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

[Kenneth G. Gordon]

On 13 Jan 2014 at 22:04, D. Platt wrote:

> Can someone let me know what type of taper is used on the 50K 
> sensitivity pot used on all the receivers in this series?  Whatever
> I'm trying to use appears to compress the usable range to the upper
> 15%-20% of rotation of the pot.

Yes. I had the same problem, which is why I went to audio-taper pots for 
both the RF gain (50K) and the AF gain (2 Megohms replacing R-20)

>  Also, I tried the McCoy 2nd IF regen
> mod

Where did you find that? I have been looking for it but have not yet found it. I 
need the details.

> and it did not help, at all.  I got the IF to adjust into
> oscillation, no problem there.  Also, it appears that the extra gain
> via the regen is too much for the BFO injection and I have to really
> reduce the sensitivity to hear things with a good note.  Yes, signals
> are there but audio output is now very low.  I could use an audio amp,
> but why?  Out the mod comes!

Here is my experience with that: first of all, one must remove the 
suppressor-to-cathode connection at the 1st IF tube socket and ground the 
suppressor directly.

Secondly, the IF cathode must be removed from the AGC line that connects 
to the RF amp cathode. However, IF regen control should still be 620 ohms 
above ground when "bottomed".

Thirdly, that IF selectivity enhancement only works if the RF gain is lowered. 
Otherwise the RF gain "swamps" the 1st IF and essentially eliminates any 
selectivity enhancement.

Fourthly, the BFO MUST be set on the correct side of the IF passband or 
SSB signals, especially, will be very weak.

Fifthly, the IFs MUST be EXACTLY aligned on top of one another. The 
regened-IF is sharp enough that if they aren't, you will get two peaks in the IF 
passband, or a very broad single peak.

I found that if you reduce the RF gain to the point that when you crank up the 
IF's regeneration, you get a decided "almost oscillating" sound, a sort of 
hissing sound at the same pitch as the BFO-offset, the selectivity then 
exhibits a very clear "single signal" effect in that one side of zero beat is 
MUCH weaker than the other.

However, I think that one could get a much better IF shape factor by simply 
adjusting the two coils in each IF transformer a bit further, say 1/4" further, 
apart, and not bother with the regen IF.

As to why the audio output appears to be lower than without the mod, I have 
not yet figured that out, but you're right, it is.

I did increase the BFO injection by adding a 1" long gimmick to the 
BFO-to-detector connection, in parallel with "C-33" and that helped a lot.

BTW, this was with an R-26/ARC-5.

> On another tack, I just finished re-capping 3 receivers and the
> results are noteworthy!  Of course, the underside looks positively
> naked without all those cans.

And the receiver works so very much better too.

>  I did change the mica across the audio
> output and a couple more.  There's no guarantee that these old mica
> caps will be OK, anymore.  Time was, one could pretty much trust them.
>  But, then again, its only been 70+ years!!!

I didn't think of that. I better take a look at my latest restoration.

Ken W7EKB