with it? Yep- some of the graphite.
So if you're trying to De-Ox-It the switches, don't do the "fireman"
thing, because if you get it
on those caps, whatever contaminant is on them is going straight inside.
There was nothing for it but to take both stacks apart and clean the
caps, one -by-one. Gause and gloves, because skin oil will hurt the
electroplate.
And these things are as fragile as blown glass,
as you'll soon see.
When disassembling the caps, I marked the hub with a little paint spot
and carefully counted the number of turns needed
to get it apart, so I could get it back together with the same
compression. Most took 2.5 turns, some 3. Once cleaned, I reassembled
the first stack and put them on the contral axel.....and the stack was
too tall. Way too tall. I can't explain how that can possibly be; I
was very careful with my turn count.
But the caps had to come back off the axel so I could tighten
the central hubs enough to re-assemble the stack.
Error #2:
I tightened them down until just lightly snug, then backed off a half
turn or so. Sounds reasonable, right? Nope.
If you tighten them even finger-tight, the central part
of the ceramic micro-fractures and, once you put the stack all back
together and push on the spider-arms
to tune them, "PLINK!" a big chunk of brown disc
breaks off and your resolution to stop cussing gets broken, too.
Out they came to try and salvage the situation....
I saw where someone in the past had soldered a jumper
from a spider to the rotor plate; I guess
the hi-Z contact problem must have a history. So I tried JB Welding the
busted plate back together,
then soldering across the broken electroplate to restore
the cap plate...... and the electroplate lifted off instantly.
Turns out you've got to use uncontaminated silver solder
to make this connection, which I don't have, of course.
Lost three caps (and 20 meters, and the 40 phone band).
Guess I'll be looking for a Freq. Multi. chassis for awhile.
I put the chassis back in and started the tune-up.
The lower six bands tuned right up- Yeah!
Then I tried to work on the upper bands.
Nada... no grid drive. Argh! The troll was ankle-grabbing again.
And again, it was oxidation. The rear bandswitch
(the fiend that has be-deviled me this whole repair)
has a dual set of contacts- one selector, one wiper-
made of spring tinned brass and sandwiched together
using a tiny bit of all-thread with nuts on each end, through the rotor
part of the switch, then secured with solder on each side. It connects
the proper tank cicuit to the 2nd Mult. for each of the upper bands.
This wiper contact / selector contact stack was hi-Z
between the two leaves, and no amount of De-Ox-it or edge scrapeing and
re-soldering would break the corrosion and allow the two contacts to
conduct. I ended-up soldering a short bit of copper braid from the
switch hub connection to the selector contact, and this solved the problem.
After a bit more more tinkering and twiddling,
I put a 20 KV 500 pFd cap between the output and
ground and tuned it up into a 50-ohm dummy load.
The troll is now putting out 120 watts carrier on 160 mtrs (1980 KC), 80
and 75, and just under
100 on 40 and 30 mtr CW. Disconnected the 500 pFd
cap for 30 meters. Could probably get more juju
out on 40 with a smaller cap. Audio sounds really good
with a quality carbon element (it ought to after all the work I did on
the audio driver chassis), even with the vacuum cleaner sound of the
dynamotor ;-).
Hooked it to a random wire about 8 feet off the ground
and worked Mason, K5YHX and Ronnie, W5SUM with good reports. Mason
provided an autotune cover panel
to replace to nasty one I had. It looks good now.
It's been a long fight, but worth the work.
The ATC is going back to its owner this weekend. I'll revisit it once I
find a good Freq. Multi. chassis (or good replacement caps), but that's
OK. I've learned a lot about this transmitter that will be
useful on other projects. I don't know which is next:
the ATB/ARB, or the Australian AR8/AT5.
Either way, it will be fun.
I hope all this yabber has been useful to someone.
73 DE Dave AB5S
______________________________________________________________
Milsurplus mailing list
Home: http://mailman.qth.net/mailman/listinfo/milsurplus<http://mailman.qth.net/mailman/listinfo/milsurplus>
Help: http://mailman.qth.net/mmfaq.htm<http://mailman.qth.net/mmfaq.htm>
Post: mailto:Milsurplus at mailman.qth.net<mailto:Milsurplus at mailman.qth.net>
This list hosted by: http://www.qsl.net<http://www.qsl.net/>
Please help support this email list: http://www.qsl.net/donate.html<http://www.qsl.net/donate.html>
More information about the ARC5
mailing list