ZX spectrum spider mod

Post Reply
Rabs
Posts: 33
Joined: Thu Feb 23, 2023 7:34 am

ZX spectrum spider mod

Post by Rabs »

Hi, starting to repair a ZX Specturm Issue 2 board. Power is ok and clock is ok but no activity on Z80 M1 pin so removed and tested the Z80 and sure enough it is dead.

So planning next steps but this issue 2 board has the spider mod. Am I OK to continue testing for now without this mod and is there a better place to install the mod rather than over the Z80?
20240225_153514 (1)~2.jpg
Thanks, any help much appreciated.
User avatar
1024MAK
Posts: 5116
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX spectrum spider mod

Post by 1024MAK »

Yes, it will work in BASIC without the spider transistor. It's machine code programs (games) that access I/O ports (the ULA or joystick interface) that would experience problems.

And yes, there are alternative positions for this transistor. I'll try to find a photo showing a better position.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
User avatar
1024MAK
Posts: 5116
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX spectrum spider mod

Post by 1024MAK »

Okay, have a look at my flickr photos here ;-)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Rabs
Posts: 33
Joined: Thu Feb 23, 2023 7:34 am

Re: ZX spectrum spider mod

Post by Rabs »

Thanks for that, replaced Z80 and now getting bus activity and M1. But screen as follows.
20240226_183035.jpg
I can see activity on all lower RAM Dout pins, so think my next step will be to socket the ROM and replace with a Diagnostics ROM. I do have a working ULA, so as this is socketed may swap anyway just in case.

Any hints or tips much appreciated.
Thanks
Rob
User avatar
1024MAK
Posts: 5116
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX spectrum spider mod

Post by 1024MAK »

That looks like a problem with the 'lower' DRAM, or a problem with the busses that means the ULA is having trouble getting screen data.

In your photo, the border is white. Does it go white at every power up (NOT reset)? Try six to ten times. If it always goes white, this indicates that the Z80 is able to read the very first part of the ROM.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Rabs
Posts: 33
Joined: Thu Feb 23, 2023 7:34 am

Re: ZX spectrum spider mod

Post by Rabs »

Thanks, border is always white? Guess I need to eliminate the lower RAM as a first bet?

Thanks
Rob
Rabs
Posts: 33
Joined: Thu Feb 23, 2023 7:34 am

Re: ZX spectrum spider mod

Post by Rabs »

Well that's a turn up for the books.
20240227_185122.jpg
I took the ULA out and placed in my other working Spectrum, and worked as expected.

Could not be bothered to swap back, so inserted the ULA from my other working Spectrum and Bob's your uncle.

Still can't quite believe it, so may swap back the ULAs. What's the betting.
User avatar
1024MAK
Posts: 5116
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX spectrum spider mod

Post by 1024MAK »

Possible poor pin/socket connection.

Are both boards issue two, or is the other board a different issue board?

Are both ULA chips the same, or are they different versions?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Rabs
Posts: 33
Joined: Thu Feb 23, 2023 7:34 am

Re: ZX spectrum spider mod

Post by Rabs »

I will put it down to the socket connection, for now. Will see if it reoccurs after a soak test.

Both issue 2 both ULA5C112E-3.

Swapped back and still working.

Will crack on with the spider mod as you suggested.

Thanks
Rob
Post Reply