The AM Forum

I'm finally getting around to building the modulator for that 100 watt, 2-FET 75 meter RF deck I built some time ago.  I had been collecting the parts for quite a while, but never started building it because I was looking for a small rack enclosure to put it in.  Well, I found one!

I went to Goodwill a few weekends ago and found this one for $4.  It was covered in stickers but they all came off with Goo Gone.  

The other picture is the 19" shelf that the Heising modulator will be built on.  Lots of iron!  That will be on the bottom of the enclosure and the solid state audio amp will sit above it.  

The iron is for:

48 vdc drain supply
12 vdc IXDD driver supply
5 vdc negative peak limiter supply
Heising choke

I'm also attaching a picture of the RF deck itself.

Jon

Most of the heavy drilling is done and almost everything is mounted.  Three LED's up front indicate if the power sources are up. Audio input from the amp comes in the left front panel, and power in/out entry is on the right.  Wiring comes next.

I had to take a flashlight and snoop around my 75 meter modulator to remember what I did.  This is the same design.  Audio amp cut-in and the drain supply voltage output are both sequenced.  A big wattage resistor is placed on the Heising capacitor to allow it to charge momentarily after the drains are turned on.  This protects the audio amp.  

You can't see it in the picture, but there's a steel bar going across the shelf behind the front row of transformers.  There's lots of iron and the shelf needed a little reinforcing.

Also, a reverse biased diode is placed on the output of the Heising choke to ground allowing the choke to discharge after un-key.

I'm regulating the negative peak limiter supply to 5.1 volts using a zener/resistor approach.  I've done the math and everything will jive.  I wanted to keep it simple rather than using a switching regulator.  The 12 volts going to the IXDD's will be unregulated.  With drive, the voltage should drop to about 12 volts.  I got the transformer from a low voltage lighting kit.

Aside from that, everything else is typical.

Jon

The wiring is about halfway done.  I used a speakON connector for the connection to the audio amp. 

Jon

...and it's all done.

Jon