Zx Spectrum +2 128K black/grey screen. First use in 25 years!

surferneil
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Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Good morning. I was a keen Spectrum gamer back in the 1980s, but I am afraid that my Spectrum +2 has been sitting in the loft, boxed up since the early 1990s!

I was just wondering if there are any simple things that I could do. This is what I get so far:
  • Tuned an old television into the signal and I get a black/grey screen.
    The power light is on, the tape player works - if I play a tape you get the wonderful sound of data!
    Pressing the reset button does very little.
    When I removed the power cable, I noticed that as you push it in partially, you sometimes see a black sharp rectangle with a white background before the cable is seated properly.
    I have removed the back carefully and blown out the dust etc. although it was pretty clean.
I'm guessing that I might need someone more technical than me to look at it, but I just thought that I would ask you knowledgeable folk first.

My lad is very excited about seeing this work again and it would be fun to try to do this for him.


Thanks very much.

Neil.
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1024MAK
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by 1024MAK »

First off, we need to work out which version of the +2 you have.

What colour is the case, black or dark grey?
Which PSU does it use, a 9V with a barrel plug that goes in the back of the computer, or a +5V/+12V/-12V PSU that uses a DIN plug to go in the back of the computer?

The outer rectangle is referred to as the border. Does it always end up white (or light grey depending on how you have set up your TV contrast and brightness)?

The middle (main) part of the display is the text and graphics screen. Is this a completely black rectangle, or are there any faint vertical red lines? You will need to have the TV on and ready before turning on power to the computer in order to see the red lines, as they may only be present for a few seconds.

Does the reset switch have no effect at all on the picture?

Mark
ZX81 Variations
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surferneil
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Many thanks for replying Mark.

The case is definitely dark grey or black.

The PSU is the big one with the DIN plug on the back.

When it is plugged in the screen is just completely dark grey. No border at all.

When I press the RESET the screen goes slightly blacker and then goes back to dark grey.

The only way I have managed to get the screen with the border is if the DIN plug is pressed in about half way. I quickly snapped the screen shots, but not sure how to attach to post!

Thanks again Neil
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1024MAK
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by 1024MAK »

surferneil wrote: Mon Aug 30, 2021 2:03 pm The PSU is the big one with the DIN plug on the back.
That makes it either a +2A or +2B, very likely with either an Z70833 ISSUE 1 board or an Z70833 ISSUE 2 board.
surferneil wrote: Mon Aug 30, 2021 2:03 pm When it is plugged in the screen is just completely dark grey. No border at all.

When I press the RESET the screen goes slightly blacker and then goes back to dark grey.

The only way I have managed to get the screen with the border is if the DIN plug is pressed in about half way. I quickly snapped the screen shots, but not sure how to attach to post!
With this model/design, you should never insert or remove the DIN power plug while the PSU is powered up. The PSU can be damaged by doing this.

It is possible that one or two of the output supplies from the PSU have failed. The +5V is present, as that’s the supply used by most of the circuitry including the tape deck.

However, it looks like the +12V supply is faulty, this feeds the video circuitry.

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Do you own or have access to a digital multimeter?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
surferneil
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Thank you Mark.
I will not plug the DIN in when it is on again!

I do have a multimeter.

I've actually got a Physics degree but my electronics knowledge is rubbish !!!!



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1024MAK
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by 1024MAK »

You may find this web page helpful ;-)

As it says, be VERY careful when testing not to short anything out!

Myself, I use a spare PCB mounting DIN socket when testing, but I very much doubt that most people will have one of these lying around.

Alternatively, as long as you are careful with the keyboard ribbons (and do be careful, as membranes for these machines are not readily available as spares), you can test the voltages on the main PCB of the computer.

All voltages are DC and all are referenced to the common 0V/GND (ground). Note, this is not the same as mains earth/ground.
An easy place to put the negative (black) multimeter lead/probe to connect to a 0V point, is the outer of the TV/UHF output on the modulator. Use this as your 0V/GND reference point.

A pin-out of a Z80A is linked to in my sig, repeated here: ZX81 Chip Pin-outs. All DIL/DIP chips have their pins ordered in a similar way. As you will see for the other chips in that topic.

The Z80A is IC2. Set your multimeter to the 20V DC voltage range (or equivalent). Measure the +5V supply on pin 11. It should be between 4.75V and 5.25V. If it is under, or over this range of values, then the PSU is faulty, or there is a problem with the cable or one of the connectors.

The colour encoder chip is IC9 (the chip will be labelled TEA2000). Measure the +12V supply on pin 11. It should be between 10.8V and 13.2V. If it is under, or over this range of values, then the PSU is faulty, or there is a problem with the cable or one of the connectors.

Be very careful when testing voltages on the pins of the chips, it’s very easy to slip the probe between two pins, and this could cause damage to that chip or others that are connected to it.

Please report your findings back here if you want more help.

Incidentally, if you intend to use this machine a lot, we recommend using a RGB SCART lead like this one . This is a good quality type. This seller also has their own website: https://www.retrocomputershack.com/. This is my preferred option. But as you can see, they are having a break at the moment.

An alternative, that is just as good, is this one. Don’t let the name put you off.

I’ve bought from both these sellers in the past, and am happy with the products.

However, ZX Spectrum computers were never designed to work with modern TVs, and the leads can’t overcome the lack of a proper +12V supply from the ZX Spectrum +2A/+2B computer. So TVs that require working automatic function switching inputs on their SCART input sockets may not work. Unfortunately, with so many different models of TV that are in use, it’s impossible to known in advance if a particular model will work okay.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
surferneil
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Brilliant Mark.
The power supply appears to be almost perfect with its voltages.
I will see if I can carry out some of those other tests that you suggest later.
Neils Power Supply.jpg
Cheers Neil.
surferneil
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Hi Mark,

It is 11.89V on the colour encoder chip so this seems good.

Pin 11 on the CPU flickered around a bit, but stayed within the correct ranges as well, around 5V.

Are there any other tests that I could do?

Thanks,

Neil.
surferneil
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by surferneil »

Definitely Issue 1 Mark.
Here are some closeups:
1.jpg
2.jpg
3.jpg
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1024MAK
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Re: Zx Spectrum +2 128K black/grey screen. First use in 25 years!

Post by 1024MAK »

Okay, so those power supply voltages are okay.

But I’m concerned where you say that the voltage on the Z80A pin 11 flickered around a bit. The +5V should be nice and steady. Maybe with the least significant digit of the multimeter changing, but none of the others. Did you have a good connection?

If this is not a power supply problem, then it’s a fault on the main board.

Do you have an audio lead for it, the type used so that you can feed sound to an external audio amplifier or audio system?
Or do you have a audio lead with a stereo 3.5mm plug at one end, again suitable to feed to an external audio amplifier or audio system?

I’m assuming that you don’t have a logic probe?
Can you solder?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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