No video output, could use some advice

Discussions about Sinclair ZX80 and ZX81 Hardware
grimmware
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Joined: Thu Feb 04, 2021 1:09 pm

No video output, could use some advice

Post by grimmware »

Hey team!

So I've been playing around a bunch with my zeddy in order to get it back to health and then beyond. My goal with this thing is really to make it useful to me and as a learning device.

So far I've built a power supply for it, done an RF->composite mod and built a new keyboard. This was all working great up until I did *something* during the removal and reinsertion of the ROM that now makes it not display anything. I don't actually have a display that natively does composite, so I've been leaning on a cheap AV to HDMI adapter that I have which is a little finnicky about signal, so it's entirely possible I'd be getting a white display on a CRT.

I've already looked at a few other posts for similar problems so I've got quite the body of diagnostic information already, so I'm going to present all of that here and now. I have a 2 channel 200MHz scope, a LogicPirate logic analyzer, cheap digital multimeter and a TL866ii ROM programmer available for any further debugging. I'm not super experienced with electronics so feel free to explain basic things to me - I won't take it personally ;)

Essentially I would like to know what should I test next and why? This is all part of the learning exercise for me :) I'm happy to post scope traces, more pictures, whatever!

Photos
Top
zeddytop.jpeg
Bottom
zeddybottom.jpeg
Keyboard (just for kicks)
zeddykeyb.jpeg
Power
Getting the right ~10V/~5V split on the 7805

ROM
Having discovered the problem *after* I removed and reinserted the ROM, I originally thought this was the problem so I built an adhoc 2364 -> 2764 adapter and dumped the ROM. It's a perfectly intact copy of the revision 1 550 ROM which I have checked with a few different methods (checksum, diffing the disasm, booting in an emulator) so I'm actually pretty confident the ROM removal is a red herring.

RF out
Tested at the end of the RCA cable, I'm getting a steady "square" wave which has a frequency of ~15.6KHz and a period of ~64us with a ~5us floor, which I believe is consistent with a correct composite signal for an all-white screen.

16 on the ULA is very similar - perhaps suspiciously so given that there's the RF -> composite adapter in between the video pin on the ULA and the actual cable.

My AV adapter is defaulting to a blue screen however, and not displaying anything. It's not the adapter as I successfully tested it with my BBC.

ULA
I've only tested the clock input and output on this and they look just fine - 6ish MHz in on 35 and 3.24MHz out on 14.

RAM
Doesn't boot on internal 1K or external 16K rampack. Both were working in the before-time, but I absolutley wouldn't exclude the possibility that I've done the same stupid thing twice and fried both.

Z80
This is where I've got the best indications that somethings not right

16/INT: Pulsing
17/NMI: High (~5V), should be pulsing
18/HALT: Low (~0V), should be pulsing
19/MREQ: Pulsing (~1.7MHz)
21/RD: Pulsing (~800KHz)
22/WR: Pulsing (mostly high)
24/WAIT: High, should be pulsing
25/BUSRQ: High
26/RESET: High
27/MI: Pulsing
28/RFSH: Pulsing

I additionally tested A14 and A15 which I believe are supposed to have some action on them (sorry, I'm basically just transcribing my notes) and they're low (~38mV which I'm going to go ahead and call ~0V).
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

So the Z80 is running but it appears it’s unable to run the ROM code. The ROM has been tested and is good.

As this happened after you had the ROM chip out of the socket, have you checked the contacts in the socket? Cheap sockets were used and although they normally don’t give problems, this is mainly due to them not being disturbed.

If any one socket contact is not making good contact with the relevant pin on the ROM chip, then the Z80 will read garbage from the ROM.

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

:!: Standby alert :!:
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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grimmware
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Joined: Thu Feb 04, 2021 1:09 pm

Re: No video output, could use some advice

Post by grimmware »

Ah that's a really good point! I'm going to go ahead and test continuity on the pins of the plugged-in ROM to other points on the board with it powered off tonight and report back. What is it about the diagnostics I've posted that leads you to that suspicion?

In the case that the connections are bad, what's the usual approach for fixing it? Cleaning with isopropyl alcohol? Replacing the socket?

Thanks Mark, I had a feeling you were going to weigh in on this from the other threads I read and I really appreciate it!
dessony
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Location: Indiana, USA

Re: No video output, could use some advice

Post by dessony »

What kind of wave do you see off the Z80's pin number 6 on an oscilloscope screen? Can you post a picture of that wave here, please? Thank you very much.

Cheers,

DesSony
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

grimmware wrote: Fri Feb 19, 2021 4:57 pmWhat is it about the diagnostics I've posted that leads you to that suspicion?
The /HALT signal on a Z80 is an output to indicate that it has encountered a HALT instruction. A pulse on /NMI or if interrupts were enabled a low pulse on /INT would allow it to continue. So either interrupts are not enabled (default at reset is interrupts are disabled) or it’s stuck in a loop where it keeps encountering a HALT instruction. Meanwhile it is executing code, as /MREQ and /RD are pulsing and it’s carrying out it’s memory refresh cycle as /RFSH is pulsing. During a HALT it rereads the same instruction address over and over again.

In any case, the Z80 can’t run the display code from the ROM, so the ULA will just produce a completely white screen without the full proper sync signal. Hence some displays will not even try to show a picture (unlike an older CRT).

If the socket contact is just dirty, IPA may help. But if the contact is broken, bent or has lost it’s springiness, then the socket needs to be replaced. A temporary bodge would be to insert a turned pin socket into the existing socket, the put the ROM into the turned pin socket. This sometimes works because the legs of the turned pin socket are slightly thicker than that of a actual chip.

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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Joined: Thu Feb 04, 2021 1:09 pm

Re: No video output, could use some advice

Post by grimmware »

dessony wrote: Fri Feb 19, 2021 6:19 pm What kind of wave do you see off the Z80's pin number 6 on an oscilloscope screen? Can you post a picture of that wave here, please? Thank you very much.
z80pin6.jpeg
What are you expecting to see here and what does it indicate for you?
1024MAK wrote: Fri Feb 19, 2021 3:27 pm As this happened after you had the ROM chip out of the socket, have you checked the contacts in the socket? Cheap sockets were used and although they normally don’t give problems, this is mainly due to them not being disturbed.
So I've just mapped out all of the ROM pins to where they contact on other chips in the circuit so that I could do a continuity test on all the pins. They all had continuity insofar as my digital multimeter stopped showing the "1" on the left and in most cases dropped to 0 or near 0 in the actual reading on the right, but notably the following two never dropped to 0 and had a "high" number:

* ROM 23 -> Z80 38 (A8)
* ROM 20 (CS) -> ULA 13 (ROMCS)

So apologies for my ignorance about using my own multimeter here, but some googling around suggests that this can imply poor continuity or just a comparatively high resistance, but the ROM 23->Z80 38 connection has a 1K resistor in between just like a whole bunch of other pins going straight to the Z80, and the CS pins have a 680 between them. This sounds like a pair of poor connections to me, I'm just a little confused because I haven't experienced this with a multimeter before.

I was especially suspicious of the CS connection so I did a dual trace on the pins on either end, and you can see that the trace is triggering on the order of mV which is orders of magnitude out from our TTL levels as I understand it!
ROMCS.jpeg
Am I right in thinking this is suspect as all hell?

Again apologies if I'm questioning the bleedingly obvious here - I have forgotten ~90% of the electronics I was taught!
dessony
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Re: No video output, could use some advice

Post by dessony »

That clock wave picture showed that it(Z80) and its clock circuit area are working right. I bet that 1024MAK noticed it also. Congratulations, grimmware!

DesSony
grimmware
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Joined: Thu Feb 04, 2021 1:09 pm

Re: No video output, could use some advice

Post by grimmware »

grimmware wrote: Fri Feb 19, 2021 7:57 pm So apologies for my ignorance about using my own multimeter here, but some googling around suggests that this can imply poor continuity or just a comparatively high resistance, but the ROM 23->Z80 38 connection has a 1K resistor in between just like a whole bunch of other pins going straight to the Z80, and the CS pins have a 680 between them. This sounds like a pair of poor connections to me, I'm just a little confused because I haven't experienced this with a multimeter before.
Ugh no dice, I measured the resistance across those pins where I was suspicious of the continuity test and they're very much within tolerances of the resistors I'm measuring across. I think the ROM socket is fine.
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1024MAK
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Re: No video output, could use some advice

Post by 1024MAK »

Because of the way that the ZX81/TS1000 computers work, there are various resistors wired into the address bus, control bus and data bus lines.

With the ROM, address lines A0’ to A8’ have 1kΩ resistors connected between the ROM/ULA part of the bus and the Z80/RAM/edge-connector.

The /ROMCS signal has a 680Ω resistor between the ULA the ROM/edge-connector. This is so external devices can override the ULA and disable ROM chip if needed.

So where there are resistors in the circuit, test up to the resistor. Then test from the other side of the resistor to the destination. Then test the resistor using a multimeter on the 2000Ω/2kΩ resistance range (or equivalent range).

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...

There is a schematic circuit diagram here. Just ignore the French video circuitry, it makes no difference when fault finding on the digital side.

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.
User avatar
1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: No video output, could use some advice

Post by 1024MAK »

Note: the post above has been edited...

On the subject of testing, what settings was your ‘scope on in that last picture?

You need to ensure that it’s on DC and the vertical channel settings/range is set to say 1V per division. Also if possible position a pointer/cursor where the 0V point is.

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.
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