New to repairing ZX's

[1024MAK]

Hi

You already have your answer to where the ROM should go.

As said above, normally there is no need to renew the electrolytic capacitors on a ZX81, one (or two if an issue three board) decouples the +9V supply (this is mainly intended for when the ZX Printer is attached), the other is the capacitor for the reset circuit, but this one has a very easy life.

The ULA does indeed do all the timing.

The following assumes connection via RF/UHF.
If you can tune into a completely black screen, this often indicates that there is a problem with the ULA.
If you can tune into a completely white (or light grey) screen, this indicates that there is a problem with either the RAM, the Z80A or the ROM.
If you get a repeating pattern, I would suspect RAM, and then ROM.

If you have an external RAM pack, clean the edge-connector using IPA (not the drink!), then a ink eraser. Then (with the power off) connect the RAM pack and try powering it up.

If you intend swapping chips, please mark them (e.g. with a sticker or insulation tape) so that you can keep track of which chip came from where, and whether it tests good or bad.

What test gear do you have?

Mark

[colday]

Hi

You already have your answer to where the ROM should go.

As said above, normally there is no need to renew the electrolytic capacitors on a ZX81, one (or two if an issue three board) decouples the +9V supply (this is mainly intended for when the ZX Printer is attached), the other is the capacitor for the reset circuit, but this one has a very easy life.

The ULA does indeed do all the timing.

The following assumes connection via RF/UHF.
If you can tune into a completely black screen, this often indicates that there is a problem with the ULA.
If you can tune into a completely white (or light grey) screen, this indicates that there is a problem with either the RAM, the Z80A or the ROM.
If you get a repeating pattern, I would suspect RAM, and then ROM.

If you have an external RAM pack, clean the edge-connector using IPA (not the drink!), then a ink eraser. Then (with the power off) connect the RAM pack and try powering it up.

If you intend swapping chips, please mark them (e.g. with a sticker or insulation tape) so that you can keep track of which chip came from where, and whether it tests good or bad.

What test gear do you have?

Mark

Mark, it's me. Colday from the Stardot forum. Lol. So I have...

Scope, multimeters, logic probe (awful), can solder and desolder like a demon.

Car is in for repair tomorrow but I have a couple of days off work as am getting a year older (again).

Shall be looking at this tomorrow again hopefully.

I did have a tinker today and the ULA marked up 8106 gives a plain blank screen in both machines.

The other ULA when swapped over gives a K prompt (for a while) which slowly scrambles itself till you can't tell it's a K prompt. It does type as you can see stuff appearing but it is totally garbled.

I'll grab some photo's of it tomorrow.

[colday]

I was having a look round for ULA's and found this.

This guy has got be be kidding right?

https://www.ebay.co.uk/itm/Sinclair-ZX8 ... SwxstgFZ37

Check out the silkscreen on the chip.

[1024MAK]

Mark, it's me. Colday from the Stardot forum. Lol.

Yah, but I have trouble remembering what test gear I have, let alone what other people have...!

I did have a tinker today and the ULA marked up 8106 gives a plain blank screen in both machines.

That one sounds a bit sick

The other ULA when swapped over gives a K prompt (for a while) which slowly scrambles itself till you can't tell it's a K prompt. It does type as you can see stuff appearing but it is totally garbled.

I'll grab some photo's of it tomorrow.

Please do...

I was having a look round for ULA's and found this.

This guy has got be be kidding right?

https://www.ebay.co.uk/itm/Sinclair-ZX8 ... SwxstgFZ37

Check out the silkscreen on the chip.

It’s not unusual for the letters to be unevenly spaced and not aligned vertically...

Unfortunately with working spare ULAs being scarce, you will find outrageous prices on eBay.

Put a wanted post up here if you are after one. I know one person on here who occasionally sells used but working ULAs.

Mark