20-10-2015, 05:41 PM
Don't overvolt the video input of your Aurora SCRF. Normal video is 1Vp-p. In professional kit black will be at 0V but it will can be up to a volt or 2 above that on domestic kit. Since video inputs are usually capcitatively coupled and DC restored this doesn't usually cause a problem.
But what about the input termination resistors? The SCRF uses a pair of 37R4 resistors that are effectively in series to give a 75R input resistance. There are tiny SM parts that I'm guessing are rated at no more than 0.1W. Hence if the input exceeds 3.8V they will be overloaded. This is unlikely to happen with any normal video feed but I've just had an Aurora on my bench to repair. The owner (nameless to protect the guilty) said it gave a testcard output but didn't respond to a video input. Both the resistors had gone open circuit. I didnt' have suitable SM parts to hand so I fitted a pair of 39R through hole parts. This is close enough.
After I had done the repair I expressed surprise at the failure and asked him if he had overloaded the input. He confessed to having experimented with an emitter follower output stage running from +12V. I suspect he biased it to approx mid point and didn't use a coupling cap.
But what about the input termination resistors? The SCRF uses a pair of 37R4 resistors that are effectively in series to give a 75R input resistance. There are tiny SM parts that I'm guessing are rated at no more than 0.1W. Hence if the input exceeds 3.8V they will be overloaded. This is unlikely to happen with any normal video feed but I've just had an Aurora on my bench to repair. The owner (nameless to protect the guilty) said it gave a testcard output but didn't respond to a video input. Both the resistors had gone open circuit. I didnt' have suitable SM parts to hand so I fitted a pair of 39R through hole parts. This is close enough.
After I had done the repair I expressed surprise at the failure and asked him if he had overloaded the input. He confessed to having experimented with an emitter follower output stage running from +12V. I suspect he biased it to approx mid point and didn't use a coupling cap.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
