Assy 250469 Rev. 4 #3

This is an Assy no. 250469 Rev. 4 shortboard. 

The machine would start up with either a black screen, less than 38911 bytes memory or ‘OUT OF MEMORY ERROR IN 0’.

This clearly indicates a fault in the RAM chips. The Dead Test Cartridge is great for indicating faulty RAM chips as it makes the screen flash in patterns depending on which RAM chip is faulty.

Based on the flashing sequence of the Dead Test Cart, it seemed to be the memory chip at U11. This was exchanged with a fresh new one.

The Dead Test Cart now indicated that the RAM chip at U10 was faulty also. Out with the bad one and in with a fresh new one.

This made the startup screen turn black and I was not able to get the blue screen with memory fault or less than ‘38911 bytes ready’.

I kept getting memory errors from the Dead Test Cart, so I took out the RAM chips and switched their places (U10 to U11 and vice versa). Still a black screen and the Dead Test Cart gave me a fault on U11/U10 in random order. I double checked the RAM chips in another Assy 250469 board and they worked fine! I therefore expected something else was causing the problem. First I tested if the Kernal/Basic chip at U4 was faulty by running the game of Jupiter Lander on cart as this bypases the Kernal.

The machine booted the game but the colors were greyd out.

A quick turn on the yellow pot near the VIC-II chip made the colors come back.

However, parts of the screen were missing (e.g. part of the ‘P’ in ‘Jupiter’).

As the game was running I assumed that the RAM chips were working (otherwise the game should not be able to run, right?) and I exchanged the Kernal/Basic ROM at U4.

However, the macine would still have a black screen at startup. I therefore took a closer look at the CIA chip (MOS 6526) at U2 as this chip may also cause a black screen. To rule out this chip, I exchanged it with a fresh working one.

That did not cure the black screen either. At this point I was getting really confused because I had a cart game running that I could control with a joystick which means that the RAM chips, the MPU, the VIC-II and the CIA’s were all working. I’d also exchanged the Kernal/Basic ROM with a working chip. I therefore went through Ray Carlsen’s list of chips that may cause a black screen (link). I basically used a brute force approach for the repair and started from the top of his list and worked my way down. First up was the Character ROM at U5.

Still a black screen 🙁 Next up were the U22 (7406 logic chip) and U3 (7408 Logic chip) chips.

Still a black screen. As I’d run out of chip that would cause a black screen, I even tried swapping the RF box with a working one.

Still a black screen. The power switch was also replaced with a brand new one.

Still a black screen. When powering up the machine, I would get a horizontal flash before the screen would go black. I therefore thought that maybe the machine did not get enough power at startup. I therefore exchanged two of the larger capacitors. At this point I was basically shooting in the dark!

Still a black screen. I then moved back to the Kernal/Basic chip as I thought that maybe there was a broken trace under the chip or something – Jupiter Lander was still able to run regardless of what was exchanged on the board. I removed the socket and double checked everything. All traces looked good and another socket was installed.

 

The black screen remained! At this point I gave up as I had run out of chips that normally would cause a black screen. I filed the board as ‘possessed’ and threw it on top of the shelf!

After a few weeks I returned to the board with restored mental energy! I once again popped in the Dead Test Cart and was getting the same old memory fault at U10/U11. Even though nothing pointed towards these chips (as the machine could play a cart game and they worked fine in another Assy 250469 board), I went on and swapped them with two other RAM chips from my spare’s box. And guess what – the Dead Test Cart would boot into the test screen, Jupiter Lander had no parts of the screen missing and the sweet blue screen had returned!

I cannot say how relieved I was that I finally got it working! How a game can be run with two faulty RAM chips (at least on this board) really confused me. That combined with the machine not booting really lead me astray. Well, at least I learned something when fixing old C64’s – accept that sometimes the cause of the fault is not logic and most importantly, never ever give up 🙂

This is the lot of pieces that were exchanged for new ones to get the board running.

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