Repairing Dorrough Loudness Meter Backlights
Dorrough audio meters are very useful for monitoring average program and peak audio levels simultaneously. I have a pair of model 280-D AES/EBU meters that I use to monitor my stream audio and ensure that I’m compressing the audio to a reasonable dynamic range. The meters I have were used units, and most of the little “wheat” lamps used to backlight the meter scale were burned out. Dorrough gives instructions on replacing these lamps, but I had a little trouble finding replacement parts so I decided to try and replace them with LEDs instead.
Dorrough quotes the lamp voltage for the 280-D at 6.3 V, so the LEDs require current limiting resistors of a value that is dependent on the size and color of the LED chosen for the replacement. I found that 3mm LEDs were about the same diameter as the lamps so they would fit into the plastic part that distributes the light behind the meter scale.
Next was to choose the LED color. I tried a blue-ish LED first and wasn’t happy with the visibility of the numbers. An amber LED worked better and matched the warm color of the original lamps.
A formula is used to calculate the resistor value based on the LED specifications:
R = (Vs - Vled) / Iled
For the amber LEDs I selected, the forward voltage is 2V and the test current is 20ma, so the recommended resistor value is 215 ohms.
However, this value yields a LED that is much too bright. The resistor value can be adjusted to larger values until the proper brightness is found. I ended up settling on a 1kOhm 1/6w resistor which provides a nice warm glow behind the meter.
The following pictures show the process of replacing the lamps. I would recommend measuring and recording the polarity of the power supply at each location with a multimeter so that when you install the LEDs they are in the correct orientation.
To ensure the LED light was distributed well behind the meter scale, I sanded down the tips of each LED and scuffed them up until they were ‘frosted’. Then I trimmed the leads and soldered a tiny resistor to each LED.
Prepping the blue PCB is important - carefully desolder the old lamps and clear out the thru-holes. The holes on one side of the PCB are very close to the display ICs and so a fine soldering tip and a steady hand are helpful to avoid melting anything. My preferred tool for clearing out the old thru-holes is a solder sucker. Tin the holes with fresh solder, and then solder in the LED/resistor assemblies, ensuring each is installed in the correct polarity. Trim the excess leads and reassemble the meter.
I cannot guarantee that this modification does not cause issues with calibration for analog meters, but I think it is unlikely. I think it is reasonable to think that it would not affect digital meters. And by the way - this mod probably voids the warranty!