Display problem

[sP1d3r]

I've been testing one of my 3 Zeddy's and the display is strange.
It's got a working 2C158E ULA which I've tested in another working Zeddy and it boots to a K and works fine.
However in it's own Zeddy, which has tested CPU, RAM and ROM and a ZXvid composite mod, the display is 'italic' lines moving to the right.
No amount of adjustment works with either pot in the mod.

2C158E ULA with ZXvid composite mod

I haven't tried another ULA, as I'm a bit hesitant to do so.
I wonder if anyone knows what this is?

[1024MAK]

First, the 7805 NEEDS a heatsink.

Secondly, I don’t have a ZXvid and don’t know any details of this board.

As long the +5V supply is within specification and as long as none of the ULA output pins are shorted, it’s worthwhile trying another ULA.

I presume this is a UK spec. board?

Mark

[Moggy]

Looks like the hand drawn PCB UK issue one at first glance.


@ sP1d3r
I know you say the RAM has ben tested but have you tried it with a memory pack attached (should you have one) as this would at least eliminate that as the external memory switches out the internal chip.

The "pots" only control brightness and contrast respectively I think.

The ZXvid, I believe, was designed by our very own Andy Rea, so perhaps he may have some input should he read this.

[Lardo Boffin]

Mutant Caterpillar Games sell these so may also be able to help.

[sP1d3r]

First, the 7805 NEEDS a heatsink.

Thanks, Mark,

Yes, I'm only testing this board at the moment and I'll replace the 7805.
The board should boot up and work without one, I'd have thought.

As long the +5V supply is within specification and as long as none of the ULA output pins are shorted, it’s worthwhile trying another ULA.

I presume this is a UK spec. board?

Yes, it's a UK spec board, I might try another ULA but I'll look over all the other components first.

@ sP1d3r
I know you say the RAM has ben tested but have you tried it with a memory pack attached (should you have one) as this would at least eliminate that as the external memory switches out the internal chip.

The "pots" only control brightness and contrast respectively I think.

The ZXvid, I believe, was designed by our very own Andy Rea, so perhaps he may have some input should he read this.

Much appreciated, Moggy, I've tried my issue 3 16K ram pack but there's only a pattern of blocks thus far, I hope that the ram pack ULA isn't fried.

My third Zeddy which was a part-assembled kit has a ZX-CCB and both the 2C184E ULA that came with the kit and the 2C158E work in it.

Mutant Caterpillar Games sell these so may also be able to help.

Cheers, Lardo, hopefully I'll find something wrong with the board that causes this, I thought I'd been over all the components but I might not have.

[sP1d3r]

First, the 7805 NEEDS a heatsink.

Secondly, I don’t have a ZXvid and don’t know any details of this board.

As long the +5V supply is within specification and as long as none of the ULA output pins are shorted, it’s worthwhile trying another ULA.

I presume this is a UK spec. board?

Mark

Since my previous post I removed the ZXvid, connected the ULA output direct to the modulator output pin the same as my Zeddy with the vLA81 and tried the same in this board but the output was similar diagonal lines.
I removed and tested both ZTX 313 transistors, also the CPU, RAM and ROM, I can only assume a board fault I haven't discovered or a duff crystal.

All components look OK, the 7805 output is 4.96V from my meter

[1024MAK]

or a duff crystal.

It’s a resonator, not a crystal. Also occasionally referred to as a stick of gum!

They do sometimes fail. As do crystals.

Do you have a ‘scope?

Or if you don’t, there are some tests that can be done with a logic probe or similar. Or failing that, a digital multimeter.

Mark

[sP1d3r]

They do sometimes fail. As do crystals.

Do you have a ‘scope?

Thanks, Mark, I have a scope and I can check for a 3.5 MHz signal on pin 14.
I wonder if these 6.5 MHz ceramic resonators are common, or whether a little PCB and a newer SMD crystal/resonator could be substituted?

The vLA81 has it's own SMD clock but it doesn't have to interface with ZX81 logic directly, as far as I can tell.

[1024MAK]

The vLA81 does indeed have it’s own clock. It completely ignores the main board resonator. In a ZX81, the master clock is the ULA clock. All timings are derived from the ULA master clock.

And yes, suitable resonators are still available from some suppliers.

Mark

[sP1d3r]

And yes, suitable resonators are still available from some suppliers.

I found these on eBay, though I haven't yet tested mine yet, appears to be the same manufacturer as the stock ceramic item.