[ARC5] 70+ year old errors - hacks

[Kenneth G. Gordon]

On 11 Jun 2013 at 10:09, gordon white wrote:

> Were these errors on A.R.C. receivers, or Stromberg-Carlson
> production?

I'll have to look, but I think it was a WE product. I'll let everyone know later 
today.

In any case, it is a CBY-46104 receiver. The "CBY" would tell us who made 
it.

As I said, it had been badly hacked in the 1970s. I restored it as best I could, 
and it is working very well now....although 160 meters doesn't have much 
activity this time of year... 

The case has two extra holes over two of the  alignment trimmers which I will 
fill with JB Weld and re-paint.

Sensitivity of 1 microvolt on AM is very easily obtained. Calibration is 
excellent, stability is excellent, selectivity is very good, audio output level is 
more than "adequate".

BTW, your statement in one of your CQ articles that if one matches the 
audio output impedances properly, one most certainly does NOT need 
additional audio amplification is most certainly correct !

To reiterate, the following had been inflicted on it: 1) the HFO had been 
converted to crystal-control, 2) the mixer had been changed from the 12K8 to 
a 12AT7 with consequent modification of the values of R-3 and R-6, 3) the 
12SR7 stage had been converted to an added audio preamp, 4) the detector 
was now a 1N34A, 5) the BFO had been changed to crystal control, 6) the 
12SR7 had been changed to a 12AX7, 7) most of the can-caps had been 
replaced with BIG orange drops, 8) there was an added AF gain control, 9) 
two holes had been drilled in the case over two of the alignment trimmers, 
10) L-14 and L-15 had been removed. 11) C-16A/B/C was gone, 12) C-32 
had been replaced with a big electrolytic, 13) the "usual" mods of RF gain 
control, BFO on/off switch, and phone jack had been added..

Filaments had been left at 24 V, though, and they had not changed any 
sockets, using external adapters for the two 12 volt replacement tubes.

The only one of these hacks I retained, other than "the usual",  was the AF 
gain control since I was curious as to just how well it worked and whether it 
was a valuable addition in any way, or not. Also since the "front panel" 
already had three holes in it, I figured I might as well use them all.

How the AF gain control was done was interesting and possibly the only mod 
that was done even remotely correctly: they connected the AF gain pot in 
place of R-20, the 2 megohm grid resistor for the 12A6, moved the blue wire 
coming from C-29 from pin 5 to pin 6 of the 12A6 socket, and connected that 
end of C-29 to the wiper of the pot. (Pin 6 of the 12A6 (and of a 6V6) is not 
used so that makes a good choice of a tie-point). They had also replaced 
R-19, a 100 K resistor in series from the detector to the 12A6 grid with a 
jumper.

Since they had used linear pots for both the RF gain and AF gain controls, 
and since they were the wrong values (25 K for the RF and 500K for the AF), 
I changed both of those to a 50 K for the RF and a 1 Meg for the AF, both 
with audio tapers. I wanted to use a 2 meg audio taper pot for the AF gain, 
but couldn't find one.

In any case, both controls work pretty well, and with both maxed out, there is 
enough audio to literally blow me out of the shack.

I returned everything else to the way A.R.C. originally designed it.

Ken W7EKB