[ARC5] BC-455 adventure

[Kenneth G. Gordon]

On 16 Jun 2012 at 16:09, Bill Cromwell wrote:

> A whole bunch of those small screws and washers that hold the sheet
> metal together are missing. This one is down to one or two screws per
> side. There has been reference recently about those and where to get
> replacements. I can do the search for exact details.

No need: they are 3-48 binder-head screws. Originals are cad-plated brass. 

However, I bought several boxes of stainless steel ones of various lengths 
from Fasteners Inc., along with inside-tooth lock washers and nuts of the 
correct sizes. 

I use the nuts with some of the screws (and lock-washers) to plug the holes 
in the sides of those receivers which have been converted to disk ceramic 
caps from the can-type jobs (flower-pots as some call them). Leaving those 
holes empty looks trashy to me.

You can stuff the flower-pots with new disk ceramics if you want to. It ain't 
hard.

You can see how I did one here:

http://www.w7ekb.com/glowbugs/Military/arc5pages.htm

Look down near the bottom of that page.

> The missing piece is just the cover over the tubes. Inside, the cover
> over the tuning cap is there. I used a pile (pun intended) of 9 volt
> batteries to put 180 volts on the B+ and heated it with a 24 volt AC
> power transformer. I was pleased that after a small warmup time with
> the heaters on there was 'insignificant' drift. I have run my R-23 and
> BC-453 receivers down to 90 volts and all worked okay.

First of all, leaving that tube-cover off results in the entire receiver running 
much cooler. In an aircraft at 30K feet, undoubtedly holding some of that 
heat in is a good idea: in a heated ham shack, not so much.

Secondly, my very informal testing determined that the best all around 
voltage to run those receivers on was almost exactly 170 VDC, but this was 
with either all replacement caps, or good "flower-pots" (a real rarity these 
days). IMHO, 250 VDC is completely unnecessary, even when these 
receivers were new.

Some of our folks here run the B+ at 24 VDC and have reported no 
problems, although the receivers did take a lot longer to warm up, and audio 
output levels were way down.

> I am going to
> build another power supply or two to take care of those receivers with
> somewhere from 90 to 150 volts (depending on what's in the junk box).
> They were designed to run from 24 (28) volts DC on those heaters so
> I'll accommodate that, too. I didn't notice any hum with the AC on the
> heaters. The unit I have was recapped so it could take full voltage
> but I think there is more to consider than just those caps.

Absolutely. For one thing, that 12K8 mixer is the single noisiest mixer tube 
ever made. Reducing the voltage significantly reduces the internally 
generated noise.
 
> This one is dressed in black. It has been stenciled "BC-455-B" in
> yellow and the metal tag says the same thing.

I am sure one or more of us can come up with a tube-cover and lid for you.

> I had thought about adding regeneration (Q-multiplier) in the IF but I
> am going to sweep align the IF to get it as narrow as I can and then
> try to use audio filtering and some other, outboard audio tricks to
> help sort signals.

Well, with an IF of 2830 KHz, you are not going to achieve selectivity much 
better than stock even with sweep-alignment of the IF. As I said, those 
receivers were purposely made with a very wide IF to deal with drifty and off-
frequency transmitters.

> If that doesn't work well enough I'll do the Q-mult
> approach. I read the specs. Not very CW friendly.

That may be, but I found them to work very well indeed on CW.

I have always considered the "ARC-5" receivers to be the very finest single-
band receivers ever even conceived, let alone built in large quantities. They 
are definitely one of my very most favorite receivers.

Oh! One more thing: on that URL I gave above, you will find the entire 
AN/ARC-5 manual as a PDF. I have a hard-copy here and have found it to 
be invaluable.

Good luck, and vy 73,

Ken W7EKB