FanPlatenotes.txt
From K1LKY

Notes about the “cobbled up fan mounting plate” I made for the R-390/URR and R-391 radios:

(See image file FanPlate.jpg)

While testing out a new-to-me R-390/URR (the “non-A R-390”), I had substituted a pair of 6080 series pass regulators for the far less common 6082 tubes normally in the radio.  I’d disconnected one lead of the normal filament supply and run a piece of hi-fi speaker wire in from an external 12 volt filament transformer to run the 6.3 volt tubes.

The speaker wire had gotten up against the 6080 tubes and the insulation melted !!  WHOAH, I thought, this is WAY TOO HOT.

So I cut out a piece of aluminum, made some holes in it with a Greenlee chassis punch and mounted a 4 inch computer fan against the side of the radio.  I found that if I slotted the screw holes, three of the frame screws and two of the bottom plate screws could be used to hold the plate to the side of the radio, with no modifications to the radio itself.

After running the fan, I found I can put my fingers on the tubes and hold them there, with  no discomfort. With out the fan, that would mean a serious skin burn!

My lash up used a simple line cord, and the fan runs all the time it is plugged in. Some folks use an external switch or power strip to run the radio to save possible power micro switch failure.  It would be quite possible to power the fan internally from the radio in a couple of ways:

1) Find a fan that would run on the radios internal DC relay supply.  Even a smaller fan than the one I used would make a big difference, so a 12 volt DC fan and a dropping resistor would work.

2) Solder a wire with a two prong receptacle into the radio main power connections after the power switch.  The small receptacles that snap onto lamp cords you get at the hardware store would work fine.

One difficulty with the fan I used is that you can’t mount the radio into a rack cabinet with the fan mounted. (There may well be thinner fans that would work well.) That’s why the screw slots allow you to slide the plate over the loosened screws and down into place once the radio is in the cabinet.  It’s not necessary to tighten the screws once the thing is in place, though a bit of thin foam weather stripping on the radio side of the plate would make the installation a bit more secure.  The vibration of the fan won’t cause any trouble.

You could simply place the fan on its edge on the bottom of a rack cabinet next to the radio without any plate if you like. This is practical way to make it work if your radio is on a shelf or table.  Alternatively a tall fan plate that hooked over the top rail of the radio would allow easy installation of the fan.

You might wonder if the fan should blow out or in.  Mine blows in, and provides additional cooling for the rest of the underside of the radio, but I don’t think it matters much.  If your fan blows in, the heat from the regulator tubes will go into the radio but the much improved flow of air will keep the whole thing cooler than originally designed.

May your radio live long and cool!

K1LKY