Switch mode power supply (SMPSU) repair in For-a FA-310P time base corrector
Thursday, August 5th, 2010
We use time base correctors and frame synchronizers all the time in the transfer and digitising of analogue video tape.
One of our more flexible and high quality units had recently developed an annoying and very obvious fault on it’s video outputs. While the unit was working there were faint but distinct horizontal lines on the video. This phenomenon is often called a hum bar and can be caused by ground loops.
In this case we isolated the unit from the rest of our installation and using a separate power point the problem was still there. Looking at the unit itself it is a very deep and heavy 1U case with two 40mm cooling fans at the rear corners. It is quite old too and being designed for continuous studio use is likely to get hot and have been on for very long periods.
The video fault appeared to be AC ripple ‘riding’ on the DC power. It was time to look at the electrolytic capacitors in the power supply.
Although I could have tested each one, all these caps were old and only rated for 3000 hrs at 85 celcius so they all had to go! Here’s a list of them:
The only one hard to find was the large 400v dump one. Most units now are thinner and taller but eBay came to rescue here.
This shotgun approach worked beautifully and the fault had gone. While tracing the exact fault is always the best way, capacitor often get a hard life and will not last indefinitely, especially in switch mode power supplies.
