Commodore 64 Assy No. 250425 Rev. A repair log with a broken character ROM at U5

Assy 250425 Rev. A #2

This machine is an Assy No. 250425 long board.

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At startup, the screen would be completely black. The board was getting power so I ruled that one out. As most of the big chips on the board had been socketed, I removed the SID (6581),  swapped the VIC-II (MOS6569R3), the 8701, the MPU (MOS6510) and the CIA’s (MOS6526) with known working ones to rule those ones out as well. Still a black screen. I then desoldered the PLA, mounted a socket and exchanged it with a fresh new one. Now I was seeing something on the screen:

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The scrambled screen looks alot like something I’ve seen before on a repair job I did on a C64C short board (Assy No. 250469 Rev. 4). On that board the problem was related to the CHARACTER ROM. So I put in a multirom IC I have bought on Ebay which holds the BASIC, CHARACTER and KERNAL ROMs. Each of the ROMs can be chosen using the small caps on top of the PCB. I like using it for faultfinding.

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It’s the little green PCB located next to the CIA’s on top of the board.

Commodore 64 Assy 250425 motherboard with swapped Basic and Character ROMs

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However, the result was the same – the screen was still scrambled.

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U13 and U25 (74LS257 or MOS7708) are two logic chips which can cause a blank screen or a garbage screen. Especially the MOS77xx series chips have a high failure rate. I therefore decided to give that a go as I had a couple of new 74LS257’s in my spare box.

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Well, that did not help either. The screen was still scrambled and I just wasted 25 minutes…

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I then scratched the back of my head and left the work bench for a while. When I came back, I took at closer look at the CHARACTER ROM again. It should be at U5, but that’s were the BASIC ROM was located at…???? For some unknown reason (I think I know who to blame, though…) I had exchanged the locations of the CHARACTER and BASIC ROM ICs when I started the repair job. Shame on me!

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Now the CHARACTER ROM is at U5 and the BASIC ROM is at U3. Exactly were they are supposed to be!

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And that’s how the machine was cured!

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2 comments on “Assy 250425 Rev. A #2

  1. Nice writeup! I once accidentally swapped the SID and PLA (they switched places in assembly 250469) which fried the SID so swapping the CHAR and BASIC roms seem a better misstake to make 😉

  2. …he he…yes, there are always worse ways to mess up a repair job – just glad I didn’t burn any ICs 🙂

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