Meter Lamp Replacement

You're a ham and you likely have a power supply or other gear which came with lighted meters ...which have, naturally, gone dark. We all moan when one of two meters burns out, and the second one will nag us until it too goes dark.
Replacing lamps is such a pain, and they'll just burn out again in a few months.

The common replacement is to convert the meter lamps over to LEDs. I have done this on several pieces of gear... too many to count. I had a new Astron that had lamps fail in one meter and set off to convert it, but felt I could improve on it some.

Leafing through the internet pages I came across some medium intensity white LEDs which were in a flat SMT package. Throwing caution to theh wind I bought some of them from Digikey.
They are 3mm wide and 5.2mm long. Part number 897-1191-1-ND. I bought ten of them, at a unit price of $0.51 each.

Removing the Astron meters is straight forward. To open them you must first use an Xacto knife and cut away the glue on the two tabs on the top of the meter. Once these are free you can remove the two screws below the meter face and carefully separate the meter.

Next the dial scale needs to be removed. There are two small screws holding it in. Be very careful lifting this out so you don't bend the needle. Lastly the lamp board is held in with one screw. Lift the lamp board up and towards the top of the meter and desolder the leads connected to it. I little heat from a soldering iron and they come out easily. Again, be careful working around the needle. Once the board is outyou can relax.

Now desolder and remove the bulbs from the board. To mount the new LEDs you will flip the board over so that the traces are facing you. Clean off the old solder so your work area is as clean and level as possible for the new LED. The LED is a perfect fit between the pads of the circuit board.

** CAUTION! Soldering in the new LED requires a tempurature controlled iron!**
A standard pencil iron WILL MELT THESE LEDs in the blink of an eye!

Place the diodes as shown in the drawing below, paying attention to the Cathode markings, and carefully solder them to the board.
(You do not need to solder the slug on the back unless you are going to run these at full brightness. It functions as a heatsink for the Anode.) They are far too bright at full current to be meter lamps.

Here you have the personal option to mount your resistor either externally or cut one of the traces and mount it internal to the meter. I mounted my resistor external to the meter, but there is lots of room on the card if you wish to do them internally. Select a resistor value that meets the brightness you desire. These LEDs can each handle 100mA of current as they are bright little things, but you'll want to tone them down to 2-5mA (total), far less then the standard LEDs. One could mount a pot so you can adjust brightness, but be sure to include a second resistor of about 82 ohms so that you protect the LEDs if you zero the pot.

Be sure to test your work before assembling everything back together. The polarity of the power in the meters, as they face you, is POS on the right side and NEG on the left side. Reattach the wires and reverse the disassembly process.

Note: Astron feeds the meter lights with 24VDC in their SS25M supply. I snipped the original positive leads and picked up 13.8VDC from the output side to power the LEDs.