Beomaster 1900 (Type 2903): Testing The New Bass, Treble and Balance Indicator Masks

The original display indicator masks for the bass, treble and balance settings on this Beomaster 1900 have deteriorated from long term exposure to the incandescent lamps. I measured up to 118°F on the tone control lamp lenses.

Since the bass, treble and balance lamps display their settings mechanically, by sliding the indicator mask across a fixed lamp source, these are a good case for switching from the hot, incandescent lamps to cooler operating LEDs.

Here is picture of the original incandescent lamps in their bare, illuminated state.

A design goal for a replacement lamp is to produce a reversible substitute part. The LED version of the lamp should just solder on to the same two board pads that the original lamps use. No modifications to the board should be necessary.
Each of the original lamps has a 68Ω current limiting resistor in series with it. For my replacement I decided to go with two LED devices and a load resistor to limit the current further and to control the illumination of the LEDs.

On my test circuit I used a 68Ω resistor (for the existing resistor in the Beomaster), two white LEDs and a 5KΩ trimmer resistor to come up with a good load resistor value. I ended up with a 1KΩ load resistor a and the results are a nice substitute for the original bulb. The illumination of the red indicator lens is nice and uniform.

My final replacement version of the LED assembly looks like this. I put clear heat shrink over the load resistor and color coded heat shrink to mark polarity of the LED (red for positive, black for negative).

Now for a quick test.

The test passes and showed me that I needed to bend the LED leads a little different in order to fit the lens cover assembly for the lamps.  Here is the board with all three replacement lamps in place.

The lens cover assembly fits back on and the new bass, treble and balance indicators are restored.
Before showing the new LED replacement lamps in action here is what the new indicator masks look like.

Here are the new bass, treble and balance indicators with the Beomaster 1900 turned on and playing.

The temperature on the indicator mask is much better now. These newly printed masks should last a long time.

Of course I will have to pull those three slider potentiometers again once Martin's new contact housings arrive. In the meantime I will put this Beomaster through a burn-in test. I will let the local classical radio station play for a few hours.