The Philips CM8833 is a colour video monitor from the late 1980s. It was a popular monitor for computers such as the BBC Micro, Commodore Amiga and Atari ST. It has inputs for RGB video signals, as well as PAL inputs and stereo audio amplifiers. I use my CM8833 with the Compukit UK101 and the Acorn Archimedes A310.
While using the CM8833 with the UK101 at the Maker Faire in Newcastle, the screen suddenly went blank, and the monitor began to whine. I switched off at once, although there was no smoke or burning smell. I recognised the whine as a switch-mode power supply going into overload.
Once I got the monitor home, I began with a visual inspection of the internals. No signs of burning were found. As usual with this type of failure, I suspected the line output stage of the monitor. But, the first step was to make sure that the power supply was OK. In this design, there's a switch-mode power supply in the form of a separate module that includes the power on/off switch. I unplugged the four-wire connection from the PSU to the main board, and carefully switched on. The three main voltages, 125V, 25V and 16V were correct at the PSU and no whine was produced. This convinced me that the PSU was OK and the fault lay in the main PCB.
A DC resistance check from the 125V supply rail to ground read about 28Ω, much too low to be reasonable. I removed the line output transistor, but it was OK. I also removed the 125V smothing capacitor, but that too was OK. At this point I wanted to find out whether the short was somewhere in the line output transformer, so I removed the 12Ω resistor that supplies power to it. The short on the 125V rail went away, so I knew that the fault must be in the line output transformer or one of a few associated components. Having removed each in turn and tested them, I was left with the conclusion that the short was indeed in the line output transformer.
The only way to fix it will be to obtain and fit a replacement line output transformer.