Where to look for signs of life on model 4 main board?

[chapmro]

I am trying to repair a model 4 with a dark screen. I posted earlier about the video board, but in absence of a way to debug that, I thought I'd work backwards. I looked on the scope at the 3 outputs from the main board to the video board. The H sync and V sync pulses are there, but I am not finding anything on the video signal line. What should that signal look like on a scope? Any chance the main board is not fried if it is not outputting the video signal? What signals might I look at to see how much of the main board is working? On power up the disk spins for while then stops, and will start again if I hit reset. Doesn't sound like the head of the disk is doing any moving. Thanks for any advice.

 

[ldkraemer]

Is this a NON Gate Array or Gate Array Motherboard?
How do you tell????? Look at the Serial Port DB-25 Connector. If it's parallel with the Base
of the Computer it is a Gate Array. Straight Down towards the Table it's a NON Gate Array.

Check your DM's (Direct Messages)!

Larry

 

[chapmro]

Is this a NON Gate Array or Gate Array Motherboard?
How do you tell????? Look at the Serial Port DB-25 Connector. If it's parallel with the Base
of the Computer it is a Gate Array. Straight Down towards the Table it's a NON Gate Array.

Check your DM's (Direct Messages)!

Larry

It is non-gate array, and I've convinced myself the motherboard is fine by borrowing a "top half" (crt and video board) from another computer,
and the video output looks fine. My original problem with this machine is that the video board blows fuses. Nothing is obviously damaged on the video board, no charred components, no shorts or open traces, and no capacitors or semiconductors show up as shorted when tested with a meter. Any ideas about how to start debugging that video board? I compared it visually to the working one I borrowed, and I can't see any discrepancies. I guess I can compare the 12v-ground resistance on the one that blows fuses versus the working one, and follow the schematic going from the power leads inward in hopes of finding the shorted component?

 

[RingingResonance]

Swap video boards with the working one to see if the CRT is any good? Is the CRT heater glowing? Is there any blue glow in the neck of the CRT? If there is a blue glow then it's gassed and you need a new tube. Be cautious of high voltages!