ZX81 rebuild

[Lardo Boffin]

Diodes and transistors.

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[Lardo Boffin]

Some big bits added.

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[Lardo Boffin]

Switched it on and sadly nothing. Well, nearly - after a couple of seconds i get a brief flash of white and then it goes black again. The time delay between switch on and flash and death roughly corresponds to how long it takes to work out that it has 16k of RAM.

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Now - where do I start with sorting this out? A beer and then bed I think. Worry about it tomorrow.

[sirmorris]

Maybe an address line stuck? All fine until you hit a certain place in the program?

[Lardo Boffin]

Thanks sirmorris. Although not sure how to test that...

I need to make sure the ZX8-CCB is correctly tuned. Although it works fine / reasonably on all the zeddies i have tried it with.

My plan is to double check what i ordered and how I labelled the bags and then triple check the soldering and for shorts. After that it is time!

Lardo

[Lardo Boffin]

I got the 6.5mhz resonator from sell my retro:-

https://www.sellmyretro.com/offer/detai ... ck%29-3469

What is the correct alignment for this? Does it matter? I went with the same as an installed one (lettering facing the ULA). The zeddy assembly guide doesn’t specify a way round, unless I missed it?

[sirmorris]

It's not polarised, any way is fine.

[1024MAK]

It's not polarised, any way is fine.

Except upside down

If you have a 'scope or logic probe, check Z80 address lines A14 and A15.
Also is you have a 'scope, test the video/TV output pin to see if it is switching between the three levels (high = white, "mid-way" = black and low= low part of the sync pulses).

Mark

[Andy Rea]

I would check for shorts near the data pins of the cpu, the track clearances are quite close around there, also the diodes near the keyboard connector some.of the traces run very close to the diode pads.

Other than it not working it looks very nice, good job so far.

Regards Andy

[Lardo Boffin]

It's not polarised, any way is fine.

Except upside down

If you have a 'scope or logic probe, check Z80 address lines A14 and A15.
Also is you have a 'scope, test the video/TV output pin to see if it is switching between the three levels (high = white, "mid-way" = black and low= low part of the sync pulses).

Mark

I managed not to go for the Australia fit...

I don't have a scope or probe. I never really intended getting this involved with hardware - it just kind of happened! Things have certainly moved on from ‘can I fit a ZX8-CCB?’ Is there a reasonably priced probe i can get? I don’t want to spend loads as it probably won’t get a huge amount of usage.