u/cryptoleadpro

I plugged my model III motherboard (pictured here) with a power supply from a model 4 into the position labeled 1.

Result: I get garbled text and the keyboard fails to respond. The reset button does not work (even though it passes a continuity test). Text seems to ghost write itself and then halts.

Question 1: Do I need power at "2" (5 pin adapter) to test this board?

Question 2: Is there a manual way to reset the board without the button?

My model 4 power supply does not have the right pinout (it has a 5 pin but colors look in wrong position) . I have a power supply for a III but it makes crackly noise and I havent recapped it yet.

Question 3: What is the bear minimum to test this board?

I plugged up a Model 3 board I had from another parts machine. The CRT and video circuit seem fine on the model 4 chassis. The Model 3 seems to just give garbled text. One of the transparent ribbon cables seems in really bad shape (falling apart). I would also like to fix this.

As far as the Model 4 motherboard there is no video signal although my oscilloscope shows some weak signals for h / v and maybe video. Using a pretty janks scope atm. the H and V seem strong the video seems weak. i get a nice puls off the middle pin. (not shown).

Any help appreciated getting these in working order. I can now confirm it is a mainboard issue on the 4.

I grabbed this model 4 at VCF east. I plugged it in when i was there and poof.. a stinker! Smells like a RIFA cap so I bought it at discount.

I took it home, opened it. Inspected everything and could not find a blown Rifa cap. Did a quick bench test of power supply and it was still working (though i didnt measure levels). When i reassembled disk drive spins for a second and I get this grey line screen. Hitting the keyboard does nothing. Reset works and drive spins again.

So any tips on where to start diagnosing this? And is there a hidden RIFA cap? I saw the smoke, smelled out the place though nothing looks blown. Maybe its just half blown?