Making the 93MR AM radio work again

16 mar 2023

The Motorola 93MR radio was used in 1958 to 1960 Rambler Americans, and would probably fit in earlier Nash Rambler wagons. It has a wider knob shaft spacing than the "big car" radios so you can't substitute.

Physical work

I got mine from eBay, in fair shape. The speaker cone was crumbling, it sounded awful, the power wire was broken off, knobs were missing. Knobs are scarce, and good knobs even more so. There was a dubious set for sale on eBay for far more than I paid for the overpriced radio.

The radio needed electronic repair (next section) but physically mostly needed cleanup.

My friend Bruce gave me a set of knobs and parts he had. The selector dial parts were near-junk, and these were the best he had. The knobs are made out of cheap thermoplastic, they're ancient, and sit out in the sun, and worst of all, the illuminated dial has a 2-watt filament lamp cooking it from the inside. This is why they are all warped, cracked, missing parts, and generally unusable.

I was able to poke the transparent section out of one with a broken base (and probably a burned-out lamp, hence not fully cooked), scraped out the old white paint from the molded digits, repained, remasked in black, etc.

I replaced the cooker-lamp with a white LED lamp with a cobbled up reflector made from aluminum tape. The plastic knobs were scrubbed and wet sanded and clear coated (I used gloss, should have used satin).

The yellow lucite pointer was intact but very ugly. I carved a new one out of clear lucite and affixed some green film I had. It needs redoing in yellow.

Knob shaft nuts

Consider yourself lucky if you have all three of the mounting shaft nuts. These are peculiar to this one radio only. They look like ordinary switch nuts, but are 11/16" diameter. The thread is 11/16-32. I am still missing one.

Two (one each shaft) mounts the radio to the instrument panel, but a third holds the fixed tuning dial to the chassis. DO NOT put it under the mounting nut, it will crack and destroy an irreplaceable part, as well is causing the pointer to scrape on the dial. Mine floats loose but undamaged until I can find another.

Replacement antenna

I should have anticipated that old style fender mount mast antennas are a thing of the past. Most of what's out there is unusable crap. I found this Hirschmann "Porsche 356 replacement antenna" on eBay for fifty bucks. It's well made and can be collapsed completely into the fender.


This is a primitive radio. It's a hybrid tube/transistor type, and uses special low-voltage-anode tubes. It has no AGC, so loud stations boom, or worse, get very choppy as strong signals overload the radio's front end.

It has two transistors, comprising the audio stages. If the radio sounds really scratchy and has low audio, replace the first audio stage, a TO-5 can PNP press-fit into a hole in the chassis. Don't attempt to extract it, just clip the leads and leave it there. Substitute any PNP transistor, believe me, it will be a better part than what's in there now.

The final audio stage, the venerable 2N176, has a wirewound rheostat to set base current. Mine had burned up from heat, lol, and crumbled into dust. I used a test pot to determine what fixed resistor will provide the correct current. In my radio this was 140 ohms, 10 watts! It gets hot! This value depends on the actual DC gain of the transistor, which varied widely (huge manufacturing tolerances; these are very early transistors) so you will need to do the same.

I de-soldered the big triple-capacitor (no small feat) and tested it; it's surprisingly fine, and capacitance higher than marked (tolerance).

(To measure final audio stage collector current, there is a one ohm resistor in the emitter circuit; measure voltage aross this resistor, 0.25V is 0.25A.)

I was able to find an 8-ohm speaker of the correct dimensions, but incompatible mounting holes. Though this sounds gross, I affixed it to the radio chassis with Permatex Ultra Gray. Radio remains unmodified and it will not rattle or bend the frame.

Lubricate the two concentric shafts (volume/tone and the dial and indicator) with "lock lube", which is essentially kerosene and graphite. A fraction of a drop of oil on the big capacitor's ball bearings.

I have the gear to align the RF section and it all went by the book. The process is iterative, as some of the tuned circuits interact. I spent an hour to get it right; you can be certain at the factory when new it got a minute or two (though by very experienced techs).

All in all, it's a fine sounding radio, I like having a tube set for the lulz. Really, the only reason I bothered with all this is to be able to listen to Navajo station KTNN in the four-corners area on road trips. Here in Los Angeles it's mostly traffic, news, and angry racist men. Also lovely spanish music stations and chinese talk and story stations.