I have a broken zx spectrum+

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daniel_dlds
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Joined: Tue Dec 10, 2019 12:17 am
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I have a broken zx spectrum+

Post by daniel_dlds »

Hello,

Its a 4s issue, made by Samsung.
I want to fix it. The machine has the correct power brick voltage, 15 V.
The voltage regulator seems to be ok also. I tested the the IC6 lower memory chip and it has the correct voltages.
The machine powers on and shows a screen with white border and black and white thick vertical bars.
With power disconnected I measured the voltage drop on TR6 and it shows 0.6 V. TR3 does not have this drop it just reads 0Volts.

Can anyone help ?
Thanks in advance
Regards

McKlaud
Posts: 277
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: I have a broken zx spectrum+

Post by McKlaud »

Hi,

Let starts for the begining.
The machine has the correct power brick voltage, 15 V.
This is not correct. No load voltage of the ZX Spectrum PSU should be not higher than 14.5V, and full load voltage should not be lower than 7.5V. The proper spec for Plus PSU is the unregulated power supply rated 9V@1.4A.
Last edited by McKlaud on Thu Dec 19, 2019 8:50 am, edited 1 time in total.
Claudius
----------

Lardo Boffin
Posts: 1185
Joined: Sat Nov 26, 2016 2:42 am

Re: I have a broken zx spectrum+

Post by Lardo Boffin »

A good modern PSU replacement for the plus is one of these:

https://www.thefuturewas8bit.com/shop/s ... irpsu.html
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Spectrum +3

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1024MAK
Posts: 2879
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: I have a broken zx spectrum+

Post by 1024MAK »

Hello Daniel

Welcome to our forum.

It helps us to help you if you can tell us which country you live in.
McKlaud wrote:
Thu Dec 19, 2019 8:07 am
Hi,

Let starts for the begining.
The machine has the correct power brick voltage, 15 V.
This is not correct. No load voltage of the ZX Spectrum PSU should be not higher than 14.5V, and full load voltage should not be lower than 7.5V. The proper spec for Plus PSU is the unregulated power supply rated 9V@1.4A.
That’s not the whole story. It’s possible if the mains voltage is unusually high, for the unloaded Sinclair PSU output voltage to be up to 16V. Or if a 220V PSU is being used on our U.K. mains voltage (which is often actually around 240V to 250V).

It’s also possible that he has ended up with a 1.2A PSU, the smaller transformer in this has a worse transformer regulation figure so it’s unloaded voltage is often up to 16V.

The more important figure is what are the voltages on the 7805 voltage regulator. Ignore the center leg/pin. Put your negative (black) probe on the metal tab (of the regulator that is bolted to the metal heatsink). Then put the positive (red) probe on each of the outer legs/pins. The left hand side should be between 8V and 12V. The right hand side should be between 4.85V and 5.15V. Be VERY careful that the probe does not slip and short anything out.

When you say that the voltages on IC6 are correct. Can you post up which pins you tested and what the actual voltages are please.

When you power up your Spectrum, does it always end up with a white border. Does the central area always display a similar pattern/screen?

What happens if you push and hold the reset button, then let go after three seconds?

Apart from a multimeter, do you have any test equipment? E.g. logic probe, logic analyser, oscilloscope...
Do you have any interfaces / expansions for you ZX Spectrum?

Mark

daniel_dlds
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Joined: Tue Dec 10, 2019 12:17 am
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Re: I have a broken zx spectrum+

Post by daniel_dlds »

Hello,

I live in Portugal where power supply is 220V.
The power brick has 14.67 V when putting the multimeter black probe inside the jack and I touch the outer cylinder with the red probe. Unloaded then. On the voltage regulator :
- left side is 11.8 V
- right side is 4.98 V

IC6
- lower left pin : 11,8V
- lower right pin : 4.97V
- upper left pin : -4.99V
- upper right pin : 0.01V

The pattern is always the same in the center when I power it up. The border I have seen is white or green. It also emits a sound similar to a tape loading but only for 4 to 5 seconds, then stops.
As to additional tools, I have a scope.

Thanks,
Regards

daniel_dlds
Posts: 4
Joined: Tue Dec 10, 2019 12:17 am
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Re: I have a broken zx spectrum+

Post by daniel_dlds »

Hello again,

I forgot to answer the question about the reset button. If I do that, when I release the button, the tape loading sound is heard again and the border changed to black. I repeated the process and the sound sounded again and the border turned again to green (as it was when I powered the zx).

The pattern in the center of the screen remains the same : thick vertical black and white bars.

Thanks again
Regards

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: I have a broken zx spectrum+

Post by 1024MAK »

Okay, thanks for the voltages. The unloaded output from the PSU is slightly higher than typical. But it’s still okay. All the other voltages are all well inside specification, so are all okay.

The Z80 CPU is for some reason not executing the correct code from the ROM. The border should always end up white and the machine should not produce any sound during start up.

At this stage, the following are suspect: ROM chip, Z80 CPU chip, but it could also be one of the RAM chips messing up the data bus.

Which chips are in sockets?

What type of ROM chip do you have? Is the top of it a flat plain surface with white screen printing (a mask ROM chip) or is it an EPROM with a window covered with a label / sticker / insulation tape?

Assuming that your oscilloscope has a bandwidth of 20MHz or greater, can you see what signals you get on the points detailed below.
There is a pin-out of a Z80 CPU in this post

On the Z80 CPU, test the following pins with your ‘scope:
Pin 14 D0
Pin 15 D1
Pin 12 D2
Pin 8 D3
Pin 7 D4
Pin 9 D5
Pin 10 D6
Pin 13 D7

Because we don’t know what code the Z80 CPU is running, don’t worry about the actual waveform pattern. What I’m interested in is the level. Is each pin continuously low (below 0.8V), or is it continuously high (above 2V), or is it toggling between logic high and logic low. Does each waveform look a bit like a square wave with fast edges as it changes logic levels, or is it just a mess?

Post up photos of any that you are concerned about.

Mark

daniel_dlds
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Joined: Tue Dec 10, 2019 12:17 am
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Re: I have a broken zx spectrum+

Post by daniel_dlds »

Hello,

All chips are soldered to the board, none is socketed. The ROM chip is not a EEPROM (it has no window).
I measured the Z80 D0 chip with a scope and get a signal bounded between 0 and 400 mV.
I have a small video on youtube that shows the scope's screen : https://youtu.be/FH0v0lO4ejU.

Thanks,
Regards

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