Repairing ZX Spectrum issue two

[Pygmalion]

Disclaimer: I have posted this question to "Sinclair ZX80 / ZX81 / Z88 Forums" but received no answer. I apologise if you have already seen this question.

I recently dusted off my ZX Spectrum, which my father bought me and smuggled to Eastern Europe in the first half of the 1980s. What a story.

I made my first programming steps in Basic and Assembler on this computer. Those were the days.

But it does not work. I checked the -5V, +5V and +12V lines and they are fine. But when I switch it on, all I see is a flashing square:

(

The pattern of the flashing square changes between resets.

Do you have any idea how I can make this thing work?

I have basic knowledge of electronics and soldering.



There are few peculiarities on the board: big cooler on ULA, unusual spider mod and something that looks like extra audio output.

[dfzx]

I recently dusted off my ZX Spectrum, which my father bought me and smuggled to Eastern Europe in the first half of the 1980s. What a story.

You should head straight over the "Introduce yourself" thread and tell it. We'd all like to hear that one.

Do you have any idea how I can make this thing work?

Yes. Well, maybe not me so much, but there are others who will.

First, explain how you measured the voltages. What points did you measure across and what voltage, exactly, did you find at each point?

What test equipment do you have? Do you have access to a diagnostics ROM device?

How original do you want to keep the board?

Does that white(-ish) border appear every time you restart the machine? Try 10 times.

[Pygmalion]

First, explain how you measured the voltages. What points did you measure across and what voltage, exactly, did you find at each point?

Well, I checked the leftmost lower RAM chip as described in a YouTube video and there were +12V, +5V and -5V in the appropriate places

What test equipment do you have? Do you have access to a diagnostics ROM device?

A good multimeter, Hantek 6022BE USB oscilloscope, FX2LP CY7C68013A USB Logic Analyzer clone, Raspberry Pi, bunch of Arduinos, basic electronics including charge pump and step-up board...

I did some I2S, SPI and parallel programming before, and I wanted to make simple RAM tester, but I couldn't find a good Arduino software to test 4116 and 4164 RAM chips.

How original do you want to keep the board?

That's a good good question. As original as possible, unless the change makes ZX Spectrum much less likely to die again in the future. For example, if recapping is needed, I'd use polymer capacitors. Also thinking about replacing 4116 chips with modified 4164 (removing pin 1, connecting pins 8 and 9).

Does that white(-ish) border appear every time you restart the machine? Try 10 times.

Yes, I tried that dozens of time and the the white border is always there. As far as I understand from YouTube videos, this indicates that Z80 works.

[1024MAK]

The topic on Sinclair ZX World is here.

Yes, the Z80 does look to be running.

Mark

[Pygmalion]

The topic on Sinclair ZX World is here.

Yes, the Z80 does look to be running.

Mark

It might be better to continue here. On the other forum every post must be approved and it takes a lot of time...

[1024MAK]

It might be better to continue here. On the other forum every post must be approved and it takes a lot of time...

I’m a moderator over there. But I do have a day job to do as well. Once you have more than three posts, your posts won’t need moderator approval.

Mark