5V MEMORY UPGRADE Troubleshooting

eems · May 31, 2021

I did this upgrade on an H2 motherboard and everything seemed to go well with desoldering and installing the sockets, connect +5V to L3, and pins 8 & 9 were at ~5V and pin 1 at 0V. Popped the chips in and garbled video.

Troubleshooting:

1. Continuity across the 8 chips: All pins besides 2*(see #2) & 14 have continuity. From underside all the traces had continuity.

2. *Strangely pin 2 on U15 and U17 have continuity. After making this connection, Pin 1 jumps to ~2 volts (with power disconnected) and if I put pin 1 to ground the voltage slowly drops back to 0V.

3. The address and controls across all 8 are fine to the VDP but going back to U15 and U17(see#2), I found that there is also continuity from those PIN 2s to AD5 (pin 5) and this causes the same 2 volt charge to pin 1.

4. Since AD5 is connected to all pin 6 there is also continuity between U 15 & U17 pin 2 with all Pin 6s.

Any thoughts on where there might be a short or other troubleshooting to be tried based on these observations

Edited May 31, 2021 by eems

eems · June 1, 2021

Added some pictures of how the image is displaying for additional information. Boot screen (Original Bios), Donkey Kong game select screen, and then started Donkey Kong game (sort of looks like abstract Zelda). Sound is fine. Could be a job for Jimmy.

eems · June 4, 2021

Update: I traced the continuity issue with U17-2 and U15-2 through the VDP (AD4/AD5) which individually trace to all P12 and P6 on each chip. With some help from a gracious community member the best course of action was to see what was going on under the holders so I popped them all off and found the likely short where it looks like U11-12 had some issues and is bridged to the run from U16-6. I'll report back in if that is the fix. If it is the lesson is inspect, inspect, inspect (or have your kids do it if your eyes are aging) and take a picture at each stage of the process.