Laser 128 Troubleshooting

[CygnusTM]

I have a sick Laser 128. Before I dive in on troubleshooting, I thought I'd see if anyone had seen similar issues, so I might get a head start. I have the base Laser 128, but it is the second rev that came out with the EX. It appears to be mostly functional, but shows a black screen. When I power it on it beeps and the drive starts up. If I hit ctrl-reset, I get a beep. Then if I type ctrl-g and enter, I get another beep. So I think it is processing, but no video. It generates a video signal on composite but the screen stays black. I think I am getting the same thing on the RGB output, but I'm not sure my hacked together RGB adapter is fully working. (Can't wait to get an RGBtoHDMI.) Has anyone else seen something similar to this? Thanks.

[Baldrick]

Is there an internal brightness potentiometer on the motherboard?

Has the composite video jack come un-soldered or broken?

[ChildOfCv]

The technical reference manual can be found here https://ia600709.us.archive.org/12/items/Laser_128_Technical_Reference_Manual_alt/Laser_128_Technical_Reference_Manual_alt_text.pdf

 

It includes a troubleshooting section.

[CygnusTM]

17 hours ago, ChildOfCv said:

The technical reference manual can be found here https://ia600709.us.archive.org/12/items/Laser_128_Technical_Reference_Manual_alt/Laser_128_Technical_Reference_Manual_alt_text.pdf

 

It includes a troubleshooting section.

Yeah, I've been reading through that. 

17 hours ago, Baldrick said:

Is there an internal brightness potentiometer on the motherboard?

Has the composite video jack come un-soldered or broken?

There are pots on the board, but I want to study the schematics and docs before messing with them too much. I know one deals with voltage regulation, but I haven't  found info on the others. At this point, I'm worried that it is the video gate array chip that is bad. That is a custom chip, so if that is bad, it's game over for this Laser. I guess I'll be breaking out the scope and multimeter and start poking.