Monday, August 25, 2008

Codar AT5

I never used one of these famous 160m / 80m transmitters. A friend of mine from college had one and I spent one lunchtime having a look at it. So small and compact, unbelievable that this was a valve rig (and unbelievable how this thing was stable looking at the chassis layout!)

I know the circuit of this transmitter back to front.

I built my own 160m transmitter back in 1970. The valve line up was EF91 (VFO), EF91 (Buffer), 5763 (PA) ECC83 (AF PREAMP), 6BW6 (AF AMP/MODULATOR) the design came from Short Wave Magazine by F.G. Rayer G3OGR. Frank Rayer produced many variations of the famous Codar AT5, basically the same circuit!

When I built mine I had all kinds of problems, particularly winding the coils and obtaining a suitable PA anode choke. I eventually got one from the shop on St Georges Rd who reluctantly let me have one. He asked me how old I was and am I really old enough to buy a transmitting choke! The modulation choke was a sod to find as well. I needed two air spaced capacitors (500pf), one dual gang I got from an old radio, but the second had to be pretty substantial. I ordered one from Modern Radio, when it came it was brand new and cost me an absolute fortune. I had it up and running long before I got my licence, but boy was it unstable...dip the PA capacitor and the VFO went up the band causing havoc! I borrrowed my transmitter to George G3ZQS and he used it for a while on 160m but drifted up and down the band! While he was using my rig we had a heck of a thunderstorm while he was working Neil (G3ZPL) and Chris (G4AGJ) his antenna got hit by lightening and the plates of the air spaced capacitors got welded together!

I just had to add this photo! This is me in back in 1970 using the rig, the front panel is actually green, made from steel made by my Dad at work. To the left is my R109A receiver and then my WS38 (which I didn't use much) The lamp on top of the R109A is my 'Dummy Load' which was used to replace the antenna to test the transmitter - a 15w bulb that lit brightly!

Needless to say when I finally got my licence I rebuilt the transmitter on a better chassis with more screening and it worked a treat!


Anonymous said...

I built F G Rayers Valve Transmitter in the 80s too.
I used an Audio output transfomer as the choke in the 6BW6 anode.

Anonymous said...

Nice one, I built the similar unit in the 1960 hand book with the tt11 OP

take care
G4LWN , Ray