ZX80 troubleshooting with oscilloscope.

Discussions about Sinclair ZX80 and ZX81 Hardware
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Erm
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ZX80 troubleshooting with oscilloscope.

Post by Erm » Sat Dec 28, 2019 4:01 pm

Hello.

I got an oscilloscope for Christmas! I won't pretend I know how to use it.

I thought I could use it to find out why my ZX80 Russian clone isn't behaving properly. Problem: there is no K cursor. When I power it on it flickers with a brief black horizontal line (which appears normal), goes completely white and that's it. It has a ZX8-CCB attached, as you can see (and I've played with the trimmers to get a stable pic):
20191228_145544_resized.jpg
All of the chips appear to be correct and round the right way. The Eprom is a 2732 and the board has little solder jumpers on the back to enable use of a 2732.

On the video out I get this signal (ignore the missing red line sections, that's down to crappy screenshots):
Image2.jpg
Which I think is normal.

Pin 6 (clock) on the Z80 looks like this:
z80clock.jpg
Which again, I believe is as it should be.

Pin 19 on the Z80 (MREQ) looks like this (but does seem a bit flickery when it's not a still like that):
z80pin19MREQ.jpg
Is there an idiots guide to what I should check?
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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1024MAK
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Re: ZX80 troubleshooting with oscilloscope.

Post by 1024MAK » Sat Dec 28, 2019 4:55 pm

A white screen often is due to non-functional RAM.

But let’s not jump to conclusions.
First off, what signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (I think this should always be high on a ZX80)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (I think this should always be high on a ZX80)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Next, test the /S (also known as /CS or/CE ) pin on the RAM chips. For the 2114 type, this is pin 8. Check also the /W signal on pin 10. Both 2114 chips should share both these signals.

While talking about RAM, can you tell me which version of the 2114 SRAM chips you have. That is the full part numbers. On original ZX80s, Sinclair used two different speed ratings, as one has to respond faster, so they used a quicker chip.

Z80 pin-out here.

Also have a read through this thread

Mark

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Erm
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Re: ZX80 troubleshooting with oscilloscope.

Post by Erm » Sat Dec 28, 2019 5:04 pm

Thanks so much Mark. I will check those things in due course.

I did also try with a known good 16k RAM pack and there was no change.

The RAM chips I got with the board are: MM2114N-2L and MM2114-3. Interesting about them having to be different, as I changed them for 2 from a ZX81 (because I knew both were ok). They are 2x TMM314APL-1 2114L2.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Erm
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Re: ZX80 troubleshooting with oscilloscope.

Post by Erm » Sat Dec 28, 2019 8:11 pm

Edit: IGNORE this post, I noticed one probe was set to AC and the other DC!

Ok, I think there is activity where there should be and highs where they should be.

I compared the dodgy new ZX80 to my known good original. Here is a pic showing the read out for pin 16 on the Z80. Yellow is the good one, red is the broken one. The pulsating shapes are the same but the broken one is all high, is it not? Should the troughs be under the centre line (I suspect they should).
z80p16comparison.jpg
Same for pin 18 of the Z80, again, both shown here:
z80p18comparison.jpg
Or am I not understanding my oscilloscope properly!
Last edited by Erm on Sat Dec 28, 2019 9:28 pm, edited 1 time in total.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Erm
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Re: ZX80 troubleshooting with oscilloscope.

Post by Erm » Sat Dec 28, 2019 9:27 pm

Ignore the above. One was set to AC, the other DC!
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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Erm
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Re: ZX80 troubleshooting with oscilloscope.

Post by Erm » Sat Dec 28, 2019 10:08 pm

Everything on the Z80 and the RAM looks the same as the known good ZX80, as far as I can tell.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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1024MAK
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Re: ZX80 troubleshooting with oscilloscope.

Post by 1024MAK » Sat Dec 28, 2019 11:24 pm

What happens if you blind type a simple program like:

Code: Select all

10 REM TEST
Then

Code: Select all

SAVE
?

You may want to monitor the mic port with your ‘scope...

Mark

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Erm
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Re: ZX80 troubleshooting with oscilloscope.

Post by Erm » Sun Dec 29, 2019 2:22 pm

Good idea. But it is proving quite tricky! I shall persevere.
I'm Chris, a Brit in Germany (Chr$ on the QL forum).

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1024MAK
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Re: ZX80 troubleshooting with oscilloscope.

Post by 1024MAK » Mon Dec 30, 2019 12:06 am

Just type SAVE then (press [ E ] then [New Line]

Mark

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