Lower RAM Query

[ulysess1966]

Hello All.

I'm new to this forum, but not new to Sinclair and the ZX Spectrum going right back to a ZX80 I was loaned (in 1980!), which got me hooked on home computing and robotics.

https://rgwni.blogspot.com

Recently someone gave me a big box of old 8-bit computers including some 48K Spectrums that somebody had been trying to fix at some point. There were 4 main boards, 2 of which had been hacked to bits and 2 that were salvagable. I did the video and regulator mods on these 2 and with a bit of experimenting identified 19 working 4116 RAM ICs so was able to get one board working and manged to load "The Secret of St Bride's" game (there's a good BBC podcast about the Silver Sisters of Burtonport if anyone is interested).

On to the other board.

This one had dead 12V and -5V lines so I replaced the 2 transistors and got that working, but it was still displaying lower RAM issues with colour blocks all over the screen. Now, having a working board with socketed ICs I swapped the working RAM and the ULA into the board with problems, but no luck. I then carefully tested the connections from the RAM to the ULA and CPU for continuity. Eventually I took out the upper RAM in case it was causing a problem and for the time being replaced those ICs with empty sockets. More testing, but still no luck.

Any ideas what I should be looking for? I have an oscilloscope and some other test gear.

All the best, Glenn

[1024MAK]

Okay, slow down a little…!

And welcome

Is the Z80 actually doing anything? When you power it up, is the border always white? You do have to try at least five times.

Why do I ask this? Because one of the first things the ROM code does is to set the border colour to white. It then proceeds to try to do a simple RAM check (to work out how much RAM there is more than to test if it’s any good), the RAM test should make the paper/ink part of the screen black.

Also are all three supply voltages okay now +5V, +12V and -5V ?

It’s helpful if you tell us which issue board this is as well.

Mark

[ulysess1966]

Hi Mark

Thanks for replying.

Yes, probabably need to slow down a bit, but I've been tinkering with these for a few weeks because the weather in my area hasn't been great.

Voltages are all okay.
Border remains black.
Board is a 4S.

Glenn

[1024MAK]

So the Z80 is either not doing anything useful, or it’s not getting valid instructions from the ROM.

First things first, go here for the pin-out of a Z80A.

Crack out the ‘scope.

Now what do you get on these Z80 pins:

Pin Function
6 . Clock (should be pulsing rapidly at about 3.5MHz, will NOT be a perfect square wave)
16 /INT (should be pulsing at 50Hz)
17 /NMI (should be steady high)
18 /HALT (should be steady high)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (may or may not be pulsing)
24 /WAIT (should be steady high)
25 /BUSRQ (should be steady high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Also, does the ULA you removed from this faulty board work in your ‘working’ board?

Mark

[ulysess1966]

Hello again Mark

I swapped out the ULA from the good board (same version number) to be on the safe side.

All of the signals from the Z80A appear to be as expected and match the signals from the good board (the scope is a bit old and needs calibrated).

What should I be looking for next and thanks again for all your help.

Glenn