ZX81 White display

Discussions about Sinclair ZX80 and ZX81 Hardware
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siggi
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Re: ZX81 White display

Post by siggi »

inaki wrote:I have checked the NMI.
It is alive for a couple of seconds and then vanishes.
The edge an frequency of the NMI pulse matches the start video signal. Is this right? The pulse is 5us wide.
I can figure out the NMI should last forever, unless fast mode is used, right ?
No, during SLOW mode the NMI occurs only "above" and "below" the visible part of the display. While the visible part of the display is output by the Z80/ULA, the NMI-generator is disabled.

If only the internal ram is faulty (but not too faulty), connecting an external ram pack may help.

HTH Siggi
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inaki
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Re: ZX81 White display

Post by inaki »

Tried with 16K pack. No changes.
gozzo
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Re: ZX81 White display

Post by gozzo »

Hmm..surely there can't be much more that can go wrong with it? Have you checked the RESET line, as it is fed from a high value resistor (220k) with a capacitor from that point to ground, maybe the capacitor is playing up when there is any appreciable voltage across it (they can test 'OK' on an ohmmeter but suddenly break down when more volts are across them - have had that happen a few times with vintage radios) and then check the tracks for breaks and shorts,and then only other thing I can think of is replace EVERYTHING(not the modulator!), but resistor and diode failure is very unlikely! I assume you have checked the 5v output from the regulator?
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PokeMon
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Re: ZX81 White display

Post by PokeMon »

inaki wrote:I have taken a look at the thread where pokemon explains the shape I should get on A15.
I get a similar pattern to the one in the pictures for a couple of seconds. Then the signals starts changing. Some seconds later the signal is flat.
So it seems clear to me that some component is heating up and then fails.
Strange - but you could maybe test it when cooling some ICs down.
As cooling spray is quite expensive over the time you can buy a cooler for 40 pin ICs, put it in the freazer for a while to cool it to -18° and then put in on a either CPU or ULA to see if it's running longer. The signal on A15 shows video execution, if this is present it is working in general but due to heating some components its moving. But if the ULA is okay you should have at least a picture for a few seconds (K cursor). Print P after startup for PRINT (which is better to see) and put contrast to minimum and brightness to maximum.

In my opinion the missing black pixel is a ULA issue. I have one which has the same fault and shows only white level and sync level. As this is a 3-state signal (or 3 level signal) there is maybe one internal driver damaged which moves the signal from white level to black level (that is my guess, never opened the ULA). :mrgreen:

But you should first fix the other issue with the A15 signal loose after a while without doing nothing.
Could be a RAM as well. If you want to increase internal RAM you should maybe do this first as SRAM parts are really cheap.
inaki
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Re: ZX81 White display

Post by inaki »

Thanks everyone to point out possible checkings. I will try to check it all.

However, having replaced every active component (every chip and the two transistors), is it not strange I get exactly the same behavior than using the original components ?
I mean, would it not point to some passive component or perhaps some broken/short track ? Of course it is possible the two ULAs I am trying are both wrong. Both sets of chips I have used come from unopened ZX81 kits. Perhaps both kits are wrong?


Said that, If I had to check the most basic ULA functions, which would be the very first point(s) to check ? I always hear about the video stuff, but is there anything else worth checking ?
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PokeMon
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Re: ZX81 White display

Post by PokeMon »

You could inspect the solderside of the board for any bad soldering.
If chips are socketed this is also a source of failure as often there have been used the very cheap sockets (no precision contacts).
If it is the same behaviour that you get the A15 signal as described which is lost after a few seconds you can check any suspicious activities on NMI, WAIT, HALT, BUSRQ, RESET, CLOCK. And not forget to check the power supply (+5V) to be good and steady.
rp55
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Re: ZX81 White display

Post by rp55 »

Perhaps check d6's 10k resistor on the data bus 5v supply. It could be that data bus works from ram/rom (bios rom load) but not from the ULA. I think that bit causes more than anything else some display.

I would assume if you're getting white screen you are getting your synch pulses (as you said you scoped). Just not getting the chars decoded off the data bus by the ULA. Only reason I can think of would be d6. And just guessing but this "behavior unknown" of d6 (or some other problem) could put the unit off into the deep end such that you never see any more pulse/vid output from ULA, i.e. not overheating any chips.
makary23
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Re: ZX81 White display

Post by makary23 »

Is the problem solved? I have the similar after about 30-60 seconds I have a white screen.
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