White screen and no cursor at all

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

Post by Papalapa » Wed May 01, 2019 7:17 pm

Hello,

Recently I purchased a damaged ZX81 and now I'm trying to repair it. The main problem (and the only one I hope) is that when the ZX81 is turned on I get a white screen but without cursor. The basic checks that I have done at the moment are:

1) PSU: Voltage is correct, around 14 volts without charge that reduces to 11 volts once connected to the ZX81 (400 mA consumption).

2) Chips +5v voltage: All of them have +5 volts in their respective pins so, internal voltages seems to be correct.

3) RAM: I connected the external Memotech 16K RAMpack and the problem remains, then I guess that the internal RAM is correct (as you know when the external RAM is connected, the internal one becomes disabled).

This is the unmodified faulty board (flat cable from the keyboard is broken but I already purchased a new keyboard):

Image

As I was initally thinking that could be a video modulator problem I installed a MOD kit that provides the video composite output with the back porch (although ULA is 2C210E and with a transitor+resistor is enough) and a 32K RAM together in the same board.

Here's the MOD board:

Image

I also replaced the original electrolitic caps and replaced the 7805 by a TracoPower voltage regulator. Here's the board after the upgrade:

Image

Unfortunatelly one more time after this upgrade the problem is exactly the same, white screen with no cursor. I use a CVBS to VGA converter to connect the composite output to the PC monitor but I also used the composite input of a VHS/DVD recorder connected to the TV and the result is the same. By adjusting the potentiometers on the MOD kit the image becomes darker or clearer so, I think that video image is correctly generated but the ZX81 do not starts to run.

Actually I'm waiting to receive a pair of ZTX313 transistors from UK because may be the CPU doesn't receives the CLOCK signal from the ULA but I have no way to test this (no oscilloscope at home). This is the easiest way before to start to thinking in CPU, ROM or ULA replacement. In this last case most probably it is not worth it to continue with the repair because I think that these parts are not easy to find.

Meanwhile I wait for the transistors, is there any other test that can I do to try to identify the problem?

Thanks a lot ;)

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mrtinb
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Re: White screen and no cursor at all

Post by mrtinb » Wed May 01, 2019 7:28 pm

A digital probe will help you find the problem if you don’t have an oscilloscope. Or even a voltmeter can help.

Andy from this forum has created a cheap ULA replacement.

https://www.sellmyretro.com/offer/details/32319

Hope he makes more soon.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Wed May 01, 2019 7:52 pm

mrtinb wrote:
Wed May 01, 2019 7:28 pm
A digital probe will help you find the problem if you don’t have an oscilloscope. Or even a voltmeter can help.

Andy from this forum has created a cheap ULA replacement.

https://www.sellmyretro.com/offer/details/32319

Hope he makes more soon.
Thank you for the information, it looks great!

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1024MAK
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Re: White screen and no cursor at all

Post by 1024MAK » Wed May 01, 2019 8:26 pm

It’s rare for the ZTX313 transistors to fail. They are fairly reliable.

You have eliminated the RAM. Assuming that the transistors are okay and that there is no track damage, or faulty resistors, capacitors, that leaves the following:
  • Faulty ULA
  • Faulty Z80 CPU
  • Faulty ROM
The mask ROM chips are also normally reliable. The ULA is often more likely to be faulty than the Z80. But sometimes the Z80 does fail. Some machines are actually found to be internally working when they only produce a white screen. The problem being the three state video output pin of the ULA having partly failed, such that it can only output one or two voltage levels instead of the three that it should produce [high = white level, medium = black video level, low = sync level].

It should be noted that as all the chips are soldered on your board, it is worthwhile carrying out some tests. An oscilloscope, logic analyser or logic probe are very useful for this. Some basic tests can be done with a decent digital multimeter.

But first, please ensure that all of the old membrane tails are removed from the connectors on the PCB.

If you don’t have a logic probe or anything better, it’s possible to knock up a simple logic probe on a piece of stripboard or matrix board.

Mark

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XavSnap
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Re: White screen and no cursor at all

Post by XavSnap » Thu May 02, 2019 2:16 am

You have eliminated the RAM
???
No, both straps are open (LK1/2)… and we had to put a 16k ram pack to test the ram >>> rom boot.

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Thu May 02, 2019 8:33 am

XavSnap wrote:
Thu May 02, 2019 2:16 am
You have eliminated the RAM
???
No, both straps are open (LK1/2)… and we had to put a 16k ram pack to test the ram >>> rom boot.
I soldered a jumper to connect LK2 but as it is under the green circuit in the picture is not possible to see it, but it is.

I'll try to find a logic probe until the transistors come and in this way I'll be able to do some tests on the chips..

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » Wed May 08, 2019 9:18 pm

Well, I already replaced TR1 and TR2 unsuccesfully. They were replaced by exactly the same transistor model but unfortunatelly the problem remains: white screen and no "K". This week-end I'll start to investigate with the logic probe if the ULA or Z80 are damaged. Bad luck :(

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Papalapa
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Re: White screen and no cursor at all

Post by Papalapa » 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. I don't know if these cheap test probes are reliables but I don't have oscilloscope.

At the moment I ordered the 6.5MHz crystal because it's very cheap and easy to replace. I cross my fingers that will be enough and I don't need to replace the ULA...

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1024MAK
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Re: White screen and no cursor at all

Post by 1024MAK » 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

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RetroTechie
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Re: White screen and no cursor at all

Post by RetroTechie » 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.

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