I recently picked up a pretty fully outfitted darkroom including an Omega Super-Chromega C-700 Dichroic Lamphouse (fundamentally the same unit as the slightly newer C760). As soon as I tried to power it up, the bulb appeared to be dead but visually looked OK. This lamphouse uses a pretty standard 85W, 82V Quartz-Halogen bulb (ANSI ESJ). According to the Instruction Manual (C700, C760), “The built-in solid state power supply eliminates the need for an external power supply” – sounds great, but a quick web search for a schematic diagram came up with nothing, so I decided to investigate a little further and look inside. What I found was a very simple AC-DC “half-wave rectifier” power supply comprised of a power resistor and rectifier diode. This essentially ‘cuts’ the bottom half of the AC sinewave, essentially resulting in 60V RMS DC. This provides about 1.4xRMS peak voltage (1.4 x 60V = 84V), very close to the 85W ESJ bulb specification.
I figured replacing these two very inexpensive and readily available components couldn’t hurt, and would take one variable out of the testing the unit with new bulbs (which aren’t exactly cheap).
The original power resistor was a 5 Watt, 1Ω sandstone resistor. Unfortunately, there were no markings on the diode to identify what it was but I decided a little ‘over-design’ wouldn’t hurt.
I purchased a generic 5W 1Ω resistor, and decided to go for a standard 1N5408 (3A, 1000V) rectifier diode – definite overkill, but will in no way affect performance. Total cost was $1.65 at my local friendly electronics shop.
Below are step-by-step directions on how to disassemble the lamphouse to get to the power supply and rebuild the power supply. WARNING: DO NOT ATTEMPT THIS PROCESS UNLESS YOU’RE COMFORTABLE WORKING WITH ELECTRONICS AND SOLDERING IRONS. ENSURE THE POWER IS DISCONNECTED, AND THAT YOU FOLLOW ALL APPROPRIATE SAFETY PROTOCOLS WHEN WORKING WITH SOLDER.
- Remove top of lamphouse by first removing thumbscrew – set lid and thumbscrew aside.
- Remove bulb cover by first removing two (2) thumbscrews – set cover and thumbscrews aside. I recommend removing the bulb at this time, setting aside in a safe (and clean) place. Be sure to only touch the edges of the bulb as touching the lens could lead to early burnout due to hot spots from skin oils.
- Remove left side of lamphouse by first removing four (4) Phillips head screws – set cover and screws aside. NOTE: One of the screws is longer than the others – note the location (top left in this photo) of the longer screw.
- Remove the bulb and power supply unit from the lamphouse by first removing two (2) Philips head screws – set screws aside.
- Underneath the bulb is the power supply consisting of the Power Resister and Rectifier Diode. Note the brown line on the diode – this identifies the Cathode side of the diode – be sure to install the new diode in the same orientation.
- New components – 5W 1Ω resistor, and 1N5408 (3A, 1000V) rectifier diode – $1.65 from my local electronics shop.
- Power supply with the new components installed – be careful when soldering!
- Reassemble the lamphouse by reversing these directions.
And voila! Your lamphouse is better than new, with brand new upgraded power supply components! This simple process should help ensure many years of trouble-free operation.