ZX81 strange keyboard problem?

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hyttju
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ZX81 strange keyboard problem?

Post by hyttju » Wed Jul 19, 2017 9:59 pm

Hi to everyone!

I have a zx81 computer. I tried it and it did not responded to my typing so well...about half of the letters and numbers came correct on screen when i typed.

Ok...i thought it was a membrane error and bought a new one. Installed it,but the problem is the same!

Example. I push the J key and it should type LOAD,but instead it types LOA5

About half of the keyboard types correct letters and numbers and the other half does not.the same number can come from 3 buttons example.

What can i do? What can cause this other than membrane?

Really want to fix this little computer.

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Wed Jul 19, 2017 10:03 pm

You would need to map out which keys are working and which not. This will aid in diagnosis. I hate to tell you that the ULA plays a large part in keyboard reading too.
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

hyttju
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Re: ZX81 strange keyboard problem?

Post by hyttju » Wed Jul 19, 2017 10:31 pm

I was also thinking that the ULA has somehow gone bad

No replacements to be found anywhere?

If not,then it was end for that computer. It gives a normal white picture with the letter K at the bottom.

Only that i can not write anything good on the screen.

So many letters writes wrong letters or number or some symbol.

Thank you anyway for answer!

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Paul
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Re: ZX81 strange keyboard problem?

Post by Paul » Wed Jul 19, 2017 10:33 pm

A friend of mine (Jens) had the same problem and we got it fixed.
I need to ask him how we fixed it.
There is hope!
kind regards Paul
In theory, there is no difference between theory and practice. But, in practice, there is.

gozzo
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Re: ZX81 strange keyboard problem?

Post by gozzo » Wed Jul 19, 2017 10:35 pm

hmm, unliklely to be membrane as it sounds like its responding correctly but not 'printing' correct characters, 'could' certainly be duff ula (aarrgghh), but maybe strange rom or ram fault...

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Wed Jul 19, 2017 10:44 pm

I would say that it's not keyboard fault, looks like somehow the ula is not giving the correct address at refresh, i would check the standing up resistors near the keyboard connector all should be 1k and none touching each other, of the do.it could be shorting out some of the alternate address lines used by the rom

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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sirmorris
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Re: ZX81 strange keyboard problem?

Post by sirmorris » Wed Jul 19, 2017 11:15 pm

If the error is isolated between the ULA and keyboard then absolutely yes - this could be an issue. If there is a problem with address or data lines between the CPU and memory then the machine won't boot because it won't be able to read the code from the ROM and you won't have a display as RAM will be inaccessible.

I had a machine with pretty much exactly this fault a couple of weeks ago. A switched ULA fixed it :(
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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1024MAK
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Re: ZX81 strange keyboard problem?

Post by 1024MAK » Wed Jul 19, 2017 11:27 pm

Okay. Strange fault. One thing that should be said before we proceed.

You said:
hyttju wrote:
Wed Jul 19, 2017 9:59 pm
Example. I push the J key and it should type LOAD,but instead it types LOA5
The ZX81 internally stores "keywords" (command words and function words) as a special number. When the code that is in the ROM finds one of these code, it expands the code to the correct characters and then prints these on the screen.
If you try to type in LOA5, the system would not accept it as a valid command.

So, as far as I can see, the keyboard is not the problem. Instead, the problem is the ROM, the ULA, the Z80 CPU or the circuitry that links them. As it is these that do the generation of the display. It is unlikely that the cause is the Z80 CPU, as most failures of this chip would prevent even getting to the "K" start screen.

So to test this, can you enter a simple BASIC program, then RUN it.
So for example:

10 PRINT "Hello World"
20 GOTO 10

RUN

Ignore any incorrectly displayed characters. If this works okay, then it confirms my thinking.

It would also indicate that the connections between the ROM, the ULA and the Z80 CPU are all okay.

So that then leaves a very strange ROM chip fault, or a ULA chip fault.

Mark

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Andy Rea
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Re: ZX81 strange keyboard problem?

Post by Andy Rea » Wed Jul 19, 2017 11:44 pm

The resistors i suggested checking have nothing to do with the keyboard, they are the 'fence' between cpu address lines and ula alternate lines...... doh.... even if it was shorted on the ula side that would almost certainly stop the correct cpu address getting through as well..

I am a fool.... sorry guys.... so that leaves....

Faulty ula, or if your lucky maybe just a bent pin or dicky comnection if the ula is socketed....
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

hyttju
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Re: ZX81 strange keyboard problem?

Post by hyttju » Thu Jul 20, 2017 9:38 am

I can not change line so it is impossible to run commands.


Here is a picture of what came up when i wrote

10 print "hello world"
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