The latest project at N6CC is “restoring” a 1965 Collins 618F-1A VHF aircraft radio. This radio was designed for light aircraft as an early frequency – synthesized radio built in 2 separate chassis. The “dashboard” chassis shown here is the RF section consisting of the receiver front-end and the transmitter RF circuits. It tunes in 50 KC increments from 118 – 135.95 MC, providing 6 watts of plate-modulated AM output on 360 channels to the antenna. In an aircraft, it connects to a remotely-located auxiliary chassis containing the 500 KC intermediate and audio frequency amplifiers, the transmitter plate modulator and the high voltage DC-DC power supplies.
The system can be configured for either 12 or 24 VDC operation, depending upon the aircraft power source. I obtained this well-traveled RF chassis at a ham swap-meet but unfortunately did not find the IF/AF power supply chassis. This radio is now obsolete in the US as of 1 JAN 1997. They are not authorized for transmission aboard US-registered aircraft as modern technology now requires 25 KC channel spacing as well as correspondingly tighter +/- 0.003% frequency stability. The earlier standard was +/- 0.005%
So I powered it up using a “universal” power supply to provide the 12 VDC filament/relay supply as well as the 125 VDC receiver B+ plate supply. I am using an R-390A receiver tuned to 500 KC as the temporary IF/AF amplifier system (stick THAT in your Piper Cub) and it works well in that role. After some switch cleaning and a quick alignment check, the receiver fired right up.
What’s next? Get a 240 VDC plate supply and modulator going for the transmitter and tune it up (into a dummy load of course). Then change the band crystal from 135 MC to 145 MC and realign that band so I can operate it on the 2 meter Ham radio band. Yes, we do occasionally operate 2 meters AM around here.
Still looking for the Collins 427D-1 Amplifier/Power Supply chassis and interconnect cable (got a spare in that scrap heap?). Until that surfaces I will keep running it on the bench the way it is. I may end up building a 500 KC IF amp and audio/squelch/modulator circuit along with a DC-DC converter to power it in the mobile. A beautifully built radio with chassis parts made of machined tooling plate rather than the sheet metal stampings you see in lesser gear. Silver-plated RF components as you’d expect. Great mechanical design to drive the display and synthesizer switches – oh, and it sounds great (well, via the R-390), very sensitive. It will look great mounted in the dashboard of the 1971 Bronco.
Update: After thinking about it for a few minutes, I thought (absent a 427D IF-AF Amplifier chassis) it would be easier to just adapt something rather than building something to replace the R-390A in this lash up. Hmmm. A CB radio has an IF of 455 KC.
Necessity is the Mother of Intervention.
“The enemy of creativity is good sense.” Pablo Picasso
So I took an old CB from the JunkBox and broke the receiver chain after the second mixer. I AC coupled in the 500 KC IF signal from the 618F receiver. I replaced the 455 KC ceramic filter in the CB with a capacitor and then re-tuned the IF transformers from 455 to 500 KC – they JUST made it. No changes to the Collins radio. Presto! The FrankenCollins (Flame Shields Up!)
My buddies won’t answer my phone calls any more….
Pay no attention to the Channel 9 display good buddy – the CB synthesizer is operable but disconnected. The CB is now just a fixed-tuned IF/AF amplifier. The CB provides plenty of IF gain and is adequately selective for 50 KC channel spacing provided by the 618F. Also, I get a free Signal Strength display, IF AGC and a Squelch circuit to boot. Fortunately the Volume and Squelch pots in the CB are the same value as those in the 618F (Mr. Murphy is on vacation) so moving those functions to the 618F front panel is a simple matter – the 618F was designed that way when using the 427D chassis.
As a bonus, the CB provides 4 watts of audio to drive an external “Public Address” speaker. When fed through a suitable modulation transformer, this should fully plate modulate the VHF transmitter PA tube. This is getting more interesting by the minute.
If that all works out, change a synthesizer crystal and re-tune the transmitter. Should make a cool AM transceiver for 2 meters until I can locate a 427D chassis. My ARC Type-12 AM set on 2 meters is getting nervous.
The Beast is pretty sensitive. I can easily copy high-altitude commercial aircraft over Nevada, inbound to SFO from over 200 miles away just using a home brew Diskcone antenna in the attic.
Update: Above is a “haywired” prototype high voltage power supply for the 618F-1A (hiding under the calculator) receiver section. The design is to use a 13.6 VDC primary power source to produce 125 VDC to power the receiver B+ circuits while mobile. The scope trace shows the switcher transistors’ base drive waveform and the resulting collector square wave drive to the custom ferrite transformer. Came out right on the money at 124.6 VDC under load. It has excess drive capacity for a 240 VDC secondary for the transmitter when the time comes. Next: a little packaging, I/O and shielding work.
Thanks again to Bill, W0BVR for providing the elusive 618F schematic needed to get the wheels turning.
I just obtained the elusive 427D-1 power supply-amplifier chassis. A friend in Denver located it for me so I am off in a new direction. I just have to fabricate the interconnecting cables (ugh!) and smoke-test it with the system. Stay tuned.
It’s done! I spent a couple of days fabricating the interconnect cable and IF cable and then a precautionary cleaning of all of the relay contacts. Fired right up – sounds good. I changed the primary power voltage selector jumpers – it now runs directly off 12 VDC versus 24 VDC. Handy…
I realized that the receiver was designed to drive 600 ohm headphones as expected, but it will also drive an 8 ohm speaker from a separate transformer secondary with 6 WATTS of audio! Wow! I wonder what the intent was for that? A speaker in the cockpit of a light aircraft or helo? Strange – but it sounds great. You can certainly hear the wide variations in distortion and the audio frequency response problems in the transmitted audio from all those aircraft. Some of them are absolutely awful, some sound very “Hi Fi”…
Strangely, even some big commercial airliners sound terrible with bad distortion and background noise. Even stranger, the ATC tower operator does not mention to the pilots that their transmitters need serious attention. Maybe they do that offline. Anyway, fun to play with. Now to decipher the frequency synthesizer design to figure out how to move the 134-135 Mc band up to 144-145 Mc ham radio band and try out the transmitter…
Again, thanks to Wild Bill in Denver for the lead on the 427D chassis and the all-important multi pin connectors!
Parts of FrankenCollins have been retired back to the junk box.