[ARC5] BC-456 Update

[Mark K3MSB]

Hi Guys

Well, this weekend I successfully got my BC-456 Modulator (including
dynamotor) running with my SCR-274N system!

Here are a few pictures:

www.k3msb.com/temp/SCR274_1.jpg
www.k3msb.com/temp/SCR274_2.jpg
www.k3msb.com/temp/SCR274_3.jpg

I completely disassembled the dynamotor, removed the old grease, did a
thorough cleaning, and applied new grease.

The modulator itself is stock and in superb condition.  I replaced all the
caps and stored the originals for possible future re-stuffing.   However, I
did re-stuff the two big square capacitors (C-54 & C-56) as they looked
easy to do, and in fact they were.

Resistance checks were good for the most part, with only a few resistors
being out of spec.   I decided to try the unit first before replacing the
out of spec resistors.

Before doing this I decided to re-do my entire setup with the hopes of
putting it into its final configuration.  As I was told (and I forget who
on the reflector said this to me) - “Mark, it’s never in its final
configuration”.  Onward I went…..

First, I put a layer of furnace sheet metal on the two shelves of my
setup.  I then carefully grounded each unit to a shelf with short wires.
The upper and lower decks are also grounded together.

The size and configuration of my setup is such that it will fit in the back
of my Jeep.  I don’t plan on doing mobile work with it, but I would like to
be able to take it to hamfests etc to show the younger generations what WW
II technology was like.   Maybe an on-the-air demo if I get adventurous….

Next, I realized I had no way to monitor the voltages and plate current
when the unit was running.  So, I pulled a small project box from the
pile-o-project-boxes that I have, cuts some holes – what would we do
without Greenlee punches – figured out shunt and multiplier resistors, and
installed the two best meters from my pile-o-panel-meters.  The meter box
can monitor Plate, Screen, and Oscillator voltages by the front panel
switch, as well as plate current.    I installed a Cinch-Jones connector in
the relevant wires running in the harness from the modulator to the
transmitter rack (J58 to J59), and that connector plugs into the back of
the meter box.    If I want to be authentic, I can remove the meter box and
use another Cinch-Jones connector to complete the connections and just drop
it beneath the top deck.   But, as my friend says, it may be more authentic
but its “cool” to watch the meters swing!

This past weekend was the WW II Reenactment and Airshow at Reading PA.  An
old flying buddy Marcus KB9MW came down for the weekend and was drooling to
get some “ARC-5” time in.   He’s an RF Technician of over 30 years
experience, so it was with some gusto that we powered up the SCR-274N.

Everything powered up without incident but I noticed that the screen and
oscillator voltages were about the same, not the 200 / 270 that I
expected.  This was the same for the 80 and 40 meter transmitters, so I
figured it wasn’t a particular unit.   I was getting about 20W out on both
bands and the Ip was around 100 mA.   That’s way too low. With my HB power
supply I was getting around 40W out and 190 mA of Ip.   Something was
bothering me about this, but I couldn’t figure it out.

So, faced with this problem, we decided to head out for some excellent
Mexican food.  While chatting at dinner it hit me – I remembered that in AM
mode the VR tube regulates the screens down to about 200V.  After eating
more chips and made-on-the-spot guacamole that we’d care to admit eating,
we headed back home.  I said I wanted to lift the cover and check the VR
tube.  Sure enough, key down and the VR tube lights up – on CW.   A quick
look at the schematics told us that wasn’t a good thing.

So, we pull the modulator, transmitter control box, and connecting cable
off the rack and proceed to do some resistance checks.   Resistance between
J54-16 and J54-7 on the modulator showed infinite as expected.  Resistance
between J55-16 and J55-7 on the transmitter control box showed the right
resistance based upon the setting of the TONE-CW-VOICE switch.   We checked
the cable and the continuity was as expected.

We then attached the cable to (only) the modulator and – whoa! – found 18.5
ohms of resistance between J54-16 and J54-7!

My friend looked at me and said something to the effect of “Let me see if
your soldering is a bad as it used to be buddy…..” and proceeded to put on
the head magnifiers and have a look.   Somewhat disappointedly, he said
“Nope, those solder joints look just fine”.   After a while he took a small
screw driver and made a slight etch between the relevant pins.  He said it
might be a very light carbon path etc.  I wouldn’t have thought of that.
“Well, of course you wouldn’t have” says my friend.  You guys should have
heard some of the cockpit banter between the two of us while we were
terrorizing the good people of north Texas decades ago!   Great times!

We reinstall the boxes and the VR tube stays out on key down!   I’m getting
about 230V on the screens and about 200 on the Oscillator.   Power is now
about 30W and the Ip is around 140 mA.   Getting better.   Weather it was a
carbon path we don’t know, but making a scratch between the pins worked.

One reason for the lower power is that upon key down, my Ep drops to
slightly under 500V.  It turns out that the 28V transformer in my HB supply
feeding the dynamotor is really only putting out 26V.  When I key down the
Ep quickly drops from about 520 to just under 500, and the power drops from
about 40W to 30W.  That doesn’t worry me as it’s a known cause and effect.

The screen voltage is still low, and it doesn’t matter what transmitter I’m
using.  We rechecked the values of the resistances in the voltage divider
in the BC-456.  R62, R63, and R65 are pretty much right on the button, but
R64 (100K) comes in at 87K, which is way out of the recommended 2%.  That
will get replaced.

BTW, the parts list shows R64 as being a carbon comp resistor!   But the
photo in the manual, as well as the part in my modulator, is a wire wound
resistor.

Ad reported in another post, I couldn’t try the modulator on AM due to the
condition of the T-17 mic, so hopefully that will occur soon.  A friend of
mine is going to Ham-Con in Dallas this weekend, so  I’ll ask him to be on
the lookout for an old CB carbon type mic or a replacement mic element.

Other things to be done:  I’m manually using a 200 pF series cap on 80M and
a 100 pF series cap on 40M to a multi-unun that takes me from 12 ohms to 50
ohms.  I have the article from ER for the matching network consisting a 200
pF cap,  a midi-ductor, and an air variable, and I plan to use that and
clean up the alligator clips etc you see in the photo.   I have the parts
so it’s time to build it.    The KW multi-unun is way overkill, but it was
on hand but will be used for the 160M Inverted-L come fall, so I’ll hand
build a smaller one.

I’m having a ball with this stuff!   Again, a huge thanks to all of you for
providing technical assistance and those free help parcels of parts that
show up from time to time!

73 Mark K3MSB