[ARC5] R-26 operation question - AGC

[Kenneth G. Gordon]

On 7 May 2017 at 14:20, J Mcvey via ARC5 wrote:

> I have been using the ARC 5 radios more of late, and i am noticing some of their 
> foibles.
> For instance , SSB seem to overload the radio very easily, even if the signal is 
> about a s-7 or s-9 level.

BFO injection too low. First of all, check the two 51K resistors which are in the line between 
B+ and the BFO OFF switch. Those often drift high and thus reduce the voltage getting to the 
BFO. 

Secondly, if you are NOT operating your receiver at the full 250 VDC (a good idea) you will 
have to do two things to improve matters: 1) you may have to move the screen voltage tap 
from the center of the two big black resistors at the rear to the "hot" end, and 2) you can 
increase BFO injection by doubling the value of the BFO to detector coupling capacitor. I 
believe that is C-3

> Strong CW signals will sound raspy too.

Yes. BFO injection too low.

> I have to reduce the gain considerably to make the voice intelligible, but then the 
> the overall audio volume is too low for my liking.

Yes.

> Aren't the ARC R-26 radios supposed to have AGC?

Well, the various models have different types of AGC, so you should look carefully at the 
different receiver-specific schematics. Some early AN/ARC-5 models had a rudimentary 
AGC, some later models had a better AGC, some took the AGC voltage off a diode in the 
same tube as the detector, some models (I think it was the "Navigation" models) took their 
AGC voltage from the diode in the 12SF7 in the last IF stage.

THAT model, in which the AGC voltage was derived from the 12SF7, was the only one which 
didn't desense the receiver when the BFO was on since the AGC voltage was derived from 
the last IF tube, and not from the detector tube.

Mike Murphy in the most recent issue of ER Mag goes into some of the details of the AGC 
and how to implement it by a simple, reversible, change.

> Is this how they work, or do 
> you think the radio needs a look at the AGC?

Yes. But also look carefully at the BFO plate voltage: yours may be low due to the associated 
resistors going high.

> This situation is a bit frustrating...

Yes.

When I rebuild my hacked up destroyed receivers to make them useable again, I will double 
the BFO coupling cap. Remember that in some models there is not physical cap, the circuit 
relying on coupling from within the tube or by nearby wiring.

My receivers, at least the "80 meter" ones, will demodulate CW and SSB very well.

Ken W7EKB

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