GRC-9 Receiver Dial and Panel Illumination

UPDATED: 5JAN14
The dial light in the GRC-9 receiver is pretty dim. I thought about powering an LED off the receiver B+ line via a dropping resistor but that would draw as much power as the entire B+ circuitry so it was not tried. At the suggestion of an ANGRY9 Yahoo Group member KA2IV, why not put the LED in SERIES with the B+ line, the current is in the right ballpark and the resulting B+ voltage drop should be minimal.

So I ran some experiments to find a brighter illumination scheme. The lamp in my radio is a #49 (violet insulator), not sure if this is the stock lamp. At 2.0 Volts it is rated at 0.5 Lumens, (but the filament string in the GRC-9 is 1.4 volts.) By way of comparison a standard 6.3 Volt #47 puts out 6.54 Lumens, big difference, 13+ times.

I tried to put a Green LED in series with the receiver B+ line. (I chose Green since the eye is most sensitive to Green, all other things being equal. I didn’t have a White LED but they are noticeably less efficient that a Green LED.) The LED dropped 2.03 volts while the receiver drew between 10 and 12 ma depending upon the loudness of the received signal. I attempted to photograph the result to compare it with the stock incandescent lamp. Very tricky photography with a mickey mouse digital camera, close-up lenses, holding everything in place and pushing the Dial Light button when testing the stock configuration. Pretty crummy, out of focus photo’s, the camera didn’t like being that close-up etc.

GRC-9 Receiver tuning dial

GRC-9 Receiver tuning dial

Above: Stock Config, no illumination, external room lights leaking in behind dial, focus check

GRC-9 Stock Dial Illumination

GRC-9 Stock Dial Illumination

Above: Stock Dial Lamp as-is, turned ON, in the dark, can’t focus properly. Not sure what this camera uses to determine best focus…Probably some chromatic aberration degrading the resolution as well.

Receiver dial with LED illumination

Receiver dial with LED illumination

Green LED in series with the B+ line. LED On, tough to focus, image is larger. Looks brighter than the previous photo but not noticeably so to my eye. This scene apparently has a lot of dynamic range and the camera can’t quite handle it. Should have minimal chromatic aberration in the camera optics. My eye is better, dynamic-range wise……

Results: Hard to tell from these photographs but to the eye, the stock lamp and this particular LED seemed to be about equally bright although that’s not what the camera recorded. Probably due to the cameras’ processing of incandescent light versus the eye’s photopic sensitivity curve. Aiming of this particular LED was also somewhat critical since it incorporates an integral molded-in lens that had a fairly narrow cone of light output. Difficult to capture because I ran out of hands in this simple experiment.

I also tried a simple aluminum foil “reflector” behind the stock lamp – it helped a little, not as much as expected but then it was not a decent “reflective optics” design for a focal point at the dial surface. Might be worth doing with a little more attention to detail.

Also, with the LED, the receiver functioned normally with this 2.03 volt loss of B+ (from a 98 volt supply) at around 10 ma. However, turning the LED on and off with the panel switch (in this experiment by just shorting out the LED), caused a frequency shift in the receiver of maybe 500 cps…If left ON continuously, it would be fine.

At least at this point, with this particular LED I think it’s not worth any small improvement. I’ll keep my sets stock and just use a pen light as I have been doing for Night Ops…A dark adapted eye is pretty good – as long as I can find my glasses in the dark!!

As a follow-on experiment, this Green LED, switch operated, powered at rated voltage and current via a dropping resistor off the receiver 4.5 volt bias battery would probably generate significantly more light. That may be worth the effort, I haven’t tried that.

UPDATE 31JAN14:

I have since thought that a 1.2 volt TL-3 style, # 112 flashlight lamp would work as a dial light. Those lamps are designed for a single 1.5 volt battery for power and are actually pretty bright inside their “focus cone”. They have the integral lens to concentrate the light and should work fine at 1.4 volts off the receiver filament line. Especially in push-to-illuminate mode. Continuously ON might be a lamp lifetime issue as others have noted. Also, it is a threaded base so maybe a defunct bayonet lamp like a #47 could be turned into a bayonet plug to fit into the existing receiver lamp socket with no radio mods. Then jury rig the lamp and “plug” with connecting wires to shine on the back of the tuning dial. I don’t try to run my receiver from a BA-48 so current drain is not an issue for me.
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Panel marking illumination:
Now on to lighting up the panel paint. Or, more Night Ops with the GRC-9….

At the suggestion of KC5MO on the ANGRY-9 Reflector, how about using an ultra violet LED to energize the phosphor paint on the control markings? Good idea!

Ultra violet light (wavelengths shorter than 400 nanometers) is very effective in exciting fluorescent or phosphorescent paint used in meters, clocks and instrument labels on military equipment without washing out the operators’ night vision. WWII aircraft had a small UV lamp using a mercury vapor source to light up cockpit instruments – so how about an LED?

So I bought 2 “ultra violet” LED’s from Radio Shack (P/N 276-014), two for 2 dollars. (I’d probably call them “near” ultra violet.) I then modified a cheap LED flashlight to power one of them. These LED’s are barely on the border of ultra violet at 395-405 nm but they are energetic enough to do the job here. These LED’s require between 3.3 and 4 volts to power them so a standard 2-cell flashlight would not work. So I took one of those “Dollar” 3 AAA cell LED flashlights and modified it so its 4.5 volts through a 47 ohm current-limiting resistor provided adequate but safe current flow through the LED. Incidentally, those “dollar” LED flashlights are a study in mass-produced cold solder joints! Sheesh!

The LED is rated at 20 ma and that’s about what I got with a drop of 3.4 volts across the LED. Depending upon your particular battery voltage, you could go with a smaller resistor for more optical power. I am being conservative. If you try to run the diode at 4.5 volts with no current limiting resistor, you will soon end up with a DED – a Darkness Emitting Diode –

I removed the 6 white LED’s and inserted the UV LED and its series resistor. I also deleted the clear plastic “lens” since UV light does not make it through many clear plastics. Presto – an “ultra violet” flashlight to experiment with. It is not particularly “bright” when looking down the muzzle of the LED but then again, maybe I can’t see all the energy. My eye isn’t bleeding just yet. As the sign in my old laser lab advised:
“Caution! Do Not Look Into Laser Beam With Remaining Eye”

Six LED flashlight parts

Six LED flashlight parts

Above: A cheap 6-LED flashlight powered by 3 AAA batteries disassembled for modification. Those 6 white-light LED’s will be removed.

Single UV LED installed with resistor

Single UV LED installed with resistor

Above: The modified PC board with the single UV LED and 47 ohm resistor.

UV LED Flashlight

UV LED Flashlight

Above: The modified flashlight. When describing “brightness” in eyeball terms you are getting into semantics because technically, you can’t see UV light – so it can’t be “bright” in photopic units. Whatever – there is enough eye response and sufficient “violet” light to be visible with the naked eye. The digital camera can clearly see it too but it does not look this bright to my eye.

Now on to experiments with the GRC-9 and anything else I could find to test it against. This included my scopes’ phosphor screen, military clocks, aircraft instruments etc.

The Plot Thickens….I have two GRC-9 sets, one is for portable field Ops and the other is mounted in my truck. In both sets the index lines on the control knobs respond well to UV and they retain residual glow. On the truck radio, the panel paint (which looks pale yellow in daylight) also responds well with a long half-life glow. The field set apparently just used ordinary white paint on the panel and it does not respond at all.

My O’scope screen lights up brightly, several radium painted dial meters don’t respond at all. (The radium’s radiation has long since damaged the phosphor paint so it does not actively fluoresce even though the radium is still present). One aircraft instrument I have fluoresces bright orange under UV illumination but when the light is off it instantly reverts back to whitish looking paint – no afterglow..

The clock in my truck formerly lived in a GRC-26 Comm shelter and it responds quite well with a very long half-life glow, probably hours. The half-life glow from my GRC-9 truck set has a much shorter time, maybe a minute but it has a very long, dim “tail” probably an hour or more depending upon your night vision adaptation. Different phosphor compounds have different persistence time constants. Here’s a blurry photo of the clock after removal of the UV exitation:

Military clock face after UV light exitation

Military clock face after UV light exitation


Above: It is very difficult to photograph scenes like this with my cheap digital camera, especially in the dark! I don’t know what the camera uses to determine best auto-focus, but this is what results. You get the idea. In trying to photograph the GRC-9 panels, same thing, although the residual glow is much less intense than the clock so I was unable to get any photo at all.

To the eye however, the residual glow from one of my GRC-9’s was quite usable. I’d call that a success. I didn’t do a photographic comparison of the response to the UV light versus an ordinary incandescent flashlight or white-light LED’s – that experimental oversight is called “sloppy science”!

Bottom line for me: I’ll call it “Plausible”. It works, it’s fun to play with and I learned a few things – and the UV light did not completely wash out my night vision – that was the goal. If you are going to go to this trouble for a practical solution, just use the regular 6-LED white flashlight as long as you don’t need your night vision immediately thereafter. Or just keep your eyes closed when “charging” the paint.
So who’s going to build a 6 LED UV flashlight and really light up the radio?

Now I can go outside at night and look for Uranium ore in my back yard….