Astron Meter lamp replacement and cooling modifications

I had one lamp burn out on my illuminated meter version of the Astron RS 50M.  This is the only Astron I have owned with bulbs in the meters.  Unfortunately, Astron does not sell replacement bulbs and expects you to buy a new meter assembly as its “non replaceable” part according to Astron.

I was in the process of replacing all the bulbs in my Palstar tuners and dummy load and decided to update the Astron as well using something that should last a long time, run cool and hopefully look better.  I ordered 10 green LED bulbs with dropping resistors and leads ready to install.  This worked great on the Palstar stuff as well as the Hercules II amp I decided to do later, but did not quite work as planned on the Astron.

Astron powers two bulbs in series prior to the regulator meaning they are about 20 volts. Something that will cook the dropping resistor on the LEDs in short order. The options are to install a large 1 watt resistor inline or tap the voltage for the bulbs off the 12 volt output of the supply.  I chose the latter just soldering to the buss that runs along the regulators and then to ground.

I believe the meters were originally glued on the top and screwed on the bottom. In my case the glue joints had cracked so just removing the screws was all I needed.   You can see the two phillips screws on the bottom in the picture above.  Once those are removed carefully hinge the cover up and remove.

If the top is glued, you many have to deal with that as well.

Once the cover is removed you will need to remove the meter face which is secured by two small phillips screws.

Behind the meter face is a small circuit board held in place with one small screw. The leads are long enough you can remove the board and remove the old bulbs and solder the new LED bulbs in place.  Since its a LED you will have to pay attention to polarity.

 

I just cut the leads short on the LED and soldered them to the back of the board and bent the leads around the top edge of the board.  I have added a Red (+) and Black (-) arrow to show where the leads from the board attach to meter in the image below. You can also see how I located the LED by bending it around the top of the board.

 

 

The new green illumination is fantastic.  Its about impossible to take a picture of it the station now as the LEDs over expose the camera even though they are not that bright in person.

 

One final modification was adding a fan. I admit my shelf spacing is not ideal but the powersupply runs hotter than I would like.  I had a large muffin fan that is very quiet so I zip tied it to the inside of the supply and wired it up to the 12 volt line on the regulators.  Moving just a small amount of air helps considerably and keeps the supply temps down.  Ideally, for maximum performance I would have cut a hole in top case and mounted the fan externally drawing hot air out. My previous 35M ran hot and it was never an issue but the 50m runs very hot.