exxos's Atari, Amiga & retro forum

I have run into bad z80 cpu's the plastic case let's in moisture at some point and we'll they ic gets effected. I've also had the problem with the earlier hitachi 74 ttl parts 2


Tom

Tomswork wrote: 28 Sep 2023 16:57 I have run into bad z80 cpu's the plastic case let's in moisture at some point and we'll they ic gets effected. I've also had the problem with the earlier hitachi 74 ttl parts 2


Tom

I've not had any Z80s fail on me until now, but I'm not entirely surprised by what you're saying.

Well I feel largely positive today, because now that the CPUs have been determined to be faulty, and the bad upper RAM in the spares board has been replaced, by hitting the spares board CPU with freeze spray, it's allowed me to test both boards for long enough to know that both lower and upper memory are good on both boards.

All that leaves me to do is drop a replacement CPU into the spares board and give it a soak test without it crashing or needing freeze spray on the CPU, and it's finished and can be dropped back into the Spectrum +, and then that is done. I still may put the original board back, but have to consider the below.

The original Spectrum plus board also needs a replacement CPU, but I'm confident it will run fine after that, but it does have another problem, in that the display is lacking in red, so the colours are off and muddy.

I think that it probably due to the RY input being incorrect at the LM. I know the ULA is good because it works fine in the spares board, and I know it isn't the LM1889, as I've swapped in a new one and it stays the same.

So all that should be is a simple circuit between the ULA and the LM, so I need to check all the circuit all along the way and look for issues.

Of course if I can fix that then it may go back in the plus, and I could also consider whichever board is left over becomes the subject of a 'new' Spectrum build, but we'll see.

Right then, the missing red.

I started by buzzing out the tracks from pin 16 of the ULA to each component en-route to pin 2 (RY) of the LM1889, and everything is going where it should go.

I backed this up by scoping out the RY input on the spares board (Video is good) and on the original + board (Video is missing red) at pin 2 of the LM1889. The same signal is the there on both boards.

Given the LM1889 works fine in the spares board, and the same can be said of the ULA, that suggests that the problem lies in the circuit that mixes everything together, so that's the next port of call.

Not today though, I've been feeling progressively more ill as the day has gone by and I've just tested positive for covid :cry:

Fingers crossed I can get over it as quick as I did last time...

@rubber_jonnie well the sucks, more of it going around than you think. Feel better soon.

rubber_jonnie wrote: 01 Oct 2023 15:35 Not today though, I've been feeling progressively more ill as the day has gone by and I've just tested positive for covid :cry:

Fingers crossed I can get over it as quick as I did last time...

Oh! Well, that's about two weeks of illness. :-(

The new variant is doing the rounds quite aggressively at the moment.

@PhilC @stephen_usher

Thanks fellas. We've both got it, so it's not a fun house to be in at the moment, not looking forward to a week of work with it, but at least I can work from home.

As far as I'm aware, you're still required to stay off work for five days after testing positive.

rubber_jonnie wrote: 01 Oct 2023 15:35 Not today though, I've been feeling progressively more ill as the day has gone by and I've just tested positive for covid :cry:

Really sorry to hear that - my wife tested positive about a week ago & is still recovering and I've been off work since Tuesday (and still having trouble with concentration and energy levels). I'm going to try and WFH for a few days to help revive myself a bit quicker...

I'm still aiming to be fit to attend the CL event next Sunday though!

Hope you feel better very soon @rubber_jonnie !

stephen_usher wrote: 01 Oct 2023 16:12 As far as I'm aware, you're still required to stay off work for five days after testing positive.

Indeed you are.

I work from home 4 days a week anyway, so taking Wednesday off to WFH isn't too difficult, however if I feel like I do right now I won't even WFH.