[Milsurplus] [ARC5] "Bandspreaded" R25/ARC-5

[Kenneth G. Gordon]

On 5 Feb 2016 at 14:33, J Mcvey via ARC5 wrote:

> This R-25 had a 3/4 completed "ham-ering"  that piqued my curiousity. I
> reversed all of the mods EXCEPT the tuning gangs which had all but three rotor
> blades removed in each section. The original setup used 75 pf in each section to
> compensate for the missing blades. Even with the trimmers unmeshed, the max
> frequency was 1.9 MHZ. Changing the compensation caps to 50 pf micas allowed
> tuning  from 1.7MHZ to 2.05 MHZ with approx 50% trimmer mesh.
> 
> I thought that  I was going to hate this mod, but I have changed my mind. It
> actually was joy to use on 160 meters. The wide spread and vernier action makes
> tuning very easy, In fact , dialing in SSB was quite easy as opposed to the
> usual "fine touch" it requires on a normal ARC set. CW was very easy.
> Sensitivity was excellent as the bench test had indicated.

Well, it sounds good so far. Thanks for keeping us informed.

> Other items to be addressed:
> Takes a long time to stop slowly drifting downward

That is interesting: perhaps the compensating caps should be tempco types. 
I know the main tuning dial in the receivers is a tempco type itself. Replacing 
tempco caps with non-tempco will contribute to drift. The regular ARC-5 
receivers were notable for their minimum drift. Also, I believe there is a small 
tempco cap in there somewhere which you may have to "adjust" upwards in 
value.

> Need to find a dial with bad paint or badly scratched to make a calibrated dial.
> This one is too good to repaint.

Yes. That would work.

> IFs are very broad. 

Hmmm...maybe you can physically move the two coils in each IF can further 
apart as Jeep detailed in a recent ER Article. Funny thing about that is that 
he and I had the same idea separately and almost simultaneously. :-)

I had not finished my experimentation with this method, having "operated on" 
only one IF can in a BC-454 here, while he did the whole shebang and wrote 
up the article for ER. Moving those coils is not difficult although it is 
time-consuming.

Remember that ultimate selectivity at the 60 dB down point is determined 
more by the IF frequency than anything you can do physically with the coils 
though. There is a simple formula for calculating that. However, the 60 dB 
down point is not really important for our work.

> Thought the ARC's had an AGC, 

Yes. They do, unless it has been "messed with". As I mentioned to Mac, the 
AGC diode is in the 2nd IF stage using the 12SF7. If your AGC is not 
working properly, then you might take a close look at the circuits to see if the 
original "ham-merer" modified it somehow. In fact, in one of the ARC-5 
service manuals, the AGC system is explained in some detail.

Also, one can add AGC to at least one more stage (I'd avoid doing that to the 
mixer though) and can modify how it is connected in order to make it more 
effective.

> but loud stations  overload the radio. It's annoying when a close station
> is talking to a distant one.

Then it is not working as it was originally designed. I'd be taking a very close 
look at the circuit. Sounds to me as though someone has messed with it.

Ken W7EKB
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