1024MAK wrote: ↑Sat Sep 07, 2019 1:56 pm
Hi Moggy
So just to be clear, the only chips you changed were the Z80 and the ROM/EPROM chips, yes?
The symptoms are a white screen only (no inverse K) and the +5V supply is present and correct on all chips.
So my questions so far are (and forgive me if I start with the obvious things that you may have already checked):
- You have checked for bent pins?
- You have put back the previously known good ZX80 ROM chip?
- You don’t give any details of the Z80 chips that you were trying, we’re all of them know good Z80s? Or did you put back the original known good Z80?
- Have you tried the Z80 chips in other machines to see if they still work?
- Have you had a close look at the Z80 socket and the ROM socket to see if any of the socket contacts have become damaged? A magnifying glass and good daylight / a good light are useful for this.
- Have you checked the underside of the board to see if there are any dry or cracked solder joints? Start with the Z80 socket and the ROM socket. Again, use a magnifying glass and good light.
I’ll be back later with some tests. It would help if you can give me some details of your multimeter. I presume you don’t have a logic probe?
Mark
Some excellent advice there Mark as always.
Only the ROM and CPU's changed but I will try the CPU in other zeddies. I have put back original ROM chip.
One point you make is about the sockets.
The sockets fitted to this board are of the type with the tiny holes so I was very careful about bent pins etc BUT whereas normally with this kind of socket a firm press is usually needed to seat the chip, the ones fitted to this 80 are so slack that chips could fall in under their own weight!! Exaggeration I know but you get the picture.
I will remove and replace with better sockets and report back.
I will try all CPU's in other zeddies (mixture of NEC Zilog and the Italian one)
The board underside is very good on this 80 solder all nice and shiny and must have been kept in a dry environment.
My multimeter is just the usual digital cheap and cheerful type volts amps and resistance etc.
5volt where 5volt should be and no dramatic pull down of the input voltage in the manner of a short etc, 12 volt drops to about a shade over 9.5v with RAM pack fitted 10volt without. which is normal for this set up.