ZX Spectrum - spares or repairs, let's fix it. Part 2.

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Following on from last weeks video its time to dig into the ZX Spectrum and see if we can't get it fixed. By the end of part 1 we had it so the machine would at least turn on all be it the display was corrupted so will it be low ram, will it be upper ram, will it be logic or will it be all 3 and perhaps more...
ZX Spectrum Schematics - spectrumforeveryone.com/techn...
Z Spectrum Diagnostic Rom - github.com/brendanalford/zx-d...
end theme - A Prehistoric Tale (1990) main theme by Jochen Hippel
CRG twitter - / yt_crg
CRG patreon - www.patreon.com/user?u=139190...
Chapters
00:00 Intro
00:18 Initial testing
04:21 Removed the Upper RAM
05:06 Removed the Lower RAM
07:07 Something's missing
12:20 Making a diagnostic ROM
18:58 Testing the diagnostic ROM
22:54 Where has the sound gone
26:00 The new lower RAM has arrived
27:48 Testing the lower RAM logic
31:06 Testing the upper RAM again
33:27 Who's a stupid boy then
34:42 Lets try that test again
36:18 ROM?
37:06 It works!
38:01 Conclusion and credits

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23 июл 2024

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Комментарии : 100

@radio-ged4626 Год назад

Very honest video. So many edits could have been done! LOL. On one hand you would think the ZX Spectrum a great example for learning how to repair PCs, but with Sinclair's money saving ideas there are many pitfalls. No mask on the top PCB tracks, unprotected circuitry vulnerable to power failure, and transistors too ready to let go and kill all your hard work. No wonder all the ICs get socketed. I think improving it is too easy ( e.g. removing 5v regulator and using good quality 5v supply, the RF to component mod is probably needed nowadays to be fair) But where's the fun in that? Better to struggle through and really learn how it was to use and repair early home PCs. Great job, thoroughly enjoyed watching this.

@johnbarleycorn_ 2 года назад

The 4116 chips are a pain in the backside. They very frequently fail and their dependence on three separate supply voltages is a nuisance; if the voltages aren't right or come up in the wrong order, the chips will give you trouble. A very simple mod that I would highly recommend is replacing the 4116s with 4164s. They are pin compatible apart from pin 1 (NC) pin 7 (5V) and pin 8 (A7). The heavy trace directly below the lower RAM chips is the +12V line and connects to the lower electrolytic capacitor to the right. Close to the capacitor, cut the rail and bridge it to the 5V line (the next heavy trace down). You can then fit 4164 chips which only need the one voltage. Yes, you are only using a quarter of the chip, but they are a little more reliable! The other helpful mod is to replace the linear regulator with a switched one or a power buck and you can be rid of the heatsink.

@CRG 2 года назад

I had originally wanted to just replace all the 4116s with one of those new RAM boards which only take 5v. The only place I could find them for sale was retroluem but the guy running it was away so decided to just go with the 4116s. Changing to 4164 would be a good idea too as ultimately I'm sure it's only a matter of time before that transistor blows again (or something else) I loose only of the voltages and find more fired ram.

@johnbarleycorn_ 2 года назад

@@CRG That's another good option. I've seen the schematics for the same idea and it's basically putting in a 62256 (or equivalent) with a 74LS541 to latch the upper part of the address on the RAS cycle. Like the 4164s, gets you away from the multi-voltage setup. Unfortunately for both solutions, while the -5V supply becomes redundant, 12V is still needed for the LM1889 to run the video output. Removing the 4116s will reduce the load on TR4, but it's worth making sure that you've a known good alternative part installed.

@GodmanchesterGoblin 2 года назад

@@johnbarleycorn_ LS541 isn't a latch - it's a tri-state buffer. I think you meant LS573 which you could used to latch the row address on the falling edge of the RAS\ signal (low active)

@miked4377 2 года назад

wow!! i bet you werent expecting such a ride like that!! but the bottom line ..you did it! great job crg!!

@CRG 2 года назад

Yeah I was thinking drop in some new ram and that'll be it sorted. I really should have tested the address bus at the start but when I seen the data bus all screwed up it took me down that path. No harm though, it was fun to learn how the faults manifest on this system. If I ever get another one maybe it'll be easier to diagnose and repair.

@miked4377 2 года назад

@@CRG 👍

@Xoferif 2 года назад

Always great to see a classic machine brought back from the scrap pile! Good work. 😃

@CRG 2 года назад

Thanks. It was a bit of a learning curve but all part of the fun.

@KolliRail 2 года назад

Well done! Great video. It's much more interesting to see you fail when troubleshooting than if all goes right first time. That way we can learn the lessons together. ;-)

@CRG 2 года назад

Thanks glad you enjoyed it. The failures or mistakes are all part of the process. As you say we all learn from it. Hopefully if I ever get another speccy it'll be easier to repair.

@PaulinesPastimes 2 года назад

Oh, it's all a learning curve isn't it. It's so easy to get blind sided and go down a rabbit hole. It's great you got it working in the end and intrigued to see what it can do. 😊✔

@CRG 2 года назад

It very much is a learning curve and perhaps if I had have checked the address bus first the repair might have got completed faster but that's all part of the fun and in the end as you say its fixed. Once I get the parts needed for the dandanator I'll come back to this machine and show it running some software. Its basic compared to its peers of the time but does have a charm to it. It'll be a while though, got a few other projects that I want to get off my desk.

@WhatHoSnorkers 2 года назад

What a fascinating journey! The Future is 8 Bit sell a divMMC device that hooks into the interface slot at the back. You can snapshot to it , load memory images and load/save to tape too. The tokenised keyboard is great as typing a LOT on the rubber keys would not have been fun!

@CRG 2 года назад

Thanks, it certainly took me on a journey but I learnt a lot so hopefully the next one will be a bit easier. I'll take a look at the divMMC, thanks for the tip.

@WhatHoSnorkers 2 года назад

@@CRG I've got one which I use with my ZX Spectrum +2. Easy way to load tapes, snapshot, and then use the MicroSD card in the PC!

@MoreFunMakingIt 2 года назад

Brilliant fix! Well done Glen 😁 The missing solder on logic chip is exactly what I did on the upcoming c64 repair I did. A real Doh! moment 🤣

@CRG 2 года назад

Thanks Lee, glad to know I'm not the only idiot missing solder points 🤣. Look forward to seeing what you do with the 64. Always enjoy a commodore video.

@Imperious685 2 года назад

Thanks for the videos, always enjoyable and informative. I would like to point out though that the diode test is measuring volts not ohms. So yes 0.2v is way too low for that transistor for either Base to Collector, or base to emitter. There are diodes that can be ok that measure that low. It's always easier when faulty parts measure short or open.

@CRG 2 года назад

Good point 😆. You knew what I meant though, that's the main thing.

@electronicengineer 2 года назад

Very good information Imperious685. Most germanium transistors will forward bias at 0.200 volt - 0.300 volt. Also Schottky & some high speed switching diodes (SMPS rectifiers) will also hover around 0.180 volt - 0.280 volt. Reverse bias should always read open/infinity though. Fred

@UpLateGeek 2 года назад

Perfect timing, I just hit refresh on the homepage looking for a video to watch in the background while I work on my PCB layout, and this was the first suggested video!

@CRG 2 года назад

Good timing on my part 😂, hope you enjoyed it.

@bufordmaddogtannen 2 года назад

Congrats on the fix. May I suggest investing a few quids on a components tester? It may save you a few headaches when buying replacement chips. I would also suggest a memory tester for when you work on other micros which use ram chips that are fully working (since the spectrum uses defective ram by design).

@CRG 2 года назад

There are a few bits of test equipment that I'd like to get and a component tester is certainly on the list. I'd like that one you see Adrian use which can test various different ICs, just can't remember the name of it right now.

@RetrogradeScene 2 года назад

I really enjoy the pace of your videos makes it easier to follow along as you try things

@tailzer42 Год назад

Enjoyed this vid and subbed, but can I just point out that when you test in diode mode, it’s volts not ohms, what you see is the forward bias voltage that the junction switches on at. So 0.7 is the junction switching on at 0.7v one way and as you say it shouldn’t the other way. Minor point but you are spot on for most things and people learn from these. Cheers

@paulcollins5334 2 года назад

Probably the best spent 40 minutes in the last month for me. Absolutely loved watching your Speccy repairs and am looking forward to more. Superb indeed!

@CRG 2 года назад

Thanks, glad to hear you enjoyed the video 👍

@nekelly123 2 года назад

Very informative vid, thanks 😊

@Ghost82uk 2 года назад

it was an epic battle with the little rubber keyed hero, well done mate

@CRG 2 года назад

Thanks, it took a bit more work than I was expecting but learnt a lot along the way. The next one will be easier, maybe...

@temporarilyoffline 2 года назад

Strong work Glen! I just did a video on "Temporarily Offline Retro Tech" channel on loading "tapes" from wav files. I bet that same trick would work with your Speccy!

@zero0ryn 2 года назад

Wow! What a journey, I would have given up by now, but your persistence has paid off. One tip (When using a DVM on diode test the reading is in Volts. The meter puts out around 2V through a series resistor then measures the voltage across the probes. You can actually use this diode test to light an LED.)

@CRG 2 года назад

Thanks. Yes I'm an idiot calling it ohms when in fact the diode test is in volts. Live and learn as they say... Noted for future reference 🙂

@skeptic10 4 месяца назад

You helped to get mine fixed as well! I tried the all RAM out and try them one at the time as mine has 6 socketed. I had replaced 74LS10 and the ZTX650 transistor already and the voltages were good. I couldn't find any bad RAM but when I put all the RAM back and tried to power it it booted to the Sinclair screen! So maybe some RAM just weren't seated properly. So one of my two ZX Spectrums is now alive. I probably have bad RAM in the other but I have to get back to it later.

@thegreatjonzini 2 года назад

Wonderful stuff! Congratulations 🎉

@CRG 2 года назад

Thanks

@MrLurchsThings 2 года назад

Nice work. For such a simple machine, they can really give you grief. Did you ever see Making it Lee’s disaster machine? 😂

@CRG 2 года назад

Thanks. Yes I've seen Lee's videos on the disaster Spectrum. He showed up in the first part of this video to give me some pointers. Plus the poor mans been tortured with countless questions while filming this part 😂

@PJBonoVox 2 года назад

Oh man, you really made a rod for your own back on this one 😂 Good perseverance and you got there in the end. Nice work!

@CRG 2 года назад

Thanks. You know me by now, there's no way I'd make it easy for myself 🤣

@GadgetUK164 2 года назад

Great job =D Regards the transistor you fitted - you may find it failed because its out of spec regards the original transistor - they all have different characteristics. eg. base current, base bias, collector current, base emitter voltage, base resistance, gain etc etc.

@CRG 2 года назад

Thanks. I was pointed to that S8050 diode as a suitable drop in replacement but checking the datasheets (which I should have done originally!) the spec does differ a fair bit between them. Think I'll need to order something more suitable as a replacement or risk it blowing again and loosing more lower ram.

@GadgetUK164 2 года назад

@@CRG The diode might be OK, more likely the transistor! I would hate for it to fail again and kill your new RAM

@LynxCarpathica 6 месяцев назад

Correction: 8:43 on the multimeter you are measuring voltage drop of the "diodes" (transistor in this case). That's not ohms, but milivolts. Silicon diodes have a voltage drop of around .7V

@johnnycats5157 2 года назад

great video!

@CRG 2 года назад

Thanks!

@Supercruiser5000 Год назад

Lol. 10 for tenacity. Enjoyed this. Poor bastard. Good job though. You have an Adrian Black Zif Speccy at least.

@rogue_soul 2 года назад

What an ordeal. Well done on getting there in the end.

@CRG 2 года назад

Thanks, just happy to have a working speccy.

@georgehunter2124 2 года назад

A wee bit of a struggle but you got there, well done Mate

@CRG 2 года назад

Thanks. It took a bit more than I was expecting but at least with that we had the opportunity to learn more along the way.

@Plan-C 2 года назад

Great video. It certainly led you a merry dance. Nice fix.

@CRG 2 года назад

Thanks, all part of the learning curve. If I ever get another one it'll be easier... maybe... probably not 🤣

@Retrohertz 2 года назад

I enjoyed that. I fixed a few of these and they can be tricky little beasts. But that's half the fun, right. ;-) I bought a dandanator card and it has ZX diagnostics onboard, so there's no need for a separate rom chip.

@CRG 2 года назад

Thanks, glad to hear you enjoyed it and absolutely the more twists it throughs at you the more fun the repair is, even if I was a bit deflated after fitting the new RAM although it certainly was a lesson to check those 157's first! The dandanator does look like a great option and I'm looking into getting parts to build one myself. In the mean time though its been suggest to try an app on my Android phone that will load the games. At least that will get me up and running.

@ianneill9188 Год назад

Cracking video! I really enjoyed your journey of discovery. I wondered if you ever confirmed if all those lower RAM chip were really faulty? I see from another comment that you did. Only 1 bad. Those faulty 157s really threw you a curved ball!

@CRG Год назад

Yeah only one bad but at least I've got loads of spares now 😆

@JamesPotts Год назад

It's not perfect, but I really like the Retro Chip Tester. It has saved me a lot of time, being able to test various ram and logic chips.

@johnriding3217 2 года назад

That was a mission!!!

@Lee-wl4kh 2 года назад

Glen you have 3 choices for loading games 1. TXDuino whech is basically a digital tape player 2. Dandinator which is like the 3rd option the divMMC 3. divMMC which someone already mentioned.

@CRG 2 года назад

Thanks for the tips. The dananator looks interesting and certainly more affordable than the divMMC. I think in the first instance I'll maybe just use the digital tape player option then in the future I might look at putting together a dandanator.

@Lee-wl4kh 2 года назад

@@CRG TXDuino is the choice I chose for now then like you I may get the Dandinator later on.

@CheshireNoir 2 года назад

Don't forget PlayZX straight from an old Android phone. That's what I used until I got my divMMC.

@kon-stan-tin 2 года назад

You should set trigger on your scope to ~2,5V to correctly display periodic signals like clock at 2:22.

@tenminutetokyo2643 Год назад

BTW you can replace the 7805 and dump the heat sink by using a TSR-1 2450.

@nickolasgaspar9660 Год назад

Great video and very helpful! Did you manage to verify your theory on those RAMs? Did that 7th RAM was the only one working from the bunch? I just got a ZX Spectrum that only worked for an hour and It would be nice to know if that pattern is indeed a way to test those RAMs. Since I don't have an IC tester and putting a bad RAM on an other Spectrum is a bit dangerous , checking them the way you did sounds like a good way to start.

@CRG Год назад

I did but believe it or not there was only 1 bad and 7 good, at least I've got plenty of spares. I still think that testing as I did, if the address logic had have been good, would show something different on screen so is worth trying. Ultimately though with my address logic bad my test didn't prove much.

@bitcoinbusker Год назад

DivMMC Future is the way to go.

@damianbutterworth2434 8 месяцев назад

Great. I`m just got into buying EBay computers to repair for fun. It`s quite addictive. Plus it`s a better return than a pension.

@MrFixiit 2 года назад

wow that was a jouney and a half but great to watch i beliexe noels retro lab uses some kind of tape cassete emulator may want to check that out.

@CRG 2 года назад

Thanks, glad to hear you enjoyed it. I've seen those cassette emulators but I think the dandanator will be a great solution in the long run. For now though I've been told there's an app of the phone that will load games. Am going to try that first while I wait on parts for the dandanator.

@PJBonoVox 2 года назад

No video this week? It's an outrage! I demand my money back! 😄

@CRG 2 года назад

Yeah sorry, been working on an A500 board and it turned into more than I expected. It'll be done for next week, I hope 😬

@mariuszdobrzyn9603 Год назад

hello how to solve the problem with jailbars on rgb scart connector in amiga600?

@CRG Год назад

Has the board been recapped? Are you using a CRT or LCD? If LCD you might solve it by adjusting the pixel clock on the screen.

@TheSudsy 2 года назад

your table top looks exactly like the Spectrum display, just saying. lol. Desktop power supply "666" the spectrum is Damien

@mariuszdobrzyn9603 Год назад

i checked a lot of receivers and even on crt tv it detects jailbars...

@RETROMachines 2 года назад

@peddersoldchap 2 года назад

What scope are you using?

@bruce_just_ 2 года назад

check out the SVI-CAS digital tape emulator. it supports multiple computers besides the Spectravideo SVI in its name, including Spectrum, Amstrad CPC, C64 with an adapter cable. The Spectrum Show did a review video on it

@garyhart6421 2 года назад

There is a prog... called OTLA which runs games quite quickly from PC to Speccy. But I suspect you were thinking of something more SD-ish like = divMMC.

@tiemanowo 2 года назад

2:27 Are you sure that this is an Oscilloscope issue? On the scope preview I see that your Trigger level is on 0V (right yellow arrow with letter T) so it can't trigger properly.

@CRG 2 года назад

Adjusting the trigger helps a bit but I can never get this scope to give an accurate reading of anything over 1Mhz. Its only rated to 10MHz but then again does me ok for the £40-£50 it cost. I do want to get a better one at some point.

@tiemanowo 2 года назад

@CRG Only 10MHz? ooh. That can cause some weird readings ;)

@heilong108 2 года назад

did you ever actually confirm the original 4116s are bad after replacing the logic ICs?

@CRG 2 года назад

Yes although I ended up cutting it from the video to save time. Originally I thought there was maybe 7 bad and 1 good but in reality there was 7 good and 1 bad. At least I've got plenty of spares now :) If I had of tested the address bus at the start rather than going down the rabbit hole of the data bus it might have saved some time and effort but ultimately there was always a fault in the ram that needed fix so I suppose regardless I was ordering chips.

@tenminutetokyo2643 2 года назад

DOOD!

@tenminutetokyo2643 Год назад

You can get new replacement speakers off eBay.

@mariuszdobrzyn9603 Год назад

yes board recapped i use LCD and there is no pixel clock option in my LG tv...

@CRG Год назад

If you can try it on another screen to see if the image is better, if the jail bars go away. If they do then the issue will be the LG TV and there is little you can do about it. If not the issue might be within the Amiga itself. Unfortunately the A600 is not a system I am that familiar with but if you joined and asked on the likes of eab.abime.net/ there are people who can help.