White screen and no cursor at all

Discussions about Sinclair ZX80 and ZX81 Hardware
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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Sun May 12, 2019 8:29 pm

1024MAK wrote:
Sat May 11, 2019 2:39 pm
Test the Z80 /MREQ, /RFSH, /RESET, A14, A15 and A6 pins with your logic probe.

There is a pin out on the forum, check the link in my sig below.

Mark
Thank you Mark, I was aware about your post with the pin-out of the chips, in fact I made a PDF document with all the information in just one page and it's very useful.

Concerning to the signals, I get the same result on the three pins you have suggested: HIGH state with no PULSE. At ULA pin 14 I get LOW state with no PULSE and this makes me think that may be the resonator is not working, although I'm not very condifent with my probe. Here's a picture:

Image

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Sun May 12, 2019 8:39 pm

RetroTechie wrote:
Sun May 12, 2019 2:01 pm
Papalapa wrote:
Sat May 11, 2019 1:42 pm
After testing with a digital probe I've found that the CLOCK signal on the pin 14 of the ULA is not present, its state is LOW and no PULSE.
That would indicate a problem with the ceramic resonator, surrounding passive components, or ULA. But have a look at the board: there's some bodge resistor there! Next to the ceramic resonator, above C6. What the hell is it doing there? :o C6 is part of the resonator circuit.

Disconnect a leg of that bodge resistor, then see if resonator circuit starts up. Btw: try and measure clock signal @ different points:
  • Z80 pin 6
  • ULA pin 14
  • ULA pin 35
  • Connection between C6 and ceramic resonator
Note that especially the circuit around the resonator is very sensitive to added loads. So a scope or logic probe touching points there could be enough to throw it 'out of whack'. Less so for ULA pin 14 or after TR2 (Z80 clock input). Also you could check if value (color band markings) of components fitted around there, are what they normally are / should be.
The resistor that you mention is R3 that is marked as "not used" in the electronic schemas, but I found that all the Issue 3 boards that I have seen on Internet have this resistor soldered ¿?. Anyway, I unsoldered the resistor and the problem remains so, I soldered the resistor again.

Image

The measuring results for the three pins were:

Z80 pin 6 = HIGH no PULSE
ULA pin 14 = LOW no PULSE
ULA pin 35 = HIGH no PULSE
Connection between C6 and ceramic resonator = OK

I checked the values of the involved resistors around the resonator and all of them are correct both, the color bands and the measured with the multimeter.

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mrtinb
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Re: White screen and no cursor at all

Post by mrtinb » Sun May 12, 2019 8:41 pm

Maybe it’s just a broken ULA like 99% of all the repairs in this forum. It seems to be the weak spot.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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RetroTechie
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Re: White screen and no cursor at all

Post by RetroTechie » Sun May 12, 2019 10:52 pm

Papalapa wrote:(..) although I'm not very condifent with my probe.
Just poke it into some other apparatus. There's a cpu in everything these days! :lol: (and clock oscillators that go with it)
The resistor that you mention is R3 that is marked as "not used" in the electronic schemas
No it's not. Afaict from the photo, it's soldered to one side of C6. Other side - not sure... ground? Anyway that's different position than R3. Wouldn't make sense anyway with empty position marked "R3" on the board. Out of curiosity, I checked: all 4 issue 3 boards I have, do NOT have this bodge resistor there. Nor R3 installed.

So it's definitely a bodge of some kind... perhaps some problem with a batch of ULA's, and they added this to the oscillator circuit to fix. I'd consider it tied to the ULA that's in there now: if @ some point you decide to replace it, take that bodge resistor out with it.

'Unfortunately' there's only a few passives there. Ceramic resonators hardly ever die in my experience, unless they're physically damaged (read: visible. Cracked etc). So indeed
mrtinb wrote:Maybe it’s just a broken ULA like 99% of all the repairs in this forum. It seems to be the weak spot.
seems most likely @ this point. It's always possible there's more than 1 problem, but regardless: first things first. No clock -> no go. :(

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1024MAK
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Re: White screen and no cursor at all

Post by 1024MAK » Mon May 13, 2019 1:20 am

I’ve got an issue 3 board (possibly produced for the French market given that it has a transistor inverting the video signal to the modulator) that has a resistor as mentioned in the posts above.

Here are some photos of it:
Extra 1k ohm resistor on an issue 3 board
Extra 1k ohm resistor on an issue 3 board
Extra 1k ohm resistor on an issue 3 board
Extra 1k ohm resistor on an issue 3 board
Extra 1k ohm resistor on an issue 3 board
Extra 1k ohm resistor on an issue 3 board
One end (fitted in one of the R3 positions) connects to 0V/GND. The other end connects to C6 and is therefore connected in parallel with the 6.5MHz ceramic resonator. The value of the resistor is 1k ohm. On this board (which going on the solder flux, looks to have had a different ULA fitted, and one of the 2114 SRAM chips changed), it looks like a either a factory fitted modification or a service centre modification...

Mark

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Mon May 13, 2019 9:03 am

mrtinb wrote:
Sun May 12, 2019 8:41 pm
Maybe it’s just a broken ULA like 99% of all the repairs in this forum. It seems to be the weak spot.
Yes it is possible but I want to explore first the "easy solutions" because the standard ULA is not easy to find and it seems that Andy has not more units ready at the moment. On the other hand mine is soldered to the PCB (without socket) and that makes a little bit difficult the replace operation. I cross my fingers and hope that the damaged part will be the resonator only :D

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Mon May 13, 2019 9:14 am

1024MAK wrote:
Mon May 13, 2019 1:20 am
I’ve got an issue 3 board (possibly produced for the French market given that it has a transistor inverting the video signal to the modulator) that has a resistor as mentioned in the posts above.
Mark
Just a silly question: on your pictures I see that the resonator is soldered in the opposite direction than mine (the printed letters are faced to the "extra" resistor while mines are faced to the TR2. I was thinking that resonators have no polarity (I'm not a technicien, it's just a hobby for me). I'm wrong? Also, why it has three pins while the electonic diagram shown only two? Is the central pin not connected?

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1024MAK
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Re: White screen and no cursor at all

Post by 1024MAK » Mon May 13, 2019 3:42 pm

The ceramic resonators are not polarity sensitive as such. However, there may be differences between the two sections of three terminal devices. It’s real intended purpose is as part of a filter circuit in television sets, hence it having three terminals. In the ZX81 circuit, one of the outside terminals is not used (the middle pin is ground). I have just checked four of mine, two issue 1 boards and two issue 3 boards. All have the terminal on the right had side (when looking at the markings) as the unused terminal. This is also the side that is marked with a black dot.

Photos of an issue 3 on Wikipedia:
Image
Image

Mark

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Tue May 28, 2019 3:29 pm

Murphy's law: After the resonator has been replaced by a new one the problem remains.

A new working Zeddy will come soon home and maybe it can help his brother by lending the ULA CPU and ROM for testing purposes. I'll prepare the soler iron because my Issue 3 board has all these parts soldered to the PCB (without socket).

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Sun Jun 02, 2019 12:09 pm

Well, as I was blocked at this point a colleague told me that I can send him the ZX81 and he will check what can be happened.

His diagnostic was that the ULA is defective. He swapped a good-know working ULA with mine and once replaced the ZX81 started to work while the other ZX81 failed to startup with my ULA so, the diagnostic is clear.

Now I'm trying to find an ULA but it seems to be impossible, even I think that @Andy stopped to build his improved ULA and unfortunatelly my ZX81 stills not working.

Anyway, as now I know which is the damaged part I'll try to find it.

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