Have I fried the ULA?

[Liveinabin]

Hallo all.
I thought it would be fun to work on my spare ZX81 - which already has a new power regulator and composite mod - but this time, to install an internal 16K unit that I had lying around. Here's my board with the 16K attached.

I thought I did an okay job on it but, on firing the 81 up:

I get this scrolling up the screen, flashing between two distinct sets of garbage (but attractively hi res) characters.
Key inputs don't do anything. I seem to remember from seeing similar faults that, when you have this graphics nonsense, it's the ULA.
Is that right? Have I killed the only custom component in this poor thing?

[mcarlson_sb]

First I'd try putting the old RAM back.
That will narrow the problem.

[1024MAK]

As you are getting a video picture, it’s not necessarily a faulty ULA. Because the Z80 microprocessor is used to feed video pixel information to the ULA, a fault with the Z80, the RAM, the ROM or any of the interconnections (address bus, control bus and data bus) will cause problems.

If you connect an audio amplifier or cassette deck with a monitor mode to the mic output of the ZX81 (turn up the volume, the output signal is fairly small), then power up the ZX81 and blind type SAVE “S”, if the Z80 is running the ROM code, you should hear it try to save data to “tape”. Note that you don’t have to enter a program.

Mark

[Liveinabin]

I’ll try SAVEing. Thanks. As for putting the old RAM back, I’m afraid I had to snip some legs. Couldn’t desolder them cleanly (and I do get the fear desoldering on Sinclair boards - I had a bad experience with a Spectrum!).
I guess I could carefully desolder this RAM board and fit some sockets.

[1024MAK]

I would suggest that you double check the PCB tracks and pads before doing any further work, it may just be a dry joint or cracked track, or maybe a solder whisker.

Mark

[Liveinabin]

Right, I tried the SAVE”S” and, sadly, there was nothing (also did it on my working 81 to gauge what I should be hearing).
Can I infer anything from this?

[Liveinabin]

I would suggest that you double check the PCB tracks and pads before doing any further work, it may just be a dry joint or cracked track, or maybe a solder whisker.

Mark

Will do.

[1024MAK]

Right, I tried the SAVE”S” and, sadly, there was nothing (also did it on my working 81 to gauge what I should be hearing).
Can I infer anything from this?

Yes, the Z80 is unable to run the code from the ROM, so it’s less likely to be a ULA fault.

Mark

[Liveinabin]

Smart. Right, I’ll find a more powerful pair of glasses and go over those traces. I’m sure to have done something dreadful

[bola_dor]

doble check your keyboard before giving the save test as failed.. you are also testing the keyboard there ... and is a known source of failure to.. If you have a second working ZX then use that keyboard with the faulty board..