A faulty 1084, Ray Carlsen saves the day, and leaves me baffled.

oracle_jedi · June 17, 2020

Ray Carlsen is an amazing engineer, a gift to our retro community and a helluva great guy.

An added bonus for me is he lives about an hour from me, maybe a bit longer if I take the scenic drive over the mountain pass.

So when I powered up my 1084 after a six month hiatus, and was presented with a garbled display, Ray generously offered to take a look. We agreed that shipping the unit was generally a bad idea, so I would drop it off at his place keeping a safe social distance in the process.

About a week later Ray contacted me to say the 1084 was fine. In fact all it had needed was an adjustment to the controls. I was puzzled, but it wouldn't be the first time I have missed something so completely obvious, and besides, my 1084 was apparently good, and I got another scenic weekend drive to go pick it up.

When I got it home though, the Amiga refused to display anything meaningful.

I then tested a Commodore 64 (svideo). Good display.

A Sord M5 (composite). Good display.

A CGA PC (digital RGB). Good display.

An Atari ST (analog RGB, sep H and V Sync) - Good display.

I tried two more machines that use analog RGB with CSYNC and none of them would display a stable image.

This used to work. I have photographic evidence this used to work. That at least is not my faulty memory.

Ray had tested his C64 and C128 to test composite and RGB and having gotten good results concluded there was no problem. He showed me the schematics of the unit and walked me through some trouble shooting. The schematic for the Commodore 1084D1 shows pins 7 (CSYNC) and 8 (HSYNC) being bridged. But on the circuit board these pins are not joined.

Tracing the motherboard, Pin 7 seems to move down to a solder pad that is adjacent to a pad for HSYNC and then stops (yellow line). The silk screen shows what I believe to be a CSYNC solder pad (green line), but it doesn't connect back to Pin 7. Note the damage to the ground plane was from an earlier repair, and has been repaired (out of shot).

Looking at the component side it doesn't appear anything bridges the two signal lines:

Is there some component missing here? I never noticed anything falling out, and nothing rattles. I can't see evidence of a missing component such as the hole that is left if you yank a resister out.

Following Ray's advice I used some wire to bridge pins 7 and 8 and the Amiga was once again able to show a stable image.

All modes now work but I am baffled how this ever worked in the first place.