Apple Multiple Scan 15AV

[Sunoo]

I have an Apple Multiple Scan 15AV that was working perfectly until the other day. It started off just fine, but about 30 seconds after powering up, it suddenly became very dark, and this has persisted since. The brightness knob also no longer seems to have any effect at all.

I'm guessing this may be the caps, but can someone confirm that? And does anyone have a list of caps for the monitor or know someone selling cap kits?

 

[techknight]

did it switch from bright to dark within a split second while using it? or did it slowly fade from one position to another? This all makes a huge difference in troubleshooting. 

sudden operational changes are usually caused by component failures. EEPROM IC, transistor, etc suddenly fails and the symptoms appear suddenly. 

Slow changes are usually the result of a cap going, or a resistor burning up changing in value. 

 

[Sunoo]

I was looking away when it happened, but my friend who was here says it happened in a split second.

 

[techknight]

Then your looking for a hard failure. 

If the brightness is digital, which I think it is, it sadly may be the video processor IC, or its associated EEPROM. But its hard to tell, you will have to know electronics and troubleshoot this thing. It could even be a loss of B+ to the CRT electron gun itself. 

 

[Sunoo]

The brightness is a knob, so I'd assume it wouldn't be digital?

 

[techknight]

Ok then the next thing you need to look for is a voltage loss somewhere. Without a reference or a schematic its going to be hard to guess what the real voltages need to be, but I would start at the collectors of the 3 drive transistors that drive the CRT itself make sure the voltage is in spec. Should be well over 100VDC. if its too low, you get this symptom. 

It has been a very long time since I serviced these CRTs when they were in active use, so my memory is rusty.