ZX Spectrum 48K issues

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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Sanxion
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ZX Spectrum 48K issues

Post by Sanxion »

Hi all

This is my first post...

I recently acquired what looks like a pristine 48K Spectrum. I managed to acquire a picture on my LCD TV, I do however have two problems:

1. The constant hissing from the TV - is that normal? I don't recall this problem when I last used a Spectrum some 30 years ago... :D

2. None of the keys work. Could the entire membrane have failed? What is the best way to check this?

Thanks

:D
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Sanxion wrote: Sat Feb 02, 2019 7:59 am 1. The constant hissing from the TV - is that normal?
How do you connect the Speccy to your TV? RF or a composite mod?
Sanxion wrote: Sat Feb 02, 2019 7:59 am 2. None of the keys work. Could the entire membrane have failed? What is the best way to check this?
Unplug the membrane, grab a piece of wire (or a multimeter set to measure current) and short individual conductors between the two keyboard connectors. For example, stick one end of the wire into Pin 1 of KB2 and successively stick the other end into each of the pins of KB1. You should be getting keypresses.

If you don’t, there’s definitely a problem. This could be something minor and mechanical—like a lifted pad, broken trace, or cold solder joint, to something more serious like the keyboard matrix diodes or even the ULA itself.


What’s the board revision?
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

Thanks for the reaponse.

I am using the rf modulator.

I tried connecting pin 1 of KB2 to the different pins in KB1 and received nothing on screen... :|

It is an Issue 3.

What should I do next?
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Just to eliminate the mechanical aspect, try the same method, but instead of the pins of KB2 use the anodes of diodes D1–82 located right above the KB2 connector (the anodes are on the left, if the board is viewed with the speaker at the bottom right), and one of them to either of the following pins of the ULA: 19, 20, 23, 24, and 26. The ULA pins are counted counterclockwise starting at the top left, which will be Pin 1. Pins 19 and 20, for example, are the bottom two pins on the left side. (The ULA is the large IC sitting next to KB1.) Try it with each of the eight diodes.

If the above produces no effect, you can try to carefully reseat the ULA in its socket (assuming it’s socketed) with the power off and then retest. If that still produces no effect, the ULA will have to be replaced.
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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1024MAK
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Re: ZX Spectrum 48K issues

Post by 1024MAK »

Sanxion wrote: Sat Feb 02, 2019 7:59 am Hi all
Hello :D
Sanxion wrote: Sat Feb 02, 2019 7:59 amThe constant hissing from the TV - is that normal? I don't recall this problem when I last used a Spectrum some 30 years ago... :D
When the RF/UHF TV aerial output is used, you need to turn the TV volume to minimum, as the ZX Spectrum does not transmit any sound signal via this connection. So a TV will produce white noise which is the hissing that you hear.

This model of ZX Spectrum produces it’s audio via a built in ‘loud’ speaker.

Mark
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Ast A. Moore wrote: Sat Feb 02, 2019 3:34 pm but instead of the pins of KB2 use the anodes of diodes D1–82 located right above the KB2 connector
Whoopsie, that’s a typo. I meant D1–D8, obviously. :D
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

I have a response when connecting between the ULA and the anodes! :D

So now do we conclude that the connector KB2 is at fault?
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Sanxion wrote: Sun Feb 03, 2019 8:38 am I have a response when connecting between the ULA and the anodes! :D

So now do we conclude that the connector KB2 is at fault?
Or KB1. KB1 pins go directly to the ULA, so you can try connecting the anodes of the diodes to individual points on KB1 instead of the ULA.

A multimeter set to continuity mode would help you track down the issue quicker (and without even turning the computer on), but yes, it’s safe to assume that the problem is purely mechanical—either there are a ton of cold solder joints on the connectors, or the connector contacts are dirty, or you’re not poking them hard/deep enough. ;)

In any event, looks like you’ve successfully isolated the issues (well, excluded any major fault, anyway) and fixing it should be fairly trivial. Which is great!
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

I also receive a response when connecting KB1 to the anodes except for 1 pin. However, there is no response when connecting the pins of KB2 to the anodes. Could that be the problem and is that where I should check for continuity? I am assuming KB2 connects to the annodes?
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

That’s right, KB1 connects directly to the ULA, while KB2 to the anodes of diodes D1–D8. Keep in mind that certain combinations will not produce any output directly (each SHIFT key pressed individually, for example).

The keyboard matrix on the Spectrum is fairly simple. Each row of ten keys is divided into two half rows connected to eight address lines via KB2. Each five keys in every half row is connected to the five keyboard inputs of the ULA via KB1. The rows are mirrored, that is the “5” key sits one the same line as the “6” key, “4” and “7” share a line, “3” and “8”, etc.

I suggest you reflow the solder joints of both keyboard connectors just to be sure, but the problem is obviously lies primarily with KB2.
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

Ok...I am confused now.

I have tried a continuity test between KB2 and the anodes starting from the left pin of KB2 to D1 to the right most pin of KB2 to D8 and I hear a beep for each one.

That suggest KB2 is fine does it not?
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

It does. Inspect the bottom of the board visually. Make sure ther are no cold solder joints.
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

I think I have identified the problem. One of the contacts on the KB2 keyboard membrane is torn and several are torn on KB1.

It appears I need a new membrane.

What would you recommend?
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Can’t think of anything off the top of my head. I’ll look later tonight when I get back home, but there are plenty options available if you look for them online.

The biggest issue will be removing the faceplate. I assume yours is the rubber-key model, not the Plus, and the aluminum faceplate on it is stuck with pretty hefty adhesive. It’s not easy to remove the faceplate without bending it.

By the way, if the membane tore close to the ends of the flex cables, it’s possible to trim the ends and expose the fresh portion of the conductive paint area.
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.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

Thank-you for your help.

I would rather buy a higher quality membrane in order that the problem doesn't occur again.
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1024MAK
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Re: ZX Spectrum 48K issues

Post by 1024MAK »

New membranes for rubber key 16K and 48K ZX Spectrums are normally available from RWAP on SellMyRetro. But he does not have any listed at the moment. You may want to ask him about availability.

There is another seller who sells membranes, link. I have never used one of these, so don’t know what the quality is.

Should you need them, Set of Molex Keyboard Membrane Connectors for Sinclair ZX Spectrum

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
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1024MAK
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Re: ZX Spectrum 48K issues

Post by 1024MAK »

There a at least two designs of faceplate. One is only stuck on. The other has some ‘clips’ as well.

RWAP sends out an instruction leaflet with his replacement membranes.

Various people have different techniques. They include using a hair dryer to heat the metal plate up to soften the glue.
Or immerse on hot water...

Use a wide metal blade, such as a blunt knife to gently lift the metal plate, slowly, so the glue can separate.

A solvent can help with the glue.

Once the plate is off, you need to remove all the old glue.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :dance
Looking forward to summer later in the year.
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

I managed to remove it quite easily with a plastic spatula and haven't damaged the fascia at all :D

I will no await the arrival of the new membrane!
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Re: ZX Spectrum 48K issues

Post by Mad Fritz »

Sanxion
Drutt
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Re: ZX Spectrum 48K issues

Post by Sanxion »

I am delighted to say that replacing the membrane has resolved my keyboard problem!

:lol:
Sanxion
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Re: ZX Spectrum 48K issues

Post by Sanxion »

This looks good...and had I not already ordered my membrane, I would have gone for this option!

Thanks for posting it.
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Ast A. Moore
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Re: ZX Spectrum 48K issues

Post by Ast A. Moore »

Sanxion wrote: Fri Feb 08, 2019 5:51 am I am delighted to say that replacing the membrane has resolved my keyboard problem!

:lol:
Excellent! May your Speccy live a long and happy life.
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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