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Dialing in the video levels on my 540B video board for my Challenger 4P. The brightness, contrast, and color levels all have to be just right to play nice on most displays. Turns out that I had accidentally connected to the wrong point on the video connector, which meant the brightness was WAY too low.

#ohio scientific #osi #challenger 4p

Debugging the 540B color video board on my OSI Challenger 4P

#osi #ohio scientific #challenger 4p

Ohio Scientific turns 50! - @commodorez

This was my VCF East exhibit! I brought my OSI-400 semi-replica system, a combination of authentic 1970s parts, and modern replica boards from the Glitchworks in a homemade wooden case. Attached to that is a common surplus keyboard from a Sanders 720 terminal, which OSI recommended for use with their 440 video board before they started manufacturing their own keyboard design.

I also brought my mini OSI-300, a modern replica of Ohio Scientific's first product. Connected to the OSI-400 was an NEC Spinwriter 5520, a printing terminal used for interaction with the memory monitor, and for debug statements from other software that used the video display.

The computer on the right with the wooden sides is my Challenger 4P, a later OSI machine featuring BASIC in ROM, 8K of RAM, and a modern replica 540B color video board. I mostly demonstrated the 4P in monochrome mode, running little BASIC programs.

The OSI-400 was running Desert Miner, the game I've been writing for the past few months in 6502 assembly. It's intended to showcase the limited capabilities of a 1976 kit-built microprocessor system. I used this opportunity to get feedback from players, and test the game's features with users who were unfamiliar with the title.

I brought along various documents and spare parts to really showcase what it was like to build a machine from a kit before appliance computers were available on the market. Overall, the exhibit went as well as could be expected, and I'm glad that people took the time to sit down and experience my perspective on Ohio Scientific.

#vintage computer festival east xxi #vcf east xxi #vcfexxi #commodorez goes to vcfexxi #ohio scientific #osi-400 #challenger 4p #nec spinwriter 5520 #desert miner #sanders 720

FOUND ANOTHER FUCKING SOLDER BRIDGE

See those areas marked in red? Those were holding back my OSI 540B replica video board from generating video. This board is of a design where solder mask was not always a given so it's very easy to bridge traces. I found one last spring (left), and I found another one this morning thru tracing the weirdness of the video signal, until I noticed that a pair of 74LS174's weren't passing any signals.

Turns out they were locked up, because master reset was pulled low by another solder bridge (right). See that middle line marked Q3? That should be similar to D3, but it wasn't.

I wasn't sure I had fixed it, until I started tweaking analog video levels again, and suddenly I had something visible on the screen! A few keystrokes later and... BASIC!

FUCK YEAH, BASIC!

And so I decided to run my usual SIN COS test:

The video signal is a bit low voltage, so it appears kinda dark. I need to tweak alot of the analog parts of the circuit, possibly modify a few things to get it playing nice. I also need to run test programs for the color video modes, the whole point of doing this replica build. Here's an original 540B built for monochrome video: