Sinclair Spectrum ZX48 mystery

Got an old ZX48, am trying to work out what the little additional board does. There also seems to be a couple of extra mods with the CPU and a small chip marked E next to the CPU.

Want to convert it to video output instead of the TV modulator. Maybe it’s a sound mod to get the beeps coming out of the TV? Anyone know anything about this board?

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Have you tried here?

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Thank you, I’ll have a thorough look at that later on this evening. I’ll nail down what version of the board I have, then go look at the specific mods my one has. The alteration to the CPU is most interesting!

So a bit of procrastination googling resulted in the following observations:

  • The dead bug (aka “spider”) on the CPU is a factory workaround for an issue with the ULA (the big gate array to the left), used on Spectrum Issue Two PCBs.
  • The wire at chip E (IC26) is there to specify the RAM type for the 32k extra RAM (the chips marked A) – they used 64 kbit rejects with one half known to be good, and the wire selects which half to use (there’s supposed to be a jumper on the PCB too, but that location is hidden by the additional board so not clear why that wasn’t used)
  • There’s a million Spectrum mods out there, so didn’t find the specific one, but the extra board appears to be connected to the speaker (white wire), ground (black wire) and composite input to the modulator (red wire), so you’re probably right that it’s an RF audio fix (which also means that it’s hard to find info on the internet, since everyone is modding in the opposite direction these days).
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Thank you so much, you’re amazing! I just knew someone here would know. Dead bug factory work around, wow… they really wanted to shove these things out fast! It started me on the right path…thanks again!

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Hey man

Long time Spectrum owner/user/hacker here (even worked for one of the magazines a lifetime ago).

The Spectrums before Amstrad started messing around with them had a composite video signal on the edge connector - I’m not sure how clean the signal is but it’s worth checking.

By the time I had a TV with composite video input I was on an Amstrad one, a +3 (which is about a metre away from me right now) and couldn’t use it. I eventually cobbled together video out a few years back - if I recall correctly it was quite simple, a couple of resistors and possibly a transistor coming from one of the chips.

For audio out, just plug straight into the Mic jack :slight_smile: the labels were for what you plug them into on a cassette recorder, not what they actually do which seems mad nowadays but there we are :smiley:


Thanks for that, a very useful diagram I will be using when I find an edge connector. Back in the day, I owned a +4. Not because I wanted one, but because my Grandad thought it was a good idea to get me one. I’d have bought a C64, my friends has ZX48s and I was always a bit jealous of them being able to just press keys for the basic commands. I could never remember the shortcuts, so was doomed to type in the program incorrectly many times.

Now I have an original ZX48 and am also building a clone board. Been a bit under the weather this last week, so no progress made.

Hi, Mark. I’m very interested on knowing more info about that small PCB on the photo, do you have its schematic or any page where it mentions that circuit?

Thanks in advance.

-= Paula Silva =-

I killed a Speccy once by trying to make adhoc connections to the board without the proper edge connector so that I could use it to control external hardware.

Yeah, the wisdom of presenting unbuffered Z80 pins to the open air was always a bit questionable. (I did that too!)