C128 Diagnostic Cartridge #325093-01 Replicated

The refurbishment and repair of the C16 and Plus/4 motherboards is largely complete. Next up is a small stack of C128 boards. Recently I received a C128 diagnostic cartridge with harness as a donation, which was also only slightly tattered.

The C-128 Diagnostic Test cartridge that was donated

It was a “C-128 Diagnostic Rev 1.4” with harness “ASSY NO. 325093-01”, which I had never heard of before. Information on this cartridge seems sparse, but I found it mentioned on Jani’s page on diagnostic carts and in an older forum thread (German).

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How to Lower Your PLA: The PLA264

In my previous post I described in detail how I created a ROM replacement for the Commodore Plus/4 computer and how I made sure that it would fit into the very tight space of the closed case.

Five in a row: The four ROM chips of the Plus/4 to the left and the PLA to the right.

The four ROM chips of the Plus/4 are sitting all in one row, and then there is a fifth IC right next to them: the 251641-02 PLA. This PLA chip is the same for all TED based Commodore computers, the C16, the C116, and the Plus/4. In the past, I used the excellent PLA16V8 from D. Mantione as a replacement. The PLA16V8 is quick to build from cheap parts that are easily available. And for a C16 with it’s roomy case, it’s a perfectly adequate solution.

The original PLA16V18 is much too tall to fit under the keyboard of the Plus/4.

With the Plus/4, however, it’s all about space again: There are only a few millimeters between the original PLA and the keyboard above it. So while the PLA16V8 works fine on a Plus/4 motherboard, it is far too tall to fit inside the Plus/4’s case.

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Four at One Blow: The ROMulator

The Idea

Recently, I’ve been repairing a few broken and slightly weathered motherboards for Commodore C16 and Plus/4 computers.

Despite its stylish design, the Plus/4 was no success for Commodore.

When I ran out of spare parts, I replaced the defective Kernal ROM on one of the Plus/4 boards with a W27C512 EEPROM. You can still source a used W27C512 easily and somewhat cheaply (on eBay, for example), making it my all-purpose tool for replacing ancient parallel ROMs.

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Building a PS/2 to 1351 Mouse Adapter

I’ve been meaning to build myself a mouse adapter for the Commodore 64 for quite some time. The recent release of the unofficial port of “Eye of the Beholder” for the C64 was a welcome excuse to finally get to work on this project.

The Mouse 1351 made by Commodore can be plugged directly into the control port of the C64 or C128 and it has two different operating modes. In joystick mode, the input from the mouse is compatible with that of a regular, digital joystick. In proportional mode, which is the default, it behaves and feels like any mouse used on modern computers.

Commodore Mouse 1351. (Image source: Wikipedia, License: CC BY-SA 3.0)

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The KERNALquattro, yet another ROM Replacement

Recently, I got hold of quite a few broken Commodore 64 motherboards. It has been a while since I last had the chance to repair one of those boards and it was fun! I dubbed this repair project the “Blank Screen Marathon” and documented the whole process in German on Forum64. I might still give a summary in English here at a later point. But today, I’d like to introduce a small side project that resulted from this repair marathon.

The 3 ROM chips on a C64 motherboard

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SyncFix64 Revisited – What about the VIC-20?

It’s been over four years now that I created the SyncFix64 module in order to connect a cheapo car TFT display to my C64. This setup has served me well and it helped me repair quite a few motherboards since then. A few years later, when I got my first VIC-20 the display again failed to show anything despite the integrated SyncFix64 module. That was kind of disappointing, but I was lacking time and motivation back then to investigate further.

Composite video signal from a C64 as opposed to…

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Building a new ZX81 Computer

The ZX81+38 is a clone of the Sinclair ZX81 home computer that replaces the ULA found in the original with standard logic ICs. That means, it can be built with all parts still readily available today. It was created by Mahjongg, introduced on a forum dedicated to Sinclair computers, and document on the web with construction files available on Github

The empty PCB as I received it.

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Pagefox – Reverse Engineered and Replicated

Pagefox is a DTP (desktop publishing) software that comes as a cartridge for the expansion port of the Commodore 64. It was developed by Hans Haberl — who also created  the graphics editor Hi-Eddi and the word processor Printfox — and it was published and sold by the small German company Scanntronik which is actually still in business today.

Pagefox cartridge pried open

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ESD Protection for the C64 Control Ports

The two 9-pin Control Ports located on the right-hand side of the C64 are used to connect joysticks and other input devices like paddles or even a mouse to the 8-bit computer. 

The two Control Ports.

The digital input pins are directly connected to one of the two 6526 Complex Interface Adapters (CIA) inside the C64, without any protection on the motherboard.

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Commodore VIC-20 S-Video Mod

Last Summer, after a delay of mere 38 years, I finally acquired my first Commodore VIC-20, or VC 20 as it was called in Germany. Not all parts of the machine are in their original state as the connoisseur will notice on first sight. I don’t care too much about that because I didn’t want it as a collector’s item but as a technical gadget to play around with.

My first VIC-20, freshly supplied with a donor keyboard.

First thing I had to do to even test the machine was to make a new video cable to connect it to a monitor. The pin-out of the A/V jack differs from that of the C64: there is a Vcc output where the luminance signal should be and there is no S-Video output.

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