Gary C
Veteran Member
Again, really just for me to record my repairs and my thinking at the time.
Hardest bit about this repair was finding the manual for the version 2
www.manualslib.com
So, the monitor in question was delivered to us as dead. Power lamp didn't illuminate.
All fuses ok but no DC across the rectifier ? All diodes tested fine out of circuit but only measuring 240V AC to earth ! Doesn't make sense. So if I am getting an AC reading on the DC side of the rectifier then I can't be seeing the rectifier actually passing current. I don't know the earthing arrangement in the workshop but it looks as if I am just seeing the line varying with ref to an isolated earth.
Ah ha !, resistor 3410 has failed. A 4.7 ohm 7W resistor would take a fair current to blow but it means there is a disconnect between the mains and the rectifier allowing my readings to float. While its a defect I suspect its a symptom not the cause.
So replace the resistor with a suitable unit (couldn't find a 7W to a 10W 4.7 ohm resistor used instead) and FLASH !, the 2A input fuse blew. Yep a symptom not the cause. So very high current is flowing and taking out the fuse. A DC resistance measurement on the input shows low resistance L to N, but that is just the degaussing coil dual PTC which could easily confuse, but for it also to have taken out the 4.7 ohm resistor, its going to be on the DC side.
Looking at the diagram, after the rectifier the DC supply splits. One through the transformer and down through the power switching transistor 7432 and the second down through the pilot transistor 7421.
So the BUT11AF transistor whipped out and yes, shorted Collector to Emitter, no wonder the fuse blew. This transistor is mounted on a heatsink but is slightly unusual in that the collector is insulated from the heatsink and in the design, the heatsink is electrically connected to the Emitter. The only suitable transistor to hand had the normal uninsulated collector so fitting would also cause a direct short. Found a Mica insulator kit and fitted the new transistor with that and power now restored, monitor running perfectly.
I do have the correct transistor on order and will swap it when I get chance.
Simple fix in the end.
Hardest bit about this repair was finding the manual for the version 2
Philips CM11342 Service Manual (Page 19 of 27) | ManualsLib
www.manualslib.com
So, the monitor in question was delivered to us as dead. Power lamp didn't illuminate.
All fuses ok but no DC across the rectifier ? All diodes tested fine out of circuit but only measuring 240V AC to earth ! Doesn't make sense. So if I am getting an AC reading on the DC side of the rectifier then I can't be seeing the rectifier actually passing current. I don't know the earthing arrangement in the workshop but it looks as if I am just seeing the line varying with ref to an isolated earth.
Ah ha !, resistor 3410 has failed. A 4.7 ohm 7W resistor would take a fair current to blow but it means there is a disconnect between the mains and the rectifier allowing my readings to float. While its a defect I suspect its a symptom not the cause.
So replace the resistor with a suitable unit (couldn't find a 7W to a 10W 4.7 ohm resistor used instead) and FLASH !, the 2A input fuse blew. Yep a symptom not the cause. So very high current is flowing and taking out the fuse. A DC resistance measurement on the input shows low resistance L to N, but that is just the degaussing coil dual PTC which could easily confuse, but for it also to have taken out the 4.7 ohm resistor, its going to be on the DC side.
Looking at the diagram, after the rectifier the DC supply splits. One through the transformer and down through the power switching transistor 7432 and the second down through the pilot transistor 7421.
So the BUT11AF transistor whipped out and yes, shorted Collector to Emitter, no wonder the fuse blew. This transistor is mounted on a heatsink but is slightly unusual in that the collector is insulated from the heatsink and in the design, the heatsink is electrically connected to the Emitter. The only suitable transistor to hand had the normal uninsulated collector so fitting would also cause a direct short. Found a Mica insulator kit and fitted the new transistor with that and power now restored, monitor running perfectly.
I do have the correct transistor on order and will swap it when I get chance.
Simple fix in the end.