[ARC5] Plate modulating a BC-696 or BC-459

[WA5CAB at cs.com]

The modulation transformer in the MD-7/ARC-5 has three output windings, 
1625 plates, 1625 screens and voice sidetone.  Modulating only the 1625 plates 
will lead to distortion.  Another example of people modifying things that 
they don't understand.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 11/01/2015 10:36:30 AM Central Standard Time, 
tarheel6 at msn.com writes: 
> If you have a stand-alone modulator capable of plate modulatingsay a 
> BC-696 or BC-459, a short note in the June 1953 issue ofRadio-Electronics (page 
> 105-106) might help if you experience alot of distortion.
> The article says:o Replace R-78 (51 ohms), that leads to the 1625 screens, 
> with aa 50,000 ohm 20 watt resistor.   o Now disconnect the screen lead 
> from pin 4 of the power inputreceptacle, and connect it to the PA B+ pin, 
> which is pin 7.
> o Replace R-76 (20 ohms) that leads to the PA coil with a 2.5 mh300 ma RF 
> choke..
> o Replace the plate bypass capacitor, C-66) with a .002 mfd,2000 wvdc 
> capacitor.
> o And finally, for AM service, connect pin 7 on the power inputreceptacle 
> to the top of the modulator's output transformer.
> The article says the distortion before these changes was likelycaused by 
> not modulating the screens (IOW, pin 4 might nothave been connected to B+ by 
> the correct amount of resistance.)And that a substantial amount of the 
> modulated voltage was bypassed to ground by the .01 mfd value of C-66.
> Was wondering what experiences others have had with plate andscreen 
> modulating the SCR-274-N series, and if you made any changes different than those 
> recommended above to get goodresults.
> If there is any interest in having a scan of the article (which includeda 
> partial schematic), please let me know.   I'll be glad to make one and send 
> it to you.Best,-Tom  KE4RHH
>