Help diagnosing issue with 48K Spectrum

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andychak
Posts: 3
Joined: Thu May 02, 2019 6:00 pm

Help diagnosing issue with 48K Spectrum

Post by andychak »

I recently got a used 48K ZX Spectrum that has a few issues.

Ive replaced all the capacitors and the sockets for the ROM and ULA (the sockets had been damaged)
However this has not solved the issue.

The system does not boot and instead displays some blinking blocks on screen. I've attached a video I took of the screen
https://www.youtube.com/watch?v=7sN-Bl8eQ9c

Any help would be appreciated!
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1024MAK
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Re: Help diagnosing issue with 48K Spectrum

Post by 1024MAK »

In your video, the Z80 is not running the start up code from the ROM.

Before going any further, I have a bunch of questions:
Which issue board is it (printed on the front edge)?
What were the symptoms before you started work?
What test gear do you own or have available?
Have you tested all the voltage rails (+9V nominal input, +5V, +12V, -5V) and are they all within the expected range?

If you have a logic probe, logic analyser or oscilloscope, can you test pin 6 on the Z80 CPU please.

A photo of the board would be good as well.

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

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There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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andychak
Posts: 3
Joined: Thu May 02, 2019 6:00 pm

Re: Help diagnosing issue with 48K Spectrum

Post by andychak »

To answer your questions

The board is issue 2
Originally when I started work it was just horizontal bars

Gear wise I have multimeter and a 2 channel 100mhz oscilloscope. I do have a cheap logic analyser somewhere as well (one of these https://www.amazon.co.uk/Hobby-Componen ... B00DAYAREW)

Power rails:
When i started the 7805 was outputting 4.8V, I've replaced it with a new one thats outputting 5V
12 V Rail is at 12.14V
-5V rail is -4.58V

Pin 6 on the CPU is giving me some strange readings, I'll do another post with a video

Photos of the board
https://ibb.co/n0z2f7L
https://ibb.co/L65ccrg
https://ibb.co/tq3TMrz
https://ibb.co/hCVThWZ
andychak
Posts: 3
Joined: Thu May 02, 2019 6:00 pm

Re: Help diagnosing issue with 48K Spectrum

Post by andychak »

EDIT: got a clean clock signal on pin 6
Image
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