Superboard II Model 600 Rev B unable to access memory beyond the first 1K

[royedmund]

Superboard II Model 600 Rev B unable to access memory beyond the first 1K

Installed two 2114 system boots into BASIC and show 255bytes free which is good.
Installed four 2114 and system boots into but still only shows 255bytes free. Tested all four 2114 chips and they appear okay.

/RS1 is held High swapped U22 74LS138PC with a new 74LS138N no change. Any suggestion about how I show troubleshoot this problem?
See attached image for the signals on U22 Pin 1,2,3,4 and 5

MEMORYPROBLEM00.jpg (400.72 KiB) Viewed 1941 times

MEMORYPROBLEM.jpg (1.06 MiB) Viewed 1941 times

MEMORYPROBLEM000.jpg (441.02 KiB) Viewed 1941 times

[BillO]

Once it determines that there is noting beyond 1K it will no longer attempt to address those locations, so /RS1 will stay high. What are you running on the system when testing /RS1?

Try running the following program:

10 POKE 1025,128:GOTO 10

You should have enough memory for that.

Set up CH1 on your scope to monitor pin 9 of U22 (right on chip) and to trigger off that when it goes low.

Then check pins 5, 1, 2, 3 and 4. Pins 5 and 4 should be low soon before pin 9 and at that time pin 1 should be low, pin 2 high and pin 3 high.

Since the first K of memory gets addressed there should be no problem with the signals on pins 4 and 5. I'd also bet that U22 is fine too.

If Pin 9 on U22 is not pulsing low when you run that program, then the problem is with one or more of the inverted address lines. If they are not L H H then trace them back to their source (U21 pins 8, 2 and 12). If they are not L H H then check that pins 9, 1 and 13 are H L L.

If pin 9 on U22 is pulsing low, check pin 8 on both U46 and U32. They should both be going low. If they are not, then you have a broken trace or a bad socket at either U22, U46 or U32.

Edit: made some tests and updated info.

[royedmund]

After hours of checking difference levels signals and checking traces, I now have 4K of RAM working, 3327 Bytes free for BASIC. The issue was caused by corrosion in the memory sockets, I have replaced the faulty sockets and now looking good.

OHIOMEMFIXED.jpg (717.56 KiB) Viewed 1914 times

[BillO]

Did you try the little program to produce the /CS pulse on the memory chips? It should have saved you a ton of time.

It's good you are doing this. These machines certainly deserve to be restored and maintained.

I've had one since the late '70s. I use it almost as much now as I did then. It has heart and soul (well no, not really) and certainly has character!

[royedmund]

I did but when I probe the 2114 chip(s) it looked okay. Later with clip leads on the chip(s) and there was no signal on pin 8 for U46, I guess the extra pressure of holding the probe helped create an okayish contact.

[BillO]

If pin 9 on the 138 was okay and the pin on the 2114 was not, then probe the pin of the 2114 socket on the back side of the board. If that is okay, the the socket is bad, if it's not okay the trace is bad. But yeah, so much can go amiss.

Good work and keep it up. Troubleshooting digital circuitry is both a challenge and its own reward.