Odd PCJR double screen output !

[drphilxray]

Hey everyone, I don’t normally post here, more of a commodore/Amiga guy with a fair share of vint PCs of course.

I just received a lovely PC Junior, with three expansion side cars, one of which is 512 K and excited for it. Also got the raphnet SD card cartridge board and it’s plugged in, from brutman.
It just outputs a weird double composite screen, no matter what I plug it into. LCD TV AV in, old CRT monitor composite in, and a newer eyoyo LCD TV. Hopefully the photos will show up.

Do I need a new motherboard? New caps? Or is there some thing on the board doing this. I’ve read some posts where NTSC output is not exactly the same megahertz as most, and your mileage may vary with different monitors.

 

[VERAULT]

New motherboard??? No we fix things here. I doubt its bad ram. I suspect the video chip. If its socketed thats an easy fix if you can source the IC.

So the motorolla 6845. https://en.m.wikipedia.org/wiki/Motorola_6845

Question is .. is it custom for IBM or can you just drop anyone in.

 

[drphilxray]

New motherboard??? No we fix things here. I doubt its bad ram. I suspect the video chip. If its socketed thats an easy fix if you can source the IC.

So the motorolla 6845. https://en.m.wikipedia.org/wiki/Motorola_6845

Question is .. is it custom for IBM or can you just droYp anyone in.

Yeah have not got further than taking cards out and seeing if it would boot differently but so far still get the double wrap around screen...
My only next step is trying composite to HDMI cheap chinese box converter I have.
Of course the video chip is not socketed! the pcjr tandy mod chip on ebay and elsewhere requires desoldering the video chip, so...
I've ordered a new mobo. I dont have the nice desoldering powered vacuum guns that so many have, just a manual thats very tedious.

 

[VERAULT]

Yeah have not got further than taking cards out and seeing if it would boot differently but so far still get the double wrap around screen...
My only next step is trying composite to HDMI cheap chinese box converter I have.
Of course the video chip is not socketed! the pcjr tandy mod chip on ebay and elsewhere requires desoldering the video chip, so...
I've ordered a new mobo. I dont have the nice desoldering powered vacuum guns that so many have, just a manual thats very tedious

How you can be in this hobby and not use a soldering iron makes no sense to me. And desoldering one ic with braid and flux is not a big job.

DONT THROW THE BOARD AWAY! send it to someone who will fix it.

 

[Eudimorphodon]

A bad CRTC isn't a terrible guess, although looking at the schematics if I had to throw a dart at the board I'd give the stink-eye to the 74LS258 multiplexers at ZM27 and ZM28 as a possible culprit as well. The video output corrupts/repeats on a 32 column cadence, which feels like a stuck address line (which could implicate the multiplexers), but the fact that it keeps alignment with the left margin every other line does kind of feel like one of the 6845's internal counters might have gone insane.

The PCjr does have a gate array chip associated with the video system that would probably doom the board to only be good for parts if that were the problem, but this looks like the issue is elsewhere. So, yeah, the board is probably repairable by someone who wants to put in the effort.