07-02-2022, 10:18 AM
After the success on the 15 that were directly ex BBC I decided to look at the one from the other batch that I'd condemned to "spares only". It had obviously been damp so I plugged it in with some trepidation. After replacing some physically damaged capacitors and open circuit preset pots it crawled into life. Horrible picture with lots of foldover at the left side.
A few measurements showed that everything was struggling. The LOP transistor was coming out of saturation during part of the forward stroke which made the LOP draw too much current which pushed the power regulator into current limit. This in turn reduced the voltage to the line drive transistor so the LOP wasn't driven hard enough. Very chicken and egg faultfinding. The -25V and +110V rails derived from the LOP were inevitably low, as was the CRT heater, also from the LOP.
I tried powering the line drive circuits from an external PSU which helped a little but clearly wasn't the whole answer. Then a resistor on the separate line scan board smoked. The -25V rectifier had gone short circuit. Replace those and the -25V smoother and was only back to where I was before. Swap line scan boards with a known good monitor. "Good" board works badly in "bad" monitor, but differently to its own board. "Bad" board works perfectly in "good" monitor. So there's a fault elsewhere. Possibly something dragging down one of the -25V or +110V rails from the LOP.
Getting annoyed with it now. It will be obvious when I find it....
A few measurements showed that everything was struggling. The LOP transistor was coming out of saturation during part of the forward stroke which made the LOP draw too much current which pushed the power regulator into current limit. This in turn reduced the voltage to the line drive transistor so the LOP wasn't driven hard enough. Very chicken and egg faultfinding. The -25V and +110V rails derived from the LOP were inevitably low, as was the CRT heater, also from the LOP.
I tried powering the line drive circuits from an external PSU which helped a little but clearly wasn't the whole answer. Then a resistor on the separate line scan board smoked. The -25V rectifier had gone short circuit. Replace those and the -25V smoother and was only back to where I was before. Swap line scan boards with a known good monitor. "Good" board works badly in "bad" monitor, but differently to its own board. "Bad" board works perfectly in "good" monitor. So there's a fault elsewhere. Possibly something dragging down one of the -25V or +110V rails from the LOP.
Getting annoyed with it now. It will be obvious when I find it....
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
