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

[Leslie Smith]

  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.  

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

  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.

  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.)

  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.

  73 de Les Smith
  vk2bcu at operamail.com


On Wed, Jan 15, 2014, at 0:52, D. Platt wrote:
> Continuing with my query and comment about the regen mod in the BC-454, 
> I pulled the manuals for both the ARC-5 and the 274N.  I was really not 
> aware of the marked differences in the two designs. Shame on me.....  
> The 274N receiver has no AGC and there's no good way to improve BFO 
> injection levels.  I have an ARC-5 version for 80 meters, so I may move 
> over to that receiver and see what might be done.  The fellow I 
> mentioned on the OMRN, in fact, did his successful mod to the first IF 
> (12SK7) vice the 2nd IF, a 12SF7. Maybe that's what I'll try, next.  
> But, I'll include the removal of AGC from that stage, too, as Ken 
> mentioned.
> 
> Jeep - K3HVG
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