There should be a small hole at the bottom front of the radio(most likely centered) behind that hole is an ALLEN type screw.simply insert the proper size allen wrench and turn it counter-clockwise.This loosens a"pawl" that holds it in the rack.once youve loosened it(about 5 turns) just pull straight out on the radio.Re-installation is of course reversed.
Ive got a Garmin GNC250XL GPS/COMM with moving map in my 150,and wouldnt get rid of it for the world.I suggest you look into one of these units.It will require rack installation,and some wiring,but its WELL worth the money...There are of course newer Panel mount GPS units out there with color etc,but the ole 250 is a great radio...tailwinds.

I got the radio out well the mechanic got it out

There are a couple of direct slide in replacement radios for Cessna on the market.  These require no rewire behind the panel or other modifications ty our system other than just slip them in the old tray.  Narco makes the Mk12D and Mk12E, both direct Cessna replacement as does Mitchell.  Both of which are currently in production and both have a better reliability than the old ARC.

Hello George,

I have a 1972 Cessna 172M equipped with original Cessna 300 Nav Com unit. The local Atlantic Avionics Company here at Halifax, Nova Scotia says that they don't service this type anymore, even though there may be very little actually wrong with the unit. If the MX 300 unit is a direct installation, then I would like to enquire if it is still available.

I will try telephoning this evening.

-Andrew

Do you still have the MX 300 for sale?

HI JOE,

HELP PLEASE.

I HAVE A 1974 C-150 WITH THE ARC MX300 NAV/COM. I FLEW A COUPLE OF WEEKS AGO AND THE RADIO WORKED FINE. LAST TIME I WENT FLYING THE RADIO WOULD NOT POWER UP. NO DISPLAYS, NOTHING. I CHECK ALL THE RADIO FUSES AND THEY ARE FINE. I AM TRYING TO REMOVE THE RADIO TO HAVE IT SERVICED. I TURN THE UNLOCKING SCREW UNTIL IT'S LOOSE BUT THE RADIO WILL NOT COME OUT. IT APPEARS TO BE STUCK ON THE LEFT SIDE. I AM AFRAID TO PRY TOO MUCH FOR FEAR OF DAMAGING THE LORAN OR THE PANEL. BEFORE I TRY TO REMOVE IT AGAIN IS THERE SOMETHING ELSE I CAN CHECK LIKE ANOTHER FUSE, LOOSE CONNECTION ETC. MY AIRPORT DOES NOT HAVE AN AVIONICS SHOP SO I'M STUCK. DO YOU KNOW OF ANY TRICKS IN REMOVING THIS UNIT.

THANKS.

I can't get the lights to turn on for the frequencies for either comm or nav, top or lower stack--they did work, but as I was playing with freqeuncies and turning on and off the master switch to save battery the lights stopped coming on.  Volume knob does seem to turn on VOR and the radio sounds on, but no lights