Why is my ZX Spectrum Faulty?

I pulled the ZX Spectrum down to show you all a new expansion but when I plugged it in to test it just showed garbage on the screen. I was sure this worked when it was last put onto the shelf but now its not so let's figure it out.
Links
Spectrum Schematics - spectrumforeveryone.com/techn...
Spectrum Parts - www.retroleum.co.uk/sinclair-...
Music
‪@Momentvm‬ Cathode Ray Gun - • Momentvm - Cathode Ray...
CRG Links
CRG X (Twitter) - / yt_crg
CRG Patreon - www.patreon.com/user?u=13919007
CRG Discord - / discord
Chapters
00:00 Intro
01:25 Diagnostics
02:27 Testing the ROM
04:43 A look at the schematics
05:29 Thinking about it
06:17 Logic Probe
07:32 ROM pin 15?
08:23 Physically broken?
10:47 Found the problem
13:08 Fixing it
14:20 Is it fixed?

[@Momentvm]

Glen, your videos are much needed freshness in the topic of retro repairs. Straight to the point, no nonsense stuff that we need. Lovely job!

[@CRG]

Thanks, this one was a simple fix but sometimes it nice to get a straight forward problem.

[@electronicengineer]

Great find on the broken data line trace and nice technique in repairing it. Fred

[@CRG]

Thanks Fred, I did consider removing the socket to just repair the trace in situ but just handier to just install the wire.

[@datriaxsondor590]

Logical path tracing method to find the break. Nicely done as usual, bud. Another beauty retro system repaired. 🍻

[@CRG]

Thanks. Where possible I like to try to think out the issue before jumping in to replace things and the spectrum is simple enough that we can logically work it where the problem is.

[@josephphillips9243]

CRG other channels would make hour plus long videos with operatic levels of melodrama and lots of exaggerated facial expressions. Yours is a genius quick and methodical diagnosis and fix. Nice, thank you and I've learned a lot from this video.

[@CRG]

Thanks, kind of you to say so. Sometimes a repair can lead you down a path but this one just worked out nice and simple.

[@pe1dnn]

Back in 1985 I didn't have an EPROM eraser so I put them without anything covering the window in direct sunlight for days on end but not a single bit flipped. They are quite resilient and just absolutely do not get erased that easily. You really need harsh hard UV light to do that. Maybe you could repeat this experiment yourself, it it quite hard to get a single bit flipped with normal light, nearly impossible. Just burn one and keep it in the sun all summer, I bet it will keep on verifying okay.

[@KarlHamilton]

It's mad how simple those scematics are compared to the stuff today.

[@CRG]

Yep, even compared to the next iteration of 16 bit machines, the spectrum here is very simple.

[@SpaceHero-Elite]

Love the pose in the thumbnail :)

[@CRG]

Deep in thought

[@stevvieb]

I had a some Amiga kickstart EPROMs with NO sticker on it's window sitting on my windowsill for almost 30 years and they still read ok. Oh This is in England though so not the sunniest of places.

[@CRG]

Not so sunny here in NI either but was worth testing to be sure.

[@WhatHoSnorkers]

Fantastic work, and it's fascinating how sensitive electronics are! Marvellous!

[@CRG]

Thanks Snorkers, always satisfying to bring a spectrum back from the dead, even if it seems it was me who killed it in the first place.

[@WhatHoSnorkers]

@@CRG I blame your evil alter-ego!

[@amanvasamantino5368]

great work

[@CRG]

Thanks

[@nickolasgaspar9660]

The heat of the desoldering process might have affected an(probably) already doggy trace. Pressing the CPU back in the socket might broken it.

[@CRG]

Possibly, I never thought of that but I guess you're correct. Maybe just need to be a little more careful in the future. Although I could have swore I tested it after fitting the new CPU but maybe not.

[@stevetodd7383]

A quick niggle, the silk screen layer is only where you see printed lettering captions on the board. The conformal coating is probably what you were thinking of, the usually green insulation layer covering all but the solder pads on a modern circuit board. This is the layer below any silk screening.

[@CRG]

Yeah that's the one, just a slip on my part.

[@AttilaAsztalos]

a.k.a. "solder mask".

[@stevetodd7383]

@@AttilaAsztalos yup, that’s the common name for it.

[@tenminutetokyo2643]

New Z80's are $17 on Mouser but they're out of stock right now. Also be aware there are 2 CS lines on the ROM and EPROM and they are different depending on which one is installed.

[@CRG]

I have a mod done to the Spectrum to use the eeprom and the bar you see linking pins 28 and pin 1 is feeding 5v into pin 1 to select that particular bank in the rom. Not essential since I have the same rom image in all 4 banks but better than leaving it floating. You could actually fit a switch and change between the different banks to select different rom images. I might look at doing that.

[@gsestream]

you said it yourself, its yours.

[@CRG]

My precious...

[@gsestream]

@@CRG indeed

[@peddersoldchap]

9:36 shouldn't the red trace go to the resistor R12 instead of R13?

[@CRG]

Well spotted, it should be to R12 as you say.

[@peddersoldchap]

@@CRG Thank you for clearing that up. I am not very experienced, and that was confusing me.

[@VEC7ORlt]

Why are you asking us? Should I look in the crystal ball before clicking the vid?

[@CRG]

Posing a question to the potential audience drives interest, why is it faulty?, click on the video to find out. Could be called clickbait of a sorts but its a flooded market and us creators need to do all we can to get people in the door. It certainly seems to have brought you here.

[@VEC7ORlt]

@@CRG called clickbait? Nah its just clickbait. So the answer to a flooded market is to add more clickbait? I wonder when the pendulum will swing the other way and actual informative titles will take over again instead of this dumb shit.

[@CRG]

But is it dumb... I'm posing a question, the same question I posed myself when I discovered it was faulty. I suppose I could have went with the title "I fixed my Spectrum" or "I fixed a broken trace" which ultimately is the result of this 15 minute video. In that case I could have made this a 30 second video and just showed the repair but then there is no story, no process of discovery to understand the fault and maybe help someone diagnose their own faulty machine. The true definition of clickbait is a title or thumbnail that promises something but does not deliver. That's not the case with my video or at least I hope not. I posed the question, then answered it. It could be argued it's clickbait but then again, isn't everything. Your comments are obviously written in a tone to try and drive interaction from me. Some might call that a troll but I wouldn't go that far . Ultimately no one is forcing you to watch the video, yet you clicked on the thumbnail and title as is. From your comments I can only assume you were disappointed with the content?

[@VEC7ORlt]

@@CRG its mild mockery of the recent (hah, been going on for years) trend of using useless red arrows pointing at obvious things, mind numbing titles, posing stupid or yes/no questions (law of headlines) - like come on man - show some finesse? How about 'A curious case of a broken trace inside my ZX Spectrum'?

[@CRG]

I actually really like that title and if we'd have had his conversation before it went live then I absolutely would have went with it. The overarching problem you describe is very evident through KZbin and lots of folk take it to the extreme with AI artwork, anime girl poses and other obviously fake nonsense in their thumbnails and I don't like the use of that as it does nothing to draw my interest but that's what its all about drawing interest. You get but only a few seconds of a person attention span to them to see the thumbnail read the title and make a decision on if to watch or not and in our group of KZbinrs posing a question is certainly one way of getting the people interested. I'll have a deeper think for a title for my next video which features the backplane you may have seen at the end of this video.

[@DimasFajar-ns4vb]

ben heck hacks

[@CRG]

He certainly does.