Patrick's Motorola R-390 Restoration Blog

The Collins designed R-390 and R-390A are considered by many to be the best tube receivers of all time. The R-390 was a "spare no expense" radio and was ultimately so successful that the military wanted a more refined, more cost effective radio ... the R-390A. There were many more R-390As made than R-390s.

I have owned a Collins R-390A radio since I was a kid in high school. After was old enough to learn more about it I learned of the original R-390 and of course wanted one ever since. Not long ago, an opportunity came up to acquire my own R-390 and of course I jumped at! The seller said the radio “powers up” whatever that means. I took it to mean that it would need plenty of work!

A Little Bit of Info About a Big Radio

The R-390 is a double conversion (triple conversion below 8Mhz) super-heterodyne receiver. It employs two RF amplifier stages, six IF amplifier stages, and two stages of audio amplification. The extra features include antenna trimmer, six bandwidth selections, three audio filters, squelch, noise limiter, AGC, and a calibration oscillator. A fairly unique feature of the R-390 series radios is permeability tuned oscillators and the RF/IF tuned circuits.

First Things First

I know from my experience with the R-390A is that I couldn’t even plug it in without tripping the GFCI. The R-390 contains a line filter with capacitors going from both hot and neutral to ground. Before plugging it in, I had to rebuild the line filter.

First Power Up!

The R-390 does not use electrolytic capacitors for the main power supply (there are electrolytics elsewhere though). This radio uses large paper-in-oil capacitors that are generally very reliable. It does however have a complex voltage regulator that is the Achilles heel of the radio. The voltage regulator uses 2 6082 tubes that generate loads of heat that tends to bake everything around them. Also if certain tubes are removed or modules disconnected, the voltage regulator will fail to operate, putting out a full 300V or more for B+ instead of 180V!

The first very first thing after powering up was to check the regulated power supply. Amazingly it read 180V! Not surprisingly though the “powers up” description was accurate and the receiver was dead! It didn’t take long to figure out why. The 3TF7 ballast tube, the most expensive tube in the radio was bad. I swapped in a solid state version I use for the R-390A, and … I received the calibration oscillator! This is very good because it meant all the stages of the radio were working to some extent. It was clear the radio still needed lots of work though. Since I had done no cleaning of the chassis or switches, moving pretty much any switch killed the audio. Also the receiver was dead below 8Mhz and the carrier meter was pegged at full scale.

Definitely lots of work to do and lots more yet to come…

If you are interested, I have written up some directions on rebuilding the gear train for the R-390 radio and put them here. The pictures are of this radio