No video output, could use some advice

Discussions about Sinclair ZX80 and ZX81 Hardware
grimmware
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Re: No video output, could use some advice

Post by grimmware »

1024MAK wrote: Fri Feb 19, 2021 10:52 pm The continuity mode on multimeters is great, but unfortunately there is no standard for what the resistance value is where the changeover occurs between it (the meter) indicating continuity and indicating open circuit... Each meter does it’s own thing...
Mark
Yeah that makes sense, I've measured the resistance on all the connections and with most of them I'd chosen test points where there were no resistors between. I'm getting 0 on most points and 686 ohms and 995 where I was expecting to see 680 and 1k so I guess that's all within tolerances. I'm pretty convinced by this stage that it's not the ROM or the ROM socket.
1024MAK wrote: Fri Feb 19, 2021 11:11 pm On the subject of testing, what settings was your ‘scope on in that last picture?
This is at the bottom left for both channels, so both channels were on 100mV which turns out to be pretty academic because I powered it up again and re-took the measurements and got this instead:
ROMCS.jpeg
I don't know if I maybe caused a short or something that caused the Z80 to get in a wedged state or if I did something else dumb which a reboot fixed but it looks a lot more like how it's supposed to (2V scale for both traces, both bottoming at 0 and capping at 4.08V).

Any ideas of where else to look or something that I've missed so far?

Thank you all so much for your help and especially your explanations, I've learned a bunch about how the ZX81 switches between onboard functionality and peripherals from the edge connectors, using resistance measurements instead of continuity on a multimeter and how to use half of the functions on my scope this evening, let alone everything I learned about the zeddy's layout and functioning! ;)
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

Remove the ROM chip, then power it up (yes, with no ROM chip fitted).

Then use your ’scope to confirm what all data lines are doing (D0 to D1), and that all address lines (A0 to A15) pulse between low and high. They each should be pulsing at a different frequency.

Mark
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grimmware
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Re: No video output, could use some advice

Post by grimmware »

1024MAK wrote: Sat Feb 20, 2021 8:24 am Remove the ROM chip, then power it up (yes, with no ROM chip fitted).

Then use your ’scope to confirm what all data lines are doing (D0 to D1), and that all address lines (A0 to A15) pulse between low and high. They each should be pulsing at a different frequency.
I've done this with the ROM in and with the ROM removed largely for my own edification, but I'll post all the results here.
Pin No/Name : ROM in : ROM out (L is "Low", P is "Pulsing")
30/A0 : L : P
31/A1 : P : P
32/A2 : L : P
33/A3 : P : P
34/A4 : P : P
35/A5 : P : P
36/A6 : P : P
37/A7 : P : P
38/A8 : P : P
39/A9 : P : P
40/A10 : P : P

1/A11 : L : P
2/A12 : L : P
3/A13 : L : P
4/A14 : L : P
5/A15 : L : P

All of the D (which I assume is incoming data from whichever memory chip is selected?) pins were pulsing with what looked to be arbitrary data on both tests.

I also ran a couple of traces on "Single" mode on my scope to verify the pulses were sometimes low, sometimes high on the address pins.

Worth noting here that I've removed the keyboard to conduct these tests. I point this out because with the ROM in, A8 and A10-15 are low and they're the pins that the keyboard row-select is on along with A9. A9 is pulsing but this is the only one of the keyboard lines with RAM connected to it in my configuration so that seems to make sense.

I can easily reconduct any of these tests with the keyboard attached if you figure it makes any difference as the headers were replaced with DuPont connectors, but the keyboard PCB I've replaced it with is pretty much just a reconstruction of the membrane circuit with extra diodes to prevent key ghosting and it worked just fine when it was working so I don't really suspect it's part of the problem. I just figure it bears calling out.
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

The keyboard being removed makes no difference. A ZX80, ZX81 or ZX Spectrum will start up quite happily without a keyboard being connected.

So from those results, if you are getting different different frequencies on the various address lines (A0 to A15) then I would say that the Z80 is okay.

It does not appear to be happy when you have the ROM chip in. I trust that you are putting the ROM chip in as per the photo in your first post?

Mark
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grimmware
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Re: No video output, could use some advice

Post by grimmware »

1024MAK wrote: Sun Mar 07, 2021 3:36 pm So from those results, if you are getting different different frequencies on the various address lines (A0 to A15) then I would say that the Z80 is okay.
ack, I'll double check that the frequencies are varying
1024MAK wrote: Sun Mar 07, 2021 3:36 pm It does not appear to be happy when you have the ROM chip in. I trust that you are putting the ROM chip in as per the photo in your first post?
Yeah, bottom of the socket with the orientation marker pointing north. I'd replace the socket but I'm planning on replacing it with a 27XX adapter anyway, I just want to get the thing back to a known-good state first. If you have opinions to the contrary I'm open to them, but as mentioned before I've already checked the continuity of the socket itself by testing from the pins.

Thanks again for your help!
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

grimmware wrote: Sat Mar 13, 2021 1:41 am I'd replace the socket but I'm planning on replacing it with a 27XX adapter anyway, I just want to get the thing back to a known-good state first. If you have opinions to the contrary I'm open to them, but as mentioned before I've already checked the continuity of the socket itself by testing from the pins.
So going on the available information, my summary so far is this (correct me if I miss anything or get anything wrong):
The +5V supply is present and correct.
You tried with a known working RAM pack and that made no difference.
The ROM code appears to be fine when read with a EPROM programmer.
All the ROM socket contacts test okay on resistance/continuity.
There is a clock signal from the ULA to the Z80.
The Z80 appears to be doing what is expected when no ROM chip is fitted.

Hmm 🤔

Okay, just going to cover some other things before we get even deeper...

Question, is the +5V supply free of any significant ripple? Use your ‘scope, connect the probe to any convenient +5V point. Select AC coupling mode. Select a 5ms/div, 1V/div. (assumes your probe is set to X1 or the ‘scope is set to X10 and the probe is set to X10). The trace should be dead flat or at least only have very very small variations. Note if the probe does not make good contact, it may well pick up mains hum.

Question, since you started this topic, have you tried the RAM pack again? If not, please do try it again.

Do you have another Zeddy?

You mentioned an adapter for 27 series EPROMs. Was it your intention to remove the existing socket and modify the PCB, or the socket? Rather than making an adapter to fit in the existing socket?

Mark
ZX81 Variations
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:!: 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.
grimmware
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Re: No video output, could use some advice

Post by grimmware »

Hey team!

Apologies for not responding to this sooner (especially after chasing Mark down in PM and asking him to respond again!). I've had various post-COVID health complications and competing time pressures weighing on me so the zeddy got pushed to the back of the desk for a while, but it's booting again!

I was speaking to a friend who used to do debugging for various RF equipment and he asked me if I'd tried reflowing the solder on the ROM socket, suggesting that I might have gotten continuity because I was pressing down on what was old solder and might have cracked, causing it to bridge whilst testing continuity but not actually connecting properly the rest of the time. I thought this was a great suggestion so today I took 10 minutes to have a pop at it and... no dice. So on that subject, I decided whilst I was at it to check the actual holes in the socket itself just in case, and after I removed the ROM chip again I saw it...

When lifting out the ROM chip previously with a screwdriver apparently I had slipped and underneath the plastic of the socket itself I'd managed to score the surface of the board, neatly cutting exactly one trace. I fired up my soldering iron and tentatively tried to bridge the gap, reinserted the ROM and fired it up and it worked!

Thank you so much for all your help and advice on this, especially Mark whose insistance that it was most likely something to do with ROM removal that had caused the problem kept me looking in that general vicinity despite the fact I'd otherwise decided that it was probably something else.

Lesson learned - sometimes you need to visually inspect *under* the IC.
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

And be very careful what you are doing with a screwdriver :lol:

Glad that you’re got your Zeddy up and running again :D

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.
grimmware
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Re: No video output, could use some advice

Post by grimmware »

1024MAK wrote: Wed May 12, 2021 1:11 am And be very careful what you are doing with a screwdriver :lol:
Hahah, quite!

Thanks again for your help! I learned loads!
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