[ARC5] ARC-5/274N/A.R.C. question

[Kenneth G. Gordon]

On 14 Jan 2014 at 8:52, D. Platt wrote:

> Continuing with my query and comment about the regen mod in the
> BC-454, I pulled the manuals for both the ARC-5 and the 274N.  I was
> really not aware of the marked differences in the two designs.

And there are even more differences yet within each series.

For instance, in the case of one or the other of the R-23 models, the BFO is 
injected into the 2nd IF stage, for instance.

And there were differences in the AGC circuitry within the AN/ARC-5 models, 
and even between models.

> Shame
> on me.....  The 274N receiver has no AGC

Well, actually SOME models DOhave a crude AGC, usually the BC-453 
models.

> and there's no good way to
> improve BFO injection levels.

There are two methods: 1) increase the value of C-33, 2) decrease the value 
of the BFO plate-dropping resistor (via Les' info).

>  I have an ARC-5 version for 80 meters,
> so I may move over to that receiver and see what might be done.

That is the model I recently refurbished for a fellow.

>  The
> fellow I mentioned on the OMRN, in fact, did his successful mod to the
> first IF (12SK7) vice the 2nd IF, a 12SF7.

I used, and have always used, the 1st IF. You need to get the "selectivity" as 
close to the mixer as possible.

> Maybe that's what I'll try,
> next.  But, I'll include the removal of AGC from that stage, too, as
> Ken mentioned.

We had a discussion some time ago concerning WHY Fred D. insisted on 
connecting the suppressor to the cathode.

There is a mention in the RCA tube manuals, that doing so gives certain 
sharp-cutoff tubes a remote-cutoff characteristic. This is valuable when you 
have a bunch of aircraft within visual distance of one another to help prevent 
overload of the receivers' inputs.

For our uses, that is less important.

I will repeat: as far as I am concerned, the ARC-5 receivers are the finest 
single-band receivers ever built, and that Fred Drake and his boys were 
sheer geniuses.

Ken W7EKB