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

[Kenneth G. Gordon]

On 15 Jan 2014 at 3:56, Leslie Smith wrote:

>   Hello Jeep,
>   The BFO circuit has a thumping great resistor in the plate circuit
>   of the BFO. Ditto for the local oscillator.  If you read the manual
>   the operating voltage of these two oscillators is around  tens of
>   volts. Maybe 24v or 30v when running on a 240 supply.

The HFO operates at 30 VDC, normally, according to the voltage charts in 
the manual. I have found this to be generally true.
  
>   When I ran one of these sets (actually a good number of these sets)
>   from a 60v B+ line the plate voltage wasn't much lower than the
>   figure published in the manual.
> 
>   This meant several thing.  First - both the LO and BFO injection
>   levels were low. I lowered the dropping resistor

Do you mean, at the BFO or the HFO? I assume, the BFO.

> from something like
>   150/330k to 15 or 33k. This immediately increased the plate current
>   and the distortion I heard on SSB disappeared.

Ah, ha. Innersting....

I had increased the BFO injection on one set by adding a 1" long "gimmick" 
capacitor in parallel with C-33. Although this most certainly did increase the 
BFO injection to a more acceptable level, at high RF gain levels it also 
"pulled" the BFO frequency. I don't like that.

>   Second, running a set with a lower B+ supply must starve the
>   oscillators for current.
>   If the measured working voltages are not too different using a 60v
>   or a 250v supply, the plate current must drop A LOT at the lower
>   voltage. There is no other possibility.

Indeed.

>   Thirdly.  The oscillators will work reliably at low plate voltages. 
>   I have seen a BFO in one set working with only 6 volts on the anode.
>   (True.  I head the beat note - I measured the plate voltage.)

Yes. It most certainly will, in my experience too.

>   Finally - A WARNING.  I'd think carefully before making ANY change
>   in these sets.
> The designers (or Fred D.) knew what they/he was about, and it's hard
> to find an an improvement that doesn't have a compensating cost.

Yes. Indeed. The only "possibly" compensating idea is that we are using 
these sets to receive modes that were not generally used in Fred D.'s 
time....like SSB.

And as one of the "Mikes" pointed out a couple of years ago, correcting my 
initially incorrect assumption, they were also never, really, designed for CW 
either, since almost all of the operation with these sets was on AM.

Besides, I WILL NOT change the circuitry unless the receiver I am working 
with has been ruined for restoration purposes anyway.

Ken W7EKB