28
May
15

ZX Spectrum Update

ZX Spectrum mainboard

ZX Spectrum mainboard

Some time ago my ZX spectrum had another failing in the video RAM department, but at the time I couldn’t easily diagnose the problem. Some parts of the machine still appeared to work such as the display circuit which could still manage to display the usual pattern when video RAM isn’t working. I put it aside to work on other projects until I could build up some knowledge.

Recently reading around the internet I came across a youtube channel with a nice guide for the basics of repairing ZX Spectrums. His name is JoulesperCoulomb, he has a number of good videos about repairing Spectrums and some other electronics such as amplifiers. He also sells some replacement memory modules for spectrums from his website. I watched the first video and decided I should check the power circuits on my unit.

Following his instructions I found a few problems with the power circuitry. Firstly the resistance through the 7805 was low by his standards at 22K ohms and the inductor seems to have a resistance value between windings that it shouldn’t. So there’s two issues to look at straight away. Further testing revealed that two transistors TR4 and TR5 have failed, which are a part of the power supply circuit. Apparently these are all common problems.

Luckily important components like the Z80 CPU and ULA seems to be working from what I can tell with my scope.

So my Spectrum is one sick puppy. The faulty power circuitry explains the problem completely, as I found out later that the -5V line to the video memory is not getting power whilst the 12V and 5V lines appear to be working. I’ll have to replace the two transistors and do some more testing. I may also replace all the electrolytic capacitors as they are all old with a few having been replaced by a previous owner. The inductor and voltage regulator I will have to investigate as measurements could have been affected by other components.

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4 Responses to “ZX Spectrum Update”


  1. May 29, 2015 at 2:14 am

    Hope you can get it working. I’ve always wanted a Spectrum but being in NA there’s 0 chance of finding one without importing which with the cost of shipping over the ocean and all that usually isn’t cheap. luckily its not a huge or heavy computer so maybe one day.

    • May 29, 2015 at 11:13 am

      I hope I can get it working to! I need to get a number of parts for it before I can go any further though, I need to get the capacitors and transistors in particular and maybe some sockets for the lower RAM. It certainly will be a good learning experience!

      The only downside of a Spectrum for yourself is that they output a PAL signal normally modulated to connect to a TV antenna socket. So it might be difficult to get a video signal on a TV in NTSC land. You can perform the composite mod which bypasses the RF modulator to get composite video out, but it will still be a PAL signal. So you’ll still need something that will accept PAL.

      It’s kinda funny, here in Australia the Spectrum is a little expensive and hard to find. I was fairly lucky to get mine at all as there seemed to be a bit of competition for them on ebay. I think the best place to get one from would have to be the UK, they are much cheaper there and you should be able to get a bunch of games on tape there as well. I should have looked there first, as it turned out even after shipping the price is better from there.

      I think there is a compatible machine in the Timex series you could try, but I don’t know anything about those.

      Sparcie

  2. May 29, 2015 at 11:39 am

    I’ve heard of the Timex version but from my research its pretty hard to find and suffers from many incompatibility issues.

    Voltage would be an issue as well as I would need a step down converter. I’ve heard of the composite mod but I do have several RGB monitors as well as a Sony PVM that accepts PAL signals so I should be fine on that front.

    I wouldn’t mind getting a hold of a UK Amiga on day as well as I’m a big Amiga fan but the bulk of games were in the UK. I’ve also run into the speed issue as I love WHDload utility which lets you install Amiga games to a hard drive. Its nice cause I’ve found Amiga floppy disk failure rate to be very high. The problem is 99% of the files out there for games and WHDload are for the PAL versions and on all my NTSC Amiga’s they run like their on crack.

    • May 29, 2015 at 12:07 pm

      If power is your main concern you could always convert another power supply that accepts local voltage. The Spectrums power input is 9v centre negative 2.1mm plug, so if you are able to arrange that without using a step-down transformer it should still be happy.

      I wouldn’t mind having an Amiga, but I don’t presently have the space. Luckily I wouldn’t have the issues you do, but I haven’t seen much Amiga software available here either.


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