Search found 26 matches
- Mon Nov 01, 2021 11:22 pm
- Forum: Hardware
- Topic: Bizarre RAM chip in a ZX Spectrum
- Replies: 7
- Views: 610
Re: Bizarre RAM chip in a ZX Spectrum
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...
- Sun Oct 31, 2021 11:08 pm
- Forum: Hardware
- Topic: Bizarre RAM chip in a ZX Spectrum
- Replies: 7
- Views: 610
Re: Bizarre RAM chip in a ZX Spectrum
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...
- Fri Oct 29, 2021 9:53 pm
- Forum: Hardware
- Topic: Bizarre RAM chip in a ZX Spectrum
- Replies: 7
- Views: 610
Re: Bizarre RAM chip in a ZX Spectrum
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, ...
- Tue Oct 12, 2021 11:50 pm
- Forum: Hardware
- Topic: Bizarre RAM chip in a ZX Spectrum
- Replies: 7
- Views: 610
Bizarre RAM chip in a ZX Spectrum
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...
- Sun Feb 21, 2021 10:32 am
- Forum: Hardware
- Topic: Testing the spectrum - soak testing and edge connector
- Replies: 4
- Views: 346
Testing the spectrum - soak testing and edge connector
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...
- Tue Feb 16, 2021 8:00 pm
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Re: Spectrum+ with potential RAM or power supply problem
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 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!
- Mon Feb 15, 2021 5:36 pm
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Re: Spectrum+ with potential RAM or power supply problem
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
My multimeter is a very cheap model, and I'm thinking of upgrading to something more robust advanced.
Iain
- Fri Feb 12, 2021 10:02 pm
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Re: Spectrum+ with potential RAM or power supply problem
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...
- Thu Feb 11, 2021 6:42 pm
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Re: Spectrum+ with potential RAM or power supply problem
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...
- Thu Feb 11, 2021 5:29 pm
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Re: Spectrum+ with potential RAM or power supply problem
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 ...
- Thu Feb 11, 2021 12:18 am
- Forum: Hardware
- Topic: Spectrum+ with potential RAM or power supply problem
- Replies: 11
- Views: 680
Spectrum+ with potential RAM or power supply problem
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...
- Fri Feb 05, 2021 3:03 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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 ...
- Wed Feb 03, 2021 11:12 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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 ...
- Wed Feb 03, 2021 5:10 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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...
- Sun Jan 31, 2021 10:51 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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...
- Sun Jan 31, 2021 11:36 am
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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...
- Fri Jan 29, 2021 1:54 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Re: Black and white screen of semi death on Spectrum+
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...
- Thu Jan 28, 2021 11:12 pm
- Forum: Hardware
- Topic: Black and white screen of semi death on Spectrum+
- Replies: 13
- Views: 1558
Black and white screen of semi death on Spectrum+
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...
- Sat Dec 05, 2020 12:18 am
- Forum: Hardware
- Topic: Issue 2 motherboard detective work
- Replies: 11
- Views: 5556
Re: Issue 2 motherboard detective work
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...
- Thu Dec 03, 2020 11:57 pm
- Forum: Hardware
- Topic: Issue 2 motherboard detective work
- Replies: 11
- Views: 5556
Re: Issue 2 motherboard detective work
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...
- Sun Nov 29, 2020 11:11 am
- Forum: Hardware
- Topic: Issue 2 motherboard detective work
- Replies: 11
- Views: 5556
Re: Issue 2 motherboard detective work
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...
- Sat Nov 28, 2020 10:26 pm
- Forum: Hardware
- Topic: Issue 2 motherboard detective work
- Replies: 11
- Views: 5556
Issue 2 motherboard detective work
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...
- Sat Nov 21, 2020 11:39 pm
- Forum: Hardware
- Topic: ZX Spectrum loading options
- Replies: 10
- Views: 5592
Re: ZX Spectrum loading options
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...
- Wed Nov 18, 2020 11:11 pm
- Forum: Hardware
- Topic: ZX Spectrum loading options
- Replies: 10
- Views: 5592
Re: ZX Spectrum loading options
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 ...
- Wed Nov 18, 2020 10:09 pm
- Forum: Hardware
- Topic: ZX Spectrum loading options
- Replies: 10
- Views: 5592
ZX Spectrum loading options
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 ...