BC-456 Modulator for SCR-274N

[k3msb]

After about a year’s worth of work I finally got my SCR-274N system on the air over the weekend and had a blast on 40 and 80 CW.  It’s complete except for the BC-456 modulator and is working pretty well.  I still have some issues to iron out, but it’s time to move onto the modulator.

Several months ago I acquired an almost pristine and unhacked BC-456.   This uses a 1625 and is a screen modulator.   

Has anybody used one of these critters?   How does it sound stock (I can guess….) ?  Any “Gotchas” to watch out for?

Thanks!

[KK4RF]

Check with Chris, AJ1G. I've heard his screen-grid modulated ARC-5 transmitter on 80 meters several times and it sounded pretty good. I'm pretty sure he uses the stock screen grid modulator that you describe. Good luck.---Marty, KK4RF---

[AJ1G]

It can sound amazing good, especially if you drive it with clean external audio.  I have been using an RM-12 military remote control unit to feed in the audio which on the air consists of a Peavy cardioid dynamic mic driving a Rat Shack octave band EQ into a small solid state utility amplifier (I got it at Nearfest a few years back for 5 bucks). It originally was used in gas stations for driving outdoor background music speakers and paging, etc.  It has a 600 ohm high impedance output that I drive the RM-12 with. 

You have to be careful on tuning up the command sets using the screen modulator.  As noted on some of the transmitters, front panel tags, the best way to do it is to tune for max output in the CW mode, and then switch modes to voice.  If you tune for max output in voice, you will end up with a severely flat topped  and clipped mod waveform.  I  have also found that using heavy loading on the transmitter by using a matching network, like the KJ4KV auto transformer circuit so that the transmitter sees a very low impedance load -  less than 20 ohms vs 50 ohms (as it was designed to) really helps the mod linearity.  I always keep an eye on the mod waveform with a scope, some slight tweaking of the the antenna coupling control while putting out some atomic yay-los can make a noticeable improvement. Or, if you can, feed in a clean test tone from something like an HP-200 audio oscillator.  I have done frequency response sweeps of the setup with the BC-456, using my HP-200 and it is very clean down to about 100 Hz, and way out to over 10 kHz.  Below 100 Hz there is some notable distortion in the the mod waveforms, but it still sounds pretty good.  The quality of "test audio" fed into the system with the transmitter driving a dummy load is pretty amazing.

Using the auto transformer matching network and the DM-33 on the BC-456, I get about 50 watts out on CW and about 20 watts on voice when adjusted for best modulation quality.   

The weak link in the command set audio quality in an otherwise original setup is the carbonium microphpnium.  I can plug one into the RM-12 in lieu of the external audio and it sounds down right awful.  The best ones to use are the T-1 handset microphone elements from an old telephone. 

[k3msb]

Hi Chris --

Thanks for your comments.   I have a few old mics lying around and I need to find out what they are but I like the idea of driving it with external audio if the "stock" method is going to cause the receiving op to cringe!

I did a search for the KJ4KV auto transformer circuit but couldn't find it.  If you have the schematics could you pass them along?   Currently I'm using a 67 pf series cap for 40M and a 200 pF series cap for 80M then onto a 4:1 UNUN to match to my 52 ohm coax.   I've also used a 100 pF series cap on 40M but it doesn't seem to work any better.  Both of these always result in the link coupling to be at max;  I'm a bit surprised at this.

Here's a shot of the station:  http://www.k3msb.com/scr274n//main.html

I currently have the 459 apart as the oscillator waveform is down right ugly and the on air tone is horrid. 

[W2PFY]

Glad to see you fellows into those rigs. I never was attracted to them as I am a high power nut case. I had a transmitter and I gave it to W2JBL a few months ago. I think it was a 75 meter transmitter. He uses his at his camp and I think his latest adventure is going mobile with a few of those?