Spectrum error

CsabaiGD
Posts: 13
Joined: Sun Aug 29, 2021 5:39 pm
Location: Hungary, at my computer basement(actually, in my grandma's cellar)

Spectrum error

Post by CsabaiGD »

Hello Everyone!

I need help to repair my ZX Spectrum 48k Issue 2.
I Think 2 years earlier accidentally i supplied 9v center positive to the computer but the psu was only connected about 2 seconds long. Yesterday, I bought a 9v Center negative supply and I Connected it to the computer. It won't display the copyright screen at all, it's stucks on garbage screen(as the picture shows below). Commonly it displays only these black lines, with the yellow border, but sometimes, the border gets white or black and rarely it gets green. Sometimes the screen shows some blinking caracters. These are most commonly purple, or yellow. But the most common is only black and white bars with yellow border, with no blinking characters. On the internet, everyone says it can be a ram problem. So, with my multimeter, i started to do some measurements.
The psu's output is 8.98v but the input of the voltage regulator is only 8.78v and its output is 5.02v which is normal, I think. The coil makes some annoying, squeaky noise, but I think its normal, too. On the lower(4116) ram chips, it displays normal voltage(the 1. pin is -5v, the 8. is 12v, the 9. is 5v) and the higher ram chips, it displays good voltage, too(I don't remember what it is but from the layout on the internet, its good.) After I removed the rom chip(it is socketed) it does not has any change on the garbage screen. The ULA getting hot fast but i think it is normal. The cpu gets hot slowly. The ram chips(both higher and lower) didn't get hot at all. The 7805 voltage regulator getting hot fast. I replaced the c44 capasitor with a new one, but it has no change. The TR4 and the TR5 transistors are not tested. After I looked to the board I see nothing is in bad shape. There is no bad trace. I found a replace z80 and some 4116 rams if i need it, but I dont want to buy it because if these items are fully working, it is totally money wasting... So someone can help me? How to diagnostise what is working and what is not? I just only have a multimeter.
Attachments
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IMG_3326.PNG
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Magyar Spectrum Egyesület büszke tagja vagyok, és mindig azon vagyok, hogy a legjobb teljesítményt nyújtsam ezügyben!
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1024MAK
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Re: Spectrum error

Post by 1024MAK »

The voltages are all within specification, so no problems there.

Going on that, either the power was never connected in reverse, or the PSU you tried was too weak to cause any damage.

The 7805 voltage regulator and its metal heatsink are designed to run hot.
The ULA also normally runs hot. The Z80A normally runs warm. The 4116 (or equivalent) DRAM often run warm.

In issue one and issue two boards, it very common for the coil to make a loud annoying noise. Again, this is normal (and actually, it’s a good sign, at it means the DC/DC converter that it is part of is working normally).

It’s not possible at the moment to tell if there is a RAM fault. As it appears that the Z80A is not running the ROM program. This could be due to a number of reasons, including the Z80A is faulty, the ROM is faulty, there is a poor/bad connection or short circuit on the board affecting the communication between these parts, or between the ULA and these parts.

You say that the ROM is in a socket. Is the Z80A in a socket?

Have you done a visual inspection of the tracks, pads etc on both the underside and the top side of the board looking for any damage or causes of short circuits?

And has any repair work been attempted before, or signs of any repair work?

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

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CsabaiGD
Posts: 13
Joined: Sun Aug 29, 2021 5:39 pm
Location: Hungary, at my computer basement(actually, in my grandma's cellar)

Re: Spectrum error

Post by CsabaiGD »

Thanks for your anwser!

"You say that the ROM is in a socket. Is the Z80A in a socket?"
No, that's not. Only the ULA and the ROM is in socket.

"Have you done a visual inspection of the tracks, pads etc on both the underside and the top side of the board looking for any damage or causes of short circuits?"

Yes. Where I found some weird-looking solder joint, I tested it with the multimeter, but everything is seems to be ok. I found a broken-looking trace below a screw, but it's working too.

"And has any repair work been attempted before, or signs of any repair work?"

This computer wasn't used a lot. After we get it, we tried it one time, and it was working. I see only the spider mod on the z80 and there is a capasitor on the input which I think it's normal but I will attach a picture here.

Because the keyboard foil is missing I think someone opened this before but as I told, the computer was working before this error. There is a point on the board, where it seems to be missing something(on the c51 point, I think), I have a picture about this.

So, bad polarity can only damage the 7805? Thanks for your anwser!

Gábor
Attachments
IMG_3338.JPG
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IMG_3334.JPG
IMG_3333.JPG
IMG_3332.JPG
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1024MAK
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Re: Spectrum error

Post by 1024MAK »

Incorrect polarity often destroys the 7805 voltage regulator, TR4, TR5 and on issue 3 boards onwards, sometimes TR7 (drives the speaker). Sometimes nothing else is damaged. Sometimes other semiconductors (chips) are damaged.

If the 7805 voltage regulator, TR4 and TR5 survive, then I would not expect anything else to be damaged due to a brief incorrect polarity if the power supply unit used was too weak to supply enough power to kill the 7805 voltage regulator, TR4 and TR5.

The C51 position is not used on issue 2 boards, hence no component is fitted. This is normal.

I’ll have a look at the photos.

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

:!: Standby alert :!:
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1024MAK
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Re: Spectrum error

Post by 1024MAK »

The capacitor near the 9V input socket {known as C74, 4.7uF 16V or 25V and fitted between R58 lead (junction with R58/R59/TR5 base) and C34 positive lead (+5V connection) with C74 positive to C34 positive lead} is part of a standard Sinclair official modification and is normal.

I can’t see any obvious problems in the photos.

One thing to check, can you have a look at the wire option links under the heatsink just to the right of the 8 way keyboard connector. What positions are they in? There are two configurations. Either they are both in the ‘N’ positions, or they are both in the ‘H’ positions.

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.
CsabaiGD
Posts: 13
Joined: Sun Aug 29, 2021 5:39 pm
Location: Hungary, at my computer basement(actually, in my grandma's cellar)

Re: Spectrum error

Post by CsabaiGD »

Thanks for your answer!

The wires are both on H position. But I noticed there is a missing component on the R57. It is normal, or that can cause this error? :shock: I show you pictures.
Attachments
IMG_3349.JPG
IMG_3348.JPG
IMG_3347.JPG
Magyar Spectrum Egyesület büszke tagja vagyok, és mindig azon vagyok, hogy a legjobb teljesítményt nyújtsam ezügyben!
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1024MAK
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Re: Spectrum error

Post by 1024MAK »

Resistor R57 has been intentionally removed from the board as per Sinclair’s modification instructions. So it’s not supposed to be fitted. Capacitor C54 is also probably not fitted.

The H/N links are correct for the ROM that you have.

Do you have any of those test connection clips that can be clipped on to component legs/leads/pins?
And can you solder?
Do you have, or have access to a logic probe?

In the meantime, please refit the heatsink. Carefully remove the ROM chip from the socket. Lever it up a bit at a time working alternatively from either end with a blunt knife or flat bladed screwdriver, but work against the socket, don’t lever against the PCB or you may damage the PCB tracks. Keep some pressure using your fingers or thumb on top of the chip to stop it suddenly escaping at one end from the socket, and then the pins at one end bending as it rotates. Bent pins are not good :(

Now power up (yes, with the ROM chip missing) and see what, if any, pattern you get on the TV screen. Take a photo and post it here.

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.
CsabaiGD
Posts: 13
Joined: Sun Aug 29, 2021 5:39 pm
Location: Hungary, at my computer basement(actually, in my grandma's cellar)

Re: Spectrum error

Post by CsabaiGD »

"Do you have any of those test connection clips that can be clipped on to component legs/leads/pins?"
Do you think for this? (last picture)

"And can you solder?"
Yes, I can(actually nicely ;) )

"Do you have, or have access to a logic probe?"
No, unfortunately I don't.

It is the same problem, after I removed the rom(1.,2. pic)...
Attachments
IMG_3357.JPG
IMG_3358.JPG
IMG_3352.JPG
IMG_3351.JPG
The clips
The clips
Magyar Spectrum Egyesület büszke tagja vagyok, és mindig azon vagyok, hogy a legjobb teljesítményt nyújtsam ezügyben!
CsabaiGD
Posts: 13
Joined: Sun Aug 29, 2021 5:39 pm
Location: Hungary, at my computer basement(actually, in my grandma's cellar)

Re: Spectrum error

Post by CsabaiGD »

One interesting thing:

When I power up the Spectrum, the black bars appears but I need to wait about 2 seconds till the colors appears.
Is it normal?

I found a 20mhz logic probe on a webshop. I need to buy it, or can I have another way to find the problem?
Magyar Spectrum Egyesület büszke tagja vagyok, és mindig azon vagyok, hogy a legjobb teljesítményt nyújtsam ezügyben!
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1024MAK
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Re: Spectrum error

Post by 1024MAK »

If the 20Mhz logic probe is reasonable value, it may be worthwhile getting.

Alternatively, there is a very simple logic probe design here: Build Your Own Simple Transistor Based TTL 5V Logic Probe ;-) But being cheap and simple means it can’t get close to a 20MHz logic probe in terms of working with high frequency signals.

The advantage of having a logic probe, is it’s then possible to test the signals on the pins of the Z80 to see if it’s doing anything. Very limited testing can be done with with a multimeter.

Based on the test results so far, I think your Z80A is faulty. I strongly recommend getting a DIL socket. Originally a NMOS type was fitted. Zilog still supply brand new Z80 chip in DIL packages, these are the lower power, more modern CMOS type. Buy from a trusted supplier. Normally they work with no changes. Or you can buy a Z80A (4MHz) NMOS type. These will be new old stock or used chips. They are available from various places, but be careful if buying from China via a trading site, as often ‘pulls’ are cleaned up and remarked with type numbers (for example, a 4MHz NMOS type may be labelled as if they is a 20MHz CMOS type).

The easiest way to remove the old chip, is to cut each and every leg as near to the plastic of the case of the chip as possible. Then lift the chip out leaving the legs behind. Then heat up the solder joint and very gently pull the leg out with fine tipped long nose pliers.

Obviously the transistor (spider) modification has to be removed first. There is an alternative position where it can be refitted to the PCB rather than on top of the Z80. But don’t worry about that now, as the spider modification is not needed for basic operation anyway.

Let me know if you want to try using your multimeter to test the Z80.

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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