Trying to fix my ZxSpectrum+

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Hyuma
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Trying to fix my ZxSpectrum+

Post by Hyuma » Sat May 18, 2019 7:04 am

Hi guys, I bought a ZX spectrum+ on some flea market here in Italy and it requires some fix. I would like so much understand what's wrong... Here a picture of the issue:



The board it's an issue 6a, I only made the video mod and that's the result. Actually I don't have the multimeter with me, but when I came back home I could make all possible tests. Thanks!
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK » Sat May 18, 2019 2:08 pm

It could be a number of things at fault, including but not limited to:
  • ULA Faulty
  • RAM fault
  • Damage to the PCB
You may need more than a multimeter to fix this.

Mark
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Hyuma
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma » Sat May 18, 2019 6:04 pm

Thanks! Next week I'll upload new pictures and keep you all updated on my step trying to fix that! Thanks again :D
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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black » Sun May 19, 2019 6:42 am

Like @1024MAK says, it is not so simple. Anyway, the mainly problems that ZX Spectrum's have, are the faulty lower & upper ram IC's. It's very difficult to find a unit with no issue in ram IC's. In case you have faulty lower ram IC's, probably you have also faulty the transistors TR4 / TR5 or both. Except all that, we have also in game the ULA & CPU, that both appear to have frequent problems. Even if you find the problem, you should also replace all electrolytic capacitors to have a healthy unit. Not because that will fix your spectrum, but because 35+ years old capacitors need to be replaced. Also it is true that some issues start from there.

As you power on you machine for several minutes... before asking :) , we can skip part of initial tests but do some of them. i can drive you, but no too far. But there is some important questions first.
  • Do you have soldering iron / desoldering pump and if yes, what is your soldering skills?
  • Do you have an other working spectrum?
If you don't have the basic skills for this, it is better to take it to a technician. ZX Spectrum pcb's are not recommended to beginners on electronics. You can easily do big damage on traces if you need to work in there.
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Hyuma
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma » Sun May 19, 2019 7:36 am

Yes I have soldering iron and pump and I have fixed an old C64 replacing the pla and one voltage regulator. I'm thinking to buy some oscilloscope kit from Amazon as a starting kit before a real one oscilloscope :)
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1024MAK
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Re: Trying to fix my ZxSpectrum+

Post by 1024MAK » Sun May 19, 2019 7:52 am

Keep in mind that oscilloscope kits and cheap (inexpensive) new oscilloscopes are mostly not suitable for helping with the repair of microcomputers like the ZX Spectrum.

There is some guidance in this post ;-)

Beyond a good digital multimeter, the next items of test gear that I recommend are a logic probe (or you can build one) and a bench power supply unit (with a regulated adjustable output control and a current limit, better still, a adjustable current limit).

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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black » Sun May 19, 2019 9:54 am

Good to hear.

Just for the story, before start the test's, keep in mind that the best way to check an unknown condition zx spectrum in the future, is to do some test before plug it on power supply. Cause you can easily do farther damage to it. That's tip for the next time as you already may pass that step.

Important:

So, if you left your spectrum on power, for several minutes, do the following test's. If you did the composite mod and just switch on your spectrum for several seconds to test the results and then switched of your spectrum immediately and never switch on the power again, skip the following steps and do ONLY the test's from the video you find at the end of the post.


Steps in case you left your spectrum on power for several minutes:

You need to test some dc voltages, and after that you post the results here.
  • First of all, test the output of your power supply "unplugged" from your spectrum . You should get 9+ volts with the negative probe on the center.
  • Then with your power supply plugged in, test the voltage regulator. With the negative probe to the center pin, measure the two other pins. You should get 9+volts and 5 volts.
  • finally test the voltages on IC6 (lower ram). With the negative probe to heatsink (the aluminium plate that absorb the heat from voltage regulator), check the voltages on pins,1,8,9 and 16.
You should get:

PIN 1: -5 volts
PIN 8: +12 volts
PIN 9: +5 volts
PIN 16: 0,00 volts

If you missing voltages on pins 1 and 8, you have faulty transistor TR4/TR5, or both. But simply change them with new ones, don't fix the problem, because missing that volts means also faulty lower ram IC's.

Tell us the results from your test's.


Steps in case you left your spectrum on power for several seconds:

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Muttley Black
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Re: Trying to fix my ZxSpectrum+

Post by Muttley Black » Sun May 19, 2019 3:02 pm

Muttley Black wrote:
Sun May 19, 2019 9:54 am
  • First of all, test the output of your power supply "unplugged" from your spectrum . You should get 9+ volts with the negative probe on the center.
Because that step may be confuse you. When i say "9+ volts" i mean 9V or greater rather than +9 volts. The original zx spectrum power supply is unregulated and the output voltage depending on how much current the computer draws. So, with the power supply not connected, the output voltage is between 12V and 16V. With the power supply connected to the computer should drop around 10 volts and 12 volts, but not below 9 volts.
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Hyuma
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma » Sun May 19, 2019 8:06 pm

Ok, on Monday or tuesday I'll do all tests and post here the results! There is a guide to build the logic probe? I would like to do that too!
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Hyuma
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Re: Trying to fix my ZxSpectrum+

Post by Hyuma » Mon May 20, 2019 11:03 am

Made some preliminary tests with multimeter:

Image

Image
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