Fixing Bad Video Quality on the Sinclair ZX Spectrum
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- čas přidán 2. 08. 2024
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We have two Sinclair ZX Spectrum+ computers with REALLY bad video image quality. This is a pretty frequent fault and is usually caused by bad electrolytic capacitors generating unstable voltages inside the computer. Here we learn how to diagnose and fix the bad video quality caused by the capacitors.
We take a good look at the voltage-generating circuitry inside the Spectrum (it generates +12V, +5V, and -5V from a 9V DC power supply) and try to understand how it all works. Then we take a look at the voltage signals with the oscilloscope and confirm that the electrolytic capacitors are most likely faulty.
Finally, we replace all the electrolytic capacitors, measure everything again, and test the final image quality and compare it to how it was at the beginning.
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Great way to teach electronic to folks and be amazed by what a few chips can do.
Your workshop is unreasonably clean, tidy and organised! =)
Hahaha... I won't lie: I like it that way. But even if I didn't, I have to keep it that way because I have a very small space. One day I'll do a mini video with the "tour" of the workshop/office.
Isn’t it. I work from home and my space is a mess, covered in prototype hardware, breadboards etc. All things I’m working on for my employer.
A video problem was the first computer repair I broke out a soldering iron for. My TRS-80 Model 100 arrived in mostly good condition, but the display flickered pretty badly. A few electrolytic capacitors had failed. Thankfully, it was easy to visually diagnose it(and some web searches pointed to failed caps being a common cause of that sort of flicker) and get the display rock solid like it should be.
At least someone who did a recap, but, showing technical information, and how it could affect each circuit.
Not like a lot of idiots, who don't even know anything about electronics, watch youtube videos and feel like gods of electronics, and every single device, for them are bad caps, "need recap" !
Great job Noel !
I'm an electronics Tech as well !
Thanks! Yes, one of the goals for the channel is not just teaching people how to do things, but to have them learn WHY they're doing those things :-)
Another very useful video with good explanations!
Awesome video! I've recently bought a zx spectrum + with that exact simptoms and now I know where to start to try the repairings. Thank you
Great! Replacing the capacitors is always a safe move and a great way to start. Check out my Tune Up video though, because some board issues have the wrong polarity marked on them, so you should always follow the polarity of the capacitor you extracted, not what's printed on the board!
Great video. Thank you!
Just found your channel, good stuff.
About the time you made this video, I was re-capping my 48K and 128K +2.
Thanks! The +2s don't have as many issues with the capacitors because they run a lot cooler, but yeah, the poor 48K, especially the Plus ones, almost always need a recap.
@@NoelsRetroLab The +2 kept crashing, resetting or sometimes there was an audio click and the picture went fuzzy. LOL. But works a treat now.
I've had two of these for decades now, but this is the first time I've ever seen anything like it.
Quite a difference those capacitors made! Nice job!
Yeah, seriously! I don't usually advocate changing capacitors just for kicks, but these poor computers really needed it!
Interesting video Noel (as always). I'd like to see a video from you explaining how to determine what a capicator's rating is and what type of new capacitor is suitable for replacing it with, thanks Noel !
Thanks Niall! There's not much to the capacitors: Use the same capacitance than the one you're replacing (the number in uF usually), and use a voltage the same or higher than the one you replaced. That's it.
Now, figuring out what capacitors to put in the first place when you design a circuit, that's much trickier and I don't even have enough experience to figure that out :-) Hopefully I'll learn that when I get more into designing my own boards.
It's quite interesting actually, I didn't know the original Spectrum had a voltage inverter. The Soviet clones of Spectrum widespread in former USSR (which I'm sure you are aware of) used the 4164 DRAM chips (Russian clone name К565РУ5) that were 5v powered and did not require 3 voltages to power. Also they used RGB output that TVs had to be modded for, hence, no RF modulator and AV output. I recentley purchased a 48K Spectrum on Ebay and I'll be aware of this problem now, thanks much for your video!
You're welcome. I didn't know the Russian clones used 4164. That would make so much sense! Is the memory organized more like in the 128K, or do they still have a separate bank for video RAM to minimize changes from the original?
@@NoelsRetroLab Yes, they do have a 16K bank for video RAM (as far as I recall it's only partially video and it's up to 0x8000 user RAM) and 32K for the rest of memory. Also, RAM performance is different in these two banks, I think original Spectrum has that effect too. As a teenager I used to write custom loaders for games, I tried to port the tape loading routine into RAM and it wouldn't work in the lower half because of screen addressing CPU cycles. )
Just wish youtube and your channel existed 35 years ago... Miss my ZX and Horace game.
Haha! But that's one of the things that makes retro electronics so compelling today: The technology we have today lets us interface with old technology like this quite easily and cheaply. 35 years ago it was state of the art and quite difficult to do a lot of these things :-) As for your ZX... it's never too late!
You can always find your beloved games, here:
worldofspectrum.org/archive/software/games
They changed the layout of the page a lot, since the '90s, but you can find it there. And then, you can just download an emulator and play it! Tell me if I helped you in any way.
I forgot about the centre negative on the Spectrum, plugged in a 9v centre plus and bang went the regulator and coil
Just done exactly the same thing, do I need a new regulator?
Hope you are not getting tired of me saying this is a great video showing how using an oscilloscope can be very useful. Is you shirt a H.P. Lovecraft reference :-D ?
Of course not! Happy to hear you're enjoying the videos! 😃And yes, you got the reference! Most people don't get it.
@@NoelsRetroLab LOL I'll keep my eye out for a Miskatonic University as well :-D
@@carnright Haha, that's my wife's t-shirt (sometimes we wear them when we play Arkham Horror for extra immersion 🤣).
Ha, I like that effect at the beginning!
😁
I have this exact problem, with the exact same interference pattern as the first machine. It also wont load from tape at all, and my theory is that all the noise in the video rf generator is polluting the sound demodulator as well.
Nice work! I have fixed image on all Sectrums that way. But I can load games only from tape player or strong audio out, and not from any phone or audio player. Maybe you can make video about that fix. Thanks
Does this apply to the TS2068 too? My composite out is quite noisy
Noel, could you put the cost of the traco Power? Or the link from where you got it? please. Usually the price is pretty more expensive than xx05, or a DC-DC Converter, but if it is worthy and prices not too crazy, could be a nice replacement. By the way... one Amperes is enough? Thinking about to plug some devices... not sure about 1 or 2 Amperes to choose in my case.
I got mine from an order I placed on Mouser a while back: eu.mouser.com/ProductDetail/traco-power/tsr-1-2450 It looks to be 5,50€ plus shipping. I wouldn't be surprised if you can find cheaper sources. There's a small DC-DC converter that's pretty inexpensive and other people swear by it, but I haven't tried it yet.
Very nice music at 11:05. What track is that?
Found it myself: opengameart.org/content/theme-song-8-bit
ZX Spectrum 16K, 48K, and ZX Spectrum+ 128K, +2, +2A, +3
CPU cycles per PAL line 224 228
PAL line time 64 μs 64.28 μs
PAL lines per frame 312 311
Frame time 19.968 ms 19.9915 ms
Frame rate 50.08 fps 50.02 fps
Display Vertical Synchronization by the Z80 maskable interrupt line
I've just got my old 48k down from the loft which I had as a kid in 83. One of the two caps you said fail often had died so I replaced it. I have done the composite mod as well, but the display is very very poor. I plugged a different power supply in without realising the positive and negative were the wrong way around (modern way around) which I think has blown something, any ideas where I should start looking?
Yes, check out the DC voltages, especially -12V in the PAL encoder (I think I have some videos exactly about that). Chances are the transistor TR4 (or TR5) is blown and not getting steady 12V. Hopefully that's all it is, although it's hard to tell the full extent of the damage ahead of time.
@@NoelsRetroLab I've ordered TR4 and TR5 Transistors and also a modern voltage regulator, I'll replace them and hopefully she'll wake up
🤘😁🤘
ohh.. Philips capacitors... time bomb :)
Oh really? I didn't know the Phillips ones were particularly bad. I'll keep it in mind. Thanks for the tip!
@@NoelsRetroLab Philips capacitors are very good, but after 20-25 years their condition deteriorates a lot. They are not dealing with the heat, but with the pulse load. (I'm upgrading old Philips CDs, all need to replace the capacitors)
My first thought when I saw this video noise: dirty power...
Definitely. Especially on the PAL video encoder it becomes very visible.
@@NoelsRetroLab It can happen even on VGA.
Probably nuked my Sinclair Z81 as a kid plugging it in while the power brick was plugged in. I bet I could fix it these days but it's of course long gone.
I don't think plugging it in while the power brick is connected should be a problem. The usual problem is connecting something to the expansion port while the computer is powered. In any case, chances are it was an easy repair. Maybe it's time to get a ZX81 back 😃
Are the blue Capacitors from Philips? Horrible things from the 80s...
I have a suggestion for a video. Updating the ram on a 48k Spectrum, transforming it in a working 128k. I found a page explaining how to to it but, unfortunately, is to complicating for my knowledge. hw.speccy.cz/128krebuild.html
Interesting! I had seen some Spectrums upgraded to something like 80K or something like that, but I'll have to look into that. Thanks for the link.
@@NoelsRetroLab And if you want to build the ultimate Spectrum + 48k-128k, it is also possible to add the AY 3-8910 Sound Generator Chip with stereo output. lotharek.pl/productdetail.php?id=229
@@NoelsRetroLab I remember my father telling me, how he upgrade his 48k Spectrum to 80K. He replaced the "faulty" 64k upper rams for true 64k ram chips, and added a switch to toggle between the two 32k pages. Crude, but worked. I think that the next step would be adding some logic to switch banks , writing to a I/O port.
the video image is too poor, you need to fix it more