R-390A HF Receiver

I have had this Collins-designed receiver for many years, it is superb on CW as one would expect and it sounds good on AM. Probably hard to beat on CW even today. Considered by many to be the very best vacuum tube receiver ever made; they have a considerable following.

A small number were built as late as 1984 for service aboard Spruance class destroyers, they allegedly cost $ 20,000 each then. Not hard to believe. History, performance specs and the dreaded “Ham-mer” mods (caveat emptor) are widespread on the internet so I won’t go into that.

My 60 year old example has had some relatively minor issues. I have owned it for over 25 years and it is a good performer. It was a ham-swap find for $150… It has a Motorola tag on the front panel (contract 14-PH-56) but it does not match the serial number on the chassis.  It also has a Stewart Warner IF module.  I have not needed to replace any tubes* in the past 25 years, it still has a noise floor lower than the usual galactic “band noise”. Maybe I’m lucky but the tuning is very smooth and easy, low torque, probably because previous owners did not gum up the gears with too much/wrong lubricant. It is in otherwise excellent cosmetic condition.

I operate CW almost daily and I prefer not to use very narrow filtering unless another signal(s) are right on top of my target. Then, using the 1kc or 100 cps filter is extremely effective without the fatiguing artifacts of the narrow crystal filters/DSP found in the modern riceboxes. I just prefer the sound of the CW signal going through the mechanical filters in the R-390A much better.

R-390A at N6CC

R-390A at N6CC

The first problem it had (when I got it) was an instability in the BFO. The BFO 455 kc frequency would occasionally “jump” or quickly “drift” about a hundred cps before it warmed up. Thirty minutes after turn-on, it settled down and was rock stable thereafter.

After checking all external components, including a tube substitution, I determined that there was some kind of component issue inside the BFO “can”; the external circuitry was per spec. So I let it warm up a bit before serious CW ops, no problem for now.

The next issue was that it eventually failed to turn OFF upon rotating the Function switch to the OFF position. That was caused by the AC power switch contacts sticking together after 60 years of slight arcing upon turn-off. They are inside a repairable microswitch that is cam-actuated by the Function switch. Easy enough to disassemble and burnish, however it required removal of the front panel. Not as bad as it sounds. Now, all is well.

Another problem was an open-circuit inside the AC line noise filter (on the line Hot side). AC power not getting to the line fuse or power switch. Strange, those should be pretty robust. An AC line voltage spike? We get those on occasion. This is a hermetically sealed box and I did not attempt to open it up so I simply bypassed it for now. Back on the air.

A very recent problem was an “opened” *3TF7 Ballast Tube (RT-510) which regulates the filament current to the series connected BFO and VFO oscillator tubes. Not sure why that happened. Hmmm, related to an AC line voltage surge? But the line filter and 3TF7 failures didn’t occur at the same time. I don’t have a spare 3TF7 and NOS units are crazy expensive so I just replaced it with 25+20 ohm, 10 watt resistors in series as a field expedient repair for now.

The leads from the new resistor terminal strip are plugged into sockets 2 and 7 of the RT-510 tube socket via gold plated connector pins. No soldering, easily reversible once I capture another 3TF7. Works fine, I can detect no stability changes in the receiver.  (BTW, the BFO frequency still “jumps” even after the ballast tube function has been replaced.)

R-390A Ballast Tube field repair

R-390A Ballast Tube field repair

OK, it’s a “Mod”. But I labeled it and it really is temporary! Improvise, Adapt, Overcome…..

On radioactive meter paint: My R-390A Receiver meters have detectable levels of radiation from the radium paint used on the scales and needles. This isotope is Ra226 and it excites the zinc sulfide phosphor material in the paint. The residual glow from mine is essentially zero these days, the phosphor has long since stopped fluorescing but the radium is still there as it slowly decays into radon gas.

The Carrier Level meter read 1.2 mR/hr, the Line meter read 0.4 mR/hr. These measurements are primarily from Gamma radiation – Alpha’s won’t penetrate the meter glass, nor will most Beta’s; sorry, no Neutrons. Measurements were made with an Eberline E-120 Geiger counter with detector probe HP-190A in contact with the glass. The usual precautions: don’t disassemble the meters or disturb the paint in any way. Otherwise there is no hazard.