[Milsurplus] bc-348 design question

[Al Klase]

OK.  I'll weigh in on this.

As Robert Downs points out, this radio was designed 15-20 years before 
SSB was a factor.  So trying to make it into an decent sideband receiver 
is a fool's errand.

On 12/11/2011 10:44 PM, John Hutchins wrote:
> When receiving SSB signals, with the BFO on, seems that strong signals
> swamp the injected BFO signal.
Wayne's right on with this one.  You might try increasing BFO injection 
by installing a small cap (10pF?) between pins 2 and 5 of the 6F7.  I 
wouldn't mess with the existing gimmick capacitor that's just a matter 
of wire placement.  In the AVC mode the increased BFO signal will drive 
the gain down, and leave the receiver somewhat deaf.  Use MVC.
> I wish I had an opportunity to  talk, read, about the design
> considerations  made by the RCA designers on the BC-348.   After some
> time looking at the circuit and playing with the restored R version;
> Questions formed -
>
> Why was the 2 IF not set to some predefined gain ,  with the cathode
> resistor to ground,  and with and AGC-AVC directly applied to the grid
> of the second IF; regardless of the MVC-AVC switch position ?
Gain "structure" and control in a receiver is very important.  I'm sure 
the guys down at RCA Camden gave this a lot of consideration.  They were 
the 700-pound gorilla of receiver design at the time.  Again, SSB was 
not a consideration.
> IMO this would have stabilized the BFO circuit since I can hear a slight
> pitch change when adjusting the MVC when listening to SSB transmissions?
Not surprising when the gain-controlled 2nd IF pentode and the BFO 
triode share the same cathode.  This probably goes almost completely 
unnoticed in CW operation.
> The military wen to the trouble of adding in  a squelch circuit.  Why
> was there not a mod for the BFO  / 2 IF circuit to make SSB reception a
> bit better?
To fix this properly you need a "product detector."  I'm partial to a 
pentagrid convertor replacing the BFO.  But that won't work for AM, so 
you need additional switching.  The real problem is making the AVC work 
properly for SSB.  The 348 does switch an extra capacitor onto the AVC 
bus when the BFO is turned on, but that's only a partial solution.

So, I might make a light reversible mod to get a bit more BFO injection, 
but anything else is going to require chopping up the whole receiver, 
and you still won't have proper IF selectivity.

Regards,
Al

-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/