Spectrum Computing

In a bizarre turn of events, it turns out the short circuit was not on the PCB, but is in fact inside the ULA. I took the ULA out of its socket just to eliminate it as a problem, and lo and behold, no short circuit on the PCB. However, testing the pins of the ULA out of the PCB showed a short betwee...

The lower RAM chips all seem to be showing the correct supply voltages, and show some signals on all the pins, except for Pin 7, which I believe is A1, one of the address lines. This is always low. That is odd. Worthwhile investigating this. On the ULA Pin 5, which is Address line 0 I think, is alw...

An update, some forward progress, but still not quite a working Spectrum... I replaced the rogue RAM chip with a 4116 RAM chip, and the voltages are now good and reading as they should. And, perhaps more excitingly, there is a picture being generated! https://i.postimg.cc/0z4WdgDc/DSC-0003.jpg Hmm, ...

I decided I don't waste enough money on dubious Ebay purchases, and so bought a pile of slightly dubious rubber key spectrums to see if any were salvageable and to provide parts for other projects if not. Sadly, no hidden gems, but they're not in as bad a condition as I worried they might be. That b...

Over the last few weeks I've been fixing the three ZX Spectrums that had been languishing in the loft for many years. Much to my amazement, and the amazement of all around me, all three of them are now working. I'm planning on probably selling two, and just keeping one, but before I sold any, I want...

This is fantastically helpful and definitely explains what was going on with my readings.

I am limited by my cheapo multimeter, and I am tempted to try to find a second hand oscilloscope but I don't know if I can quite find the space for one...

The issues with living in a flat!

I would be very interested in learning why!

My multimeter is a very cheap model, and I'm thinking of upgrading to something more robust advanced.


Iain

Success! But also a mystery... I did the composite mod, and the spectrum now produces a beautiful, crystal clear image. So, seems like there was an issue with the modulator. However, there still seems to be an unusual voltage on the LM1889... This website suggests that pin 17 should show 0.8V DC, i...

I'll admit to never having tried to composite mod a spectrum. I know it's relatively easy to do, so I might give this a go. Is it best to do the simple composite mod, using a wire only as I've seen composite mods using either a 100uF capacitor or a transistor? What are the benefits of using the extr...

I forgot to mention, I've tested the ULA in another, working machine, and it seems to be fine. As for the modulator, I have 4.8V on the 5V line, and on the video signal wire it shows 2.96V. I've also checked the voltages on the LM1889 and they are all normal, except for pin 17, which shows 8.58V. I ...

I've been testing and repairing the three Spectrums that had been languishing up in the loft, and I have left the most difficult until last. It's an issue 4S motherboard, with 48k of RAM. Having a look at the board it looks as if it has had some repair work carried out in the past. It also has an am...

So all seems well. It's stable, switches on and off properly, and displays in glorious technicolour. That being said, the picture is a bit off, but this is possibly just down to age and the fact that it's an RF connection. https://i.postimg.cc/DWFPmsKS/DSC-0095-1.jpg There is some bleeding from the ...

Um, I don’t recall reading about you having replaced TR1 and TR2. What type did you fit? If pin 13 still has the same voltage as before and if you have found nothing wrong with any of the passive components, then replacing the SN94459 with an LM1889 is a reasonable next step. However, I don’t have ...

I have replaced X2, but sadly the same problem persists. I think my next step is to replace the SN94459 with an LM1889. I know the LM1889 has more pins, although I assume that the extra pins, while there are holes on the PCB for them, aren't actually connected to any part of the circuitry? Are any o...

Heat is just one way for a bad or broken solder joint to do or not do contact. Simply moving the computer or flexing the board even with a very tinny almost imperceptible movement, can make or break a connection that might be touching by a hair. Suggestion: Poke the board around, using your own fin...

Is the +12V supply (normally around 11V) to the IC14 (SN94459) definitely okay? If this voltage drops, it will cause loss of colour. Although that alone does not explain the complete loss of video picture. The monochrome picture signal does not pass via IC14 (SN94459) but instead goes from IC1 (ULA...

You can check if you are loosing the color frequency clock, used to trigger the PAL color pulse. The board as 2 crystals, one is 14Mhz, that's the main crystal, used for video and subdivided for everything else, including ULA and CPU clocks. But next to this Crystal there is another one of 4.4336 M...

I am having some issues with a ZX Spectrum+ that appears to have some problems with a potentially unreliable video display. There are two main problems: 1) The screen only displays in black and white, and does not display colour. I know this could be an issue with the TV I'm using (it's a Sony flats...

Are the wire links for selecting the upper memory type present? I remember having a similar problem with an upgraded Spectrum where the previous owner forgot to install those. What type of memory do you have? They should match the brand and type of the memory as there are four options. Edit: pictur...

Could be. If you get 127, it means the very first byte of the upper memory is bad. What do you get if you execute this: POKE 32768,0 PRINT PEEK 32768 POKE 32768,255 PRINT PEEK 32768 So this gave quite intriguing results. I ran the code with 0, 255, 85 and 170. The first time I ran it I got the foll...

So, when I execute that line of code I get: 32767 Also, having done a bit of digging, I stumbled across this page, which suggests that some Issue 2 Spectrums were assembled as 48k machines, but only sold as 16k machines owing to faults with the higher memory. https://www.dataserve-retro.co.uk/conten...

I've been refurbishing an old Spectrm that I bought in a Boy Scout jumble sale when I was a kid. When I first bought it I didn't know much about electronics or computing, but given that it wouldn't load 48k games, I assumed that it was a 16k model. Now, many years down the line I've run test softwar...

There's a plugin for Winamp which allows to load tap directly from a PC: https://mega.nz/#!ox9lnRCC!tnH4nRdRxxStCjKtUixQR16I-VahKBpqALVOhOdv7P8 Thanks for this suggestion, it worked a charm! I haven't used Winamp for years but loaded the plugin, and after a bit of tweaking of volume levels I was ab...

Thanks for the quick answer! I am tempted by something like for my own use, as the tape loading experience can be a massive pain, as I have discovered. That being said, I still want to try to be able to test tape loading on the computers I have. Ultimately, I probably will need to sell some of them ...

Hi all! I recently re-capped my first ZX Spectrum motherboard and replaced the 7805 with something more modern that runs less hot. Maybe not the most necessary mods, but I mainly did it just as an experiment and to refresh my rusty soldering skills. It's an issue 2 board, so I also had to tweak the ...