low memory fault test

[amateus]

Hi Mark,

The ULA from the faulty works on the good one
The ULA from the good one has the same result on the faulty one.

This means the ULA is working correctly. I've also checked the voltages on pin 13 and pin 14 and they're fine.

Anything else I should check before replacing the remaining 3 low ram chips?

Thank you,
António

[1024MAK]

So, I’m concerned that your result of the test I detailed in this post resulted in a similar screen display to trying the computer normally.

Can you please try this test again. But before powering on, when the jumper is connected to R33, if you test pin 27 on the ROM with the power off, it should have continuity to +5V.

Also, which ROM chip is fitted (make and part numbers)?

Mark

[amateus]

So I soldered a piece of wire to jumper 25 (underside, that is, the jumper on the back of the board). I connected the wire to the +5V on 7805.

I have continuity between rom pin 27 with +5v with the wire connected (tested on +5V onm a low ram). If I remove the wire I no longer have continuity.

Turned on the spectrum with the wire connected and the screen was the same.

The ROM chip has the following printed:

AMI 8427AFK
356-201802
Austria

Thank you,
António

[1024MAK]

Okay, when powering on, does the border always end up white?
I do mean turning it off, waiting for at least 30 seconds, then powering up again. Do this test multiple times. At least ten times.
If you have any of the temporary wires still connected, remove them first.

Mark

[1024MAK]

When I try the ROM disable test on a broken issue 2 board (that is awaiting modification) and which has no functioning lower RAM, I don’t get a nice diagonal pattern, but for the first 10 to 20 seconds, the picture does change rapidly.

At this stage I would suggest doing some testing with a logic probe or oscilloscope, but as you don’t have these, the next step is to replace the remaining original lower RAM chips.

Then try powering up and see what happens.

If you have, or can get the bits, it’s possible to build a very basic logic probe for not much money, see this topic.

Mark

[amateus]

I did the turn on /off, and the screen border is always white.

One thing I noticed is that the ROM gets really really hot. Could it be a problem?

Regarding the logic probe, I don't think there's any problem in getting the components, have to check that.

Ok, I'll remove the chips this weekend and we go from there.

Thank you all the help Mark.

António

[1024MAK]

If ROM is getting really hot, yes, that’s a problem. The ROM normally runs slightly warm or cool.

But your symptoms are confusing, as the border always going white does indicate that the Z80 is reading the code in the ROM at reset and is getting at least to the bit where the ROM code tells the Z80 to write the data to tell the ULA to make the border white.

Mark

[amateus]

I agree.

I have ordered a z80 and a ROM from Retroleum.

Yesterday I stayed for a quite some time powering on/off the board and I finally got some black borders, around 5 in oh 100 boots. Not sure if it's usual or not, but I didn't like it. Today I got a black border on the 2nd reboot. This never happened before that I could see, as I've told you.
I inspected the solders but couldn't see anything. Also the remaining original ICs (low and high) have normal temperature.

This, plus the temperature, leads me to believe that the ROM could be faulty.

I'm going to desolder the ROM, install a socket and install a retroleum rom to check. Eventually I may have to desolder z80 as well, but will start with the ROM. Hopefully the diag rom that comes with the new ROM can also shed some light on this board.

Thanks Mark, I'll be in touch

António