TRS-80 model 4 video problem.

[chapmro]

I was given a Model 4 which keeps blowing the video board fuse. I was told that something "shorted" in the power supply (who knows?). The power supply fuse does not blow, and if you disconnect the PS and give it AC, the output voltages look ok. If you hook up everything except the connector to the video board and power it up, the voltages on the PS board still look good, the machine spins the disk a while, then stops, and if you press reset the disk spins again, so I think the motherboard may be at least partially working, if I can get anything out of the crt. Given what the previous owner told me, since I had another power supply, known good. I tried swapping it in place of the original PS, but no change. So I bought another video board on ebay advertised as "model IIIII/4" video board. It's a model III board, and while the edge connector and the plug on the end of the crt fit, the connectors are different to the coils around the crt some distance toward the screen from plug (I am ignorant about crts). The model 4 has a 4-pin connector to the video pcb for the coils and a bullet connector for ground, while the model III video board has 4 spade lugs. Can anyone help with (1) debugging the model 4 video board? or (2) adapting the model III video board to work on the model 4 crt? Thanks very much.

 

[chapmro]

To use the M3 video board with an M4, looking through the model IIII/4 tech ref manual, it looks like the yoke wires are in two pairs, one for vertical, one for horizontal. I can check for which two wires make up each pair with the continuity tester on the multimeter. It looks to me like I should make the following connections to adapt the M3 video board to the M4 crt

pin on M4 crt yoke connector -> label on m3 video board lug wire
====================== =========================
ground -> e509
connected to r321/L301 junction -> e504
pin 8 of an5763 (vert out) -> e601
pin 10 of an5763 -> e602

Is that right? and then of course hook up the connector on the side of the tube, the edge card, the ground, the round connector on the end of the crt. Will there be fireworks if I get it wrong?