600D replica problem (test 6)

[andrewh]

That ROM causes my screen to flicker:

IMG_2303.MOV

(8.31 MiB) Downloaded 163 times

I can also see a rather odd waveform on U40 pin 30 (D3):

IMG_2304.MOV

(5.16 MiB) Downloaded 160 times

I see something very similar on pin 28 (D5):

Screenshot 2022-10-05 at 13.31.13.png (76.03 KiB) Viewed 1804 times

This is more interesting - the green trace is pin 30 (D3) and the yellow trace is pin 18 (A9):

Screenshot 2022-10-05 at 13.52.32.png (81.39 KiB) Viewed 1804 times

How would this be possible?

[Mark]

The screen flicker is an artifact of the constant access of the 6502 to the video RAM. The raster gets blanked momentarily during R/W multiple times per scan line. The program is constantly polling the keyboard & writing to the screen causing the flicker. It's annoying, but operating as expected.

Based on the screen image, the access to RAM at $00xx and $01xx is corrupt although we don't know how (stuck bit or write fail etc.)

In addition data line D3 is stuck high on video RAM since all characters have binary 8 value bit set. However reading D3 works from the ROM so it is not stuck high on the CPU side -- the program is running. So it's possible it has something to do with pin 12 of U25 (8T28) not being connected to pin 14 and the D3 line from the 6502, or pin 13 is somehow wired high on the video RAM data bus. According to the schematics, pin13 U25 (8T28) is wired to to pin 11 U39 (RAM) and pin 2 of U41 (CHARGEN).
On the schematics I have, U40 (2114) is used for D4 to D7.

[Klyball]

Just curious where this board came from, if its from the gerbers then your fine. But there were 10 original boards that were missing a few traces.

[andrewh]

So I have two separate problems? I'll take a look at the video first.

All those pins appear to be connected correctly. U25 pin 13 is not completely stuck high:

Screenshot 2022-10-05 at 17.11.05.png (81.75 KiB) Viewed 1792 times

If I can identify where those pulses are coming from, that might help.

The board came from the Gerber.

[andrewh]

It works!

0A95CC30-CF48-4C5E-A6DA-1A0271ED5C22.jpeg (2.23 MiB) Viewed 1783 times

So I worked out that the strange waveform on pin 30 was caused by 4V DC still being on the pin when all of the ICs on the data bus were removed. There was a solder bridge between U36 pin 8 and the D3 track.

The picture looks a little more stretched than expected. Is this normal?

Many many thanks for all your help.

[BillO]

Yay!

Picture looks fine for being displayed on a wide screen monitor.