[ARC5] OT? bc-348 bfo question

[Christopher Bowne]

My Scott SLRM was initially very poor on CW due to low BFO injection levels.  A review of the set's schematic showed that there was in fact, no designed in deliberate coupling between the BFO and the IF signal chain, it apparently just worked on stray coupling. A gimmick cap from the BFO  tube cathode pin  (as I recall, that gave the best signal level) to the 2nd IF  tube grid pin improved things tremendously.  Just removed the tubes, inserted the gimmick leads in the sockets, and reinserted the tubes.  Did that when I was a JN back in 1967,  the gimmick is still in there.

I always wondered if the lack of direct coupling was in keeping with the low RF radiation  design of the set for use on ships in convoys.

Another set that was initially bad on CW due to low BFO to IF signal ratio was my WS19 MKII.  There it was due to lack of an RF gain control, which BTW was included in the MK III versions.  I added a  gain control pot in the RF amp cathode circuit from a command set control box into the upper right corner of the front panel, the only place to put it.  I only afterward found out that that is where the MKIII control was added, so its kind of a retroactive field change.




________________________________
 From: D. Platt <jeepp at comcast.net>
To: arc5 at mailman.qth.net 
Sent: Saturday, September 14, 2013 8:29 AM
Subject: Re: [ARC5] OT? bc-348 bfo question
 

All of you guys have it right.  The BFO injection needs to be upwards of 25% to 50% of the received signal level, at the point of mixing.  Unfortunately, the BFO signal is fixed and one must use the RF gain to bring that ratio back into some sort of reasonable number, but at the expense of gain.  Increasing the BFO injection will certainly help.  Its only when signals are at or near the receiver MDS that too much BFO can be a negative factor.  Truth told, I really don't know what the actual ratios are in these old radio sets.  I read somewhere that only a very small percentage of signal voltage is required to provide a usable CW note.   I'd be curious to know what results you find in the BC-348.  I know that in receivers such as the S-40( ) and the SX-99, twisting the gimmick to twice the turns does, in fact, help.

Jeep - K3HVG
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