Zx Spectrum Plus does not respond to keyboard commands

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Chookie
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Zx Spectrum Plus does not respond to keyboard commands

Post by Chookie »

Hi

I recently changed out the original keyboard membrane as I suspected this was the reason for loss of keyboard functionality. To my disappointment I found that the same keys did not function with the new membrane. I began to fault find starting with the keyboard ribbon connectors, after checking for dry joints and finding nothing at fault, I started working back to the resistors and diodes associated with the KB 1 & 2 connectors.

I now have no function from the keyboard at all and the screen remains fixed on the usual copyrighted Sinclair Research screen.

I'm unsure here but could it be a ULA fault at the heart of all this ?

All help and advice is most appreciated.
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Ast A. Moore
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Ast A. Moore »

Just to be absolutely sure it’s not just the membrane, use a piece of wire and manually short a few lines between KB1 and KB2. I.e. stick one end of the wire into any contact of KB1 and try poking all the contacts of KB2 one after another with the other end. You can also use your multimeter to do this by switching it to current measurement. (NOTE: This will effectively put a short between the probes, so be sure to switch it back to voltage/resistance afterward.)
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Chookie
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Chookie »

Hi

Thanks for the reply. I will give that a try and see what happens, failing that it's the ULA then ?
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Ast A. Moore
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Ast A. Moore »

Chookie wrote: Sun Feb 25, 2018 10:38 pm Hi

Thanks for the reply. I will give that a try and see what happens, failing that it's the ULA then ?
Not necessarily. Could be the 74LS157 multiplexer on the address line (although since the machine gets to the Sinclair copyright message, addressing probably works fine. Could be something else. Take the mechanical elements out of the equation first: the membrane, connectors cold solder joints, etc.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
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RMartins
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by RMartins »

Chookie wrote: Sun Feb 25, 2018 4:50 pm ... To my disappointment I found that the same keys did not function with the new membrane. ...
By this reference, I would say, you have fault somewhere between the connectors and the ULA, because apparently some keys do work, right ?

Try trace the keyboard lines, until they reach the data bus.

If you have a digital data analyser, you can check if the keyboard read commands (IO port read), is being correctly generated, and also check if it's being correctly decoded, by probing the data at the right time.

You can have some fault at the IO Req line, for example, or a faulty CPU, or a faulty ULA.
Just start checking, where the signal originates (CPU), and follow your path through the ULA and Keyboard ports/lines.

Can you describe EXACTLY which keys do not work ?
If it's an entire 5 key half row, or a specific "vertical" column, it's easier to identify the faulty line(s).
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Chookie »

Hello again

To date I have replaced the membrane and verified all key operations using a multi meter on the ribbons. I've performed continuity checks from the KB1 & 2 connectors up to the associated diodes and resistors, and carried out checks on all those components.

I did have almost full use of the keyboard commands with the exception of the quote and edit key commands. These keys were also lifeless on the last membrane that was in the computer. Then all of a sudden I lost all keyboard functions and despite shorting KB1 & 2 contacts with a piece of wire I got no response. The screen maintains its Sinclair Research logo and appears to be lifeless.

Regards
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RMartins
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by RMartins »

Did you check if the CPU IO request is reaching the ULA, as I previously suggested ?

Do you have any external interface, (DIVIDE, DIVMMC Retroleum Smart Card, or Interface II compatible Cartridge), that you could use to tests if there is any sound comming from the ULA, without requiring any key input ?

This would be a simple indicator, that the CPU is able to write IO into the ULA.

Do you have A Comcon Joystick interface ?
It allows to input keys externaly, on a 48K machine, using a joystick, once you set the keys you want to map.

Another simple test you can do, is wiggle the ULA in place or remove the ULA and re-seat it again.
Could just be a simple bad contact on the IO Request or READ/WRITE lines reaching the ULA.
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Chookie »

Hi

Sorry been away for a few days.

Unfortunately I don't have any of the peripherals you mentioned only a kempston interface.

I've checked the paths from the CPU IO to the ULA and this is what I've found..

From CPU I checked continuity from IO REQ on pin 20 to the right side of resistor R27 and got nothing but after checking further it made circuit on pin 10 instead. From ULA I checked pin 33 to left side of resistor R27 I got nothing but again after further checks it made circuit on pin 30 instead. Is this the fault ? I checked this a couple of times thinking I had got my pin sequence wrong way round, that said I still think I've not noticed something.

Out of interest I checked continuity on a number of other paths from the ULA to the CPU such as M REQ, INT and CLK etc. and all these are good.I am also noticing that the CPU is starting to get warm although I'm unsure if this normal.


Regards
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Ast A. Moore
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Ast A. Moore »

Hmm. I was going to suggest you check the INT line, but seeing that it checks out, I’m a little puzzled. What’s the logic board issue number?
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Chookie »

Hi there

The board is an issue 3
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Ast A. Moore
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by Ast A. Moore »

Just for kicks and giggles, test TR3 (above the KB1 connector, in the middle of a bunch of resistors and capacitors). Also check C67. Oh, and I’m sure you already did, but just to be sure, check that there’s 5V on R25’s lead that’s opposite the one connected to the TR3’s collector.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
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balford
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Re: Zx Spectrum Plus does not respond to keyboard commands

Post by balford »

This is going to be a ULA problem, unfortunately. What happens is that one of the KBx lines in the ULA gets stuck low, so the ROM code thinks multiple keys are permanently held down, therefore no keypresses can be registered. If you run a diagnostic program that displays the status of port 0xFE then you can actually see the stuck bit associated with the broken line.

Issue 3 machines with 6C001E-6 ULA's are especially prone to such issues as these ULA's are quite fragile - I've got a broken one with exactly the same fault in my spares box.
Fixing ZX Spectrums in the 21st Century
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