ZX Spectrum 48K Repair - ROM Woes
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- เผยแพร่เมื่อ 24 พ.ย. 2024
- This 48K gave me a lot of trouble, and it was an easy fix!
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I bought an issue 3B "fixer-upper opportunity" on eBay and after seeing every failure mode (and replacing every IC and transistor as I chased problems around the entire board) I'd all but given up. Thanks for a really interesting and informative video, I've picked up a couple of useful tips and maybe even the inspiration to try again with this poor hopeless Spectrum! 🤞🏼
Thanks! Keep us posted how it goes, always interested
Brilliant TV!
I knew about the Hitachi/NEC ROM jumpers, (aren't they switching 2 pins on the ROM?) and selecting those wrong should of course prevent the ROM to be read properly. But I would never have guessed that it will cause a complete lower RAM failure... Good to know, thanks for sharing!
The ROM can still be read properly, the problem is that it doesn't go off the data bus when it's supposed to, which means trouble when moving data between the other chips
Yeah, this was a bit odd; after all, IF the Lower RAM chips had indeed all failed, the Speccy shouldn't be able to show any screen image whatsoever. I'm glad you managed to track down the root of the problen in the end, good job.
Aside from that, i THINK i have read somewhere that if those 4116s in the lower RAM area don't get all three voltages correct, there's a risk of them dying. I guess you were lucky there.
Yes that's true they can die, but I can't believe they would ALL die in the exact same way! What I still don't understand is why the upper RAM passes the test, I had planned to explain that in the video but couldn't figure it out
TR5 would actually be better as a higher gain transistor rather than ZTX751. BC556 is plenty good there. The critical one is TR4, as it has to have a high current rating. I normally use ZTX653 there, it's just a higher voltage version of the original (and what Diodes Inc recommend as the replacement). Generally all of the transistors in the spectrum can be replaced with BC546/BC556 types, apart from TR4. The only other one that might need to be slightly higher current is TR7, and in that case a BC327 is good (or even another ZTX653 if you want to keep your stock neat)
The main reason the ZTX213/313 were used is because they were made by Ferranti originally, so Sinclair got some sort of a price deal on them with the ULA. That part of Ferranti got spun out into Zetex (the name was taken from the part code on the transistors!) and are now part of Diodes Inc
Extremely knowledgeable as ever Peter thanks for sharing, and thanks for helping me bottom out these repairs in the group
But now knowing you didn't have bad ram, did you put the old chips back in and save the lower ram board for later?
That is an option but I can't face doing any more work on this board! :)
Brilliant work as always, lovely and tidy and very well explained!
What did the pope say about them solder joints
Hello! Do you sell any of these Spectrums? I’m after a rubber 48k I want to display on the wall (and play every now and then). Yes, I’m starting my midlife crisis. Love your content.
Hi. Yeah I normally sell the ones I've repaired/refurbed. Drop me an email HappyLittleDiodes@gmail.com
@@HappyLittleDiodes happy days. Will do.
Did you have an "office space" printer scene moment with them links off camera?
I see you noticed the massive Mafia Pliers in the side of the shot