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Commodore PC 20-III CGA Composite

The Commodore PC 20-III is a XT-class 8088 clone PC running at 4.77 to 9.54MHz with 640KB of RAM. It also features an on-board Paradise PCV4 chip that provides CGA graphics on both D-Sub/RGBI and RCA/Composite connectors. While the RGBI worked fine, I could not get any picture with the composite connection.

I measured with both an oscilloskope and a multimeter and the composite signal was around 5 volts, which is not correct at all. Fortunately, circuit diagrams for this PC can be found on Bo Zimmerman's web pages, where I found something interesting:

Commodore PC 20-III Circuit Diagram Snippet

There was a high chance the 5 volts came from the main VCC voltage rail through this transistor, which could be shorted. I confirmed a short circuit between collector and emitter using a multimeter and proceeded to remove the transistor. As expected the short was now gone. I did not have any 2N3904 in my stash, but used a BC549B as a replacement, which should be equivalent. Here is the replaced transistor in the middle:

Commodore PC 20-III Circuit with Transistor

I powered on the PC and measured with an oscilloscope again, which now shows a proper composite video signal:

Commodore PC 20-III Composite Video Signal

I also ran the 8088 MPH demo to test for more colors on CGA:

Commodore PC 20-III Composite Video Signal

Topic: Repair, by Kjetil @ 11/12-2021, Article Link