The AM Forum

I'm going to build this circuit below this weekend and wanted to get a little last minute feedback before I do.  It's a CB to 75 meter receiving converter using a single mosfet. 

The antenna is connected to one of the gates via a resonant 75 meter tank.  The tank inductor is in the picture attached and is made out of a cardboard form and electrical tape using 24 guage wire. 

3.870 Mhz-ish is mixed with the 23 Mhz oscillator to center around channel 10.  Course tuning is accomplished through changing channels, and fine tuning is done via a variable capacitor in the oscillator.

The ARRL handbook is saying that there should be around 6 volts PP on the gate from the oscillator.

I know CB's aren't high fidelity devices, but I need a receiver and I found a CB at Goodwill for $6. 

Jon
KA1TDQ

From the subject I had not idea what you were doing.... It should read 75 Meter to CB AM receive converter.

Interesting idea, lets us know how it works.

Yeah, imagine that... technicians and engineers throwing a converter in-line to their 75 meter receivers in order to listen to CB high-fi. 

There could be a market for that.  They could be sold at truck stops.

Jon

For a minute I thought it was a trick question:

The answer is a ham ticket.....

73GD

That looks like a pretty simple circuit, I might have to try it here, I have an older 23 channel CB that has a somewhat decent receiver section, but won't transmit. The nice thing is it has a fine tune control built right in to it too (it's ssb capable in addition to AM).

I would suspect overload, distortion, and lack of real selectivity will be your biggest problems. Some CB's require the mike to be plugged in in order to hear the receiver. Make sure you don't key the transmitter.

The concept is not new. In the 70's, Clegg made a solid-state HF to 2 meter receive converter called the  AB-144 "All-Bander".

My friend KC8COM sells a cheep kit that allows RX only of many HF bands with any regular CB radio.

 Multiband Converter from Cycle 24 Kits will allow you to receive the 40, 20, 17, 15, and 13 meter bands on an ordinary cb radio and the 80, 49, 40, 20, 15, and 10 meter bands on an am broadcast receiver with no modification to the radio. Get your own here:
http://www.cycle24kits.com/
The converters consist of an sa612an, 5 selectable band pass filters, 6 selectable crystals for the local oscillator, a VXO circuit for fine tuning, and a simple transmit protection circuit on the output to protect against accidental "key-ups" when using the converter with a cb or 10 meter radio. I've also included a place on the board for an lm-386 audio amplifier to allow the converter to be used as a direct conversion receiver.

I did this video for someone showing a "HAM" class at a local Tech school a few years back.

http://www.youtube.com/watch?v=qyv14j5mH3w

Well, here it is.  No smoke...

My CB by itself has a terrible hum from the speaker even with the volume turned all the way down.  But, when I plug my converter in, I am able to tune the input tank for a pronounced hiss peak.

Also, when you turn my converter on and off, you can hear a definite change in the his level (hum is there no matter what).

Anyway, I'm going to have to wait until Monday when I can take this in to work and get some hard numbers on it with some real test equipment.  I'll inject about a -110 dbm 75 meter signal into it and see what I get out on channel 10.  I'll also see how much the VXO pulls the crystal. 

Jon
KA1TDQ

 Jon,

   That hum is most likely from 60hz power ripple in the 12V feeding the CB radio. I see you used half wave rectification in the converter box. Any chance you can give that CB 'cleaner' power, maybe from a battery to see if that gets rid of the hum?

  Also I wonder if your 7984 tube based creations survived your move. I just noticed your U-Tube videos seem to be gone. My favorite was the one where your XYL was showing off the ham station while the tune "Listen to the AM radio" was playing. That was fantastic.  ;D

Jim
WD5JKO

I still have the 7984 transmitter, but everything else is gone/tore apart. I just couldn't fit everything into the car. I could kick myself for that too because I had an awesome station. I'll never have the money again to buy all that heavy tube stuff again. To xmit again I will probably have to go solid state simply due to cost. We're trying for a baby so finances have other priorities.

I do have two small crystals for 3885 and 3880 which are HC-17 (I think). I thought about modifying Steve's VFO and just using a crystal oscillator into a FF and then driving the logic gates without the frequency divider. I could probably do class E, but it would have to be in stages... vfo, RF deck, power supply, modulator, etc.

YouTube stuff is gone, but I'll make some new stuff eventually.

I'm typing on my phone too, so there may be mistakes.
Jon
KA1TDQ

The 23.3 Mhz oscillator is working but the mixer isn't.  Fry's only had one 2-gate MOSFET and wanted $10 for it, so I put in 2 JFETs.  When I was building it I caught myself making several wiring errors and probably one slipped by.  I think I'll order a bunch of 2-gaters from Newark and give it another try.

Jon
KA1TDQ