Piffpaffpoltrie Gives an Old Amp a New Look with a Smart LED Backlighting Upgrade
Drop-in boards deliver a low-power, even replacement for dying incandescent bulbs.
Pseudonymous maker "Piffpaffpoltrie" has written up a recent project to overhaul a vintage audio amplifier — an in doing so offered a guide on how to dramatically improve aesthetics by replacing dying incandescent backlights with modern LEDs.
"What I describe here is an upgrade/repair of a vintage Technics SU-7 amp from about 1983, which means that it is more than 40 years old now, and still going strong! It has only one flaw: the beautiful, backlit, analogue power meters are only partially illuminated, because some of the old-fashioned, incandescent, oblong festoon bulbs are burnt out," Piffpaffpoltrie explains. "As is commonly known, the main feature, or rather, the flaw of incandescent lamps, apart from their low efficiency, is their limited lifetime."
These days, the majority of lighting is handled by LEDs drawing a fraction of the power of an incandescent of the same brightness — but with the internals of an amplifier, it's not quite as easy as just swapping out a bulb. "When I realized how the bulbs were mounted here on two dedicated PCBs, short pieces of an LED strip presented themselves immediately," the maker explains. "One single group of three LEDs has just the right length to be used as a replacement for two of the festoon bulbs.
"When zooming in on the circuit diagram of the amp's service manual you see that the four bulbs in question (two each on the M and N PCBs) are powered from the 11VAC winding of the mains transformer. LEDs unfortunately cannot be used with AC (alternating current) [so] some more modifications are required for using them here."
To get the LEDs running as replacement bulbs, Piffpaffpoltrie designed a bridge rectifier with smoothing capacitor — plus resistors to drop the roughly 15VDC output down the 12V required by the LED strips, or lower in order to reduce what turned out to be an overwhelming brightness. The rectifier, plus LED strip, was assembled on prototyping board shaped to be a drop-in replacement for the original lighting boards — delivering a clear, bright, and uniform backlight to the amp's analog dials.
The full project write-up is available on Piffpaffpoltrie's Instructables page.