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

[D C _Mac_ Macdonald]

True that, Robert, but it seems that VERY few folks have an MD-7 or even the modulation transformer from one. 
 
I still need to get to the modulator task which lies somewhere around 2,597 or so in my priority list of desires! 
 
* * * * * * * * * * * 
* 73 - Mac, K2GKK/5 * 
* (Since 30 Nov 53) * 
* k2gkk hotmail com * 
* Oklahoma City, OK * 
* USAF & FAA (Ret.) * 
* * * * * * * * * * * 
 

 
> Date: Sun, 1 Nov 2015 13:07:40 -0500
> To: arc5 at mailman.qth.net
> Subject: Re: [ARC5] Plate modulating a BC-696 or BC-459
> From: arc5 at mailman.qth.net
> 
> 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