Can I fix an eBay spares repairs ZX81 with an AliExpress Oscilloscope?

I finally got myself a new Oscilloscope but not wanting to spend to much money I found one on AliExpress for only £40. I also recently bought a ZX81 from eBay that was sold for spares or repairs so can I use one to fix the other?

Music - Cathode Ray Gun by @Momentvm

Chapters
00:00 Intro
01:02 First look and unboxing
04:57 PSU test
06:26 Opening the ZX81
07:13 Look around the ZX81 board
08:35 Powering up
09:55 Testing with the scope
13:06 Composite mod and thanks to @MoreFunMakingIt
19:22 What's on the screen
19:44 More scope tests
22:39 Down the ram rabbit hole
26:41 Swapping the CPU
28:52 Keyboard test and repair
32:12 Will it load something?
34:05 Ear input repair
35:28 Will it load something now?
40:01 Conclusion and credits

[MOS]

Hi Glen first time watcher. I generally hate YT thumbnails, but “Cheap vs Faulty” is fkn hilarious and an awesome setup for a video. A+ work mate, happy the algorithm recommended your channel. Cheers from Australia

[CRG]

So it seems I did a bit of a dumb in this one. The component that I accused of being a resistor on the earphone input more than likely was a capacitor.

Regardless though it had to be replaced as the old component was broken, or certainly broken when it came off. No harm done but certainly a lesson learnt on my part as I didn't know capacitors could come in that type of package!

[SockyNoob]

I'll definitely need to look into this scope and others like it.

[MoreFunMakingIt]

Cracking video Glen! It had everything, suspense, drama, the lot. A real hero's journey.

[desiv]

Great vid. One of the nice things about having an old CRT monitor around is that it helps you adjust the audio based on the size of the wavy bars when trying to load... (Which you can't see on an LCD as the screen just blanks)
I was pleased when my TZXDuino worked with the default volume level. (I was thinking I'd have to use a small amp...)
Yeah, did the 16K internal, composite mod, and replaced the 7805 with a Traco on my TS1000...

[mbunds]

Even when these were new, I hated the ghosting the Timex Sinclair produced on the TV through its cheap RF modulator, so I bypassed the modulator in the Sinclair, and the tuner in the TV. Nowadays, I just grab the composite feed connected to the RF moduator in the Sinclair and feed it to a composite-VGA converter.

Back when black and white glass tube TVs were still available, I would feed this composite signal directly to the composite circuitry after the I.F. section of the television.

[Edsshed]

With regards to the keyboard issues that you had, the zx spectrum keys are read in batches of four, thus hjkl and bnm, are all read together, that means that two of the connectors from the ribbon went connection ting correctly. Keep up the great work.

[MrLurchsThings]

Nice work. The internal 16k memory is such a great mod and that’s the way I went. As for the jailbars, heh, for a ZX81 that’s “good” image quality.

[envoycdx]

Love it, the sheer joy when the audio input works.. all good! :)

[kirkpreston]

I have fond memories of the zx81. I was shouting at the screen regarding the keyboard ribbon fix and the issue loading the game as the speccy had to have the right sound level to load games

[weepingscorpion]

Great video, Glen. There is a ZX81 in our attic. I should check if it still works but like you I may have to do the composite mod. And it wouldn't be tape loading without faffing about with the volume setting. :) But that KungFu cartridge caught my attention and as a Commodore kid with some Spectrum on the side I'm looking forward to seeing you build that.

[LifeSizeTeddyBear]

I suspect the trimming of the five-wire section of the keyboard ribbon might not have been necessary. When you tested the keyboard and found that it was not all working, there were two groups of five keys that were not operational -- HJKL[NEWLINE] and BNM.[SPACE] which suggests that it is actually two connections in the 8-way strip that were not making it through because of the way that the matrix of keys is organised. Most likely it was the strengthening and/or reinserting of that 8-way ribbon that actually got the keyboard to behave.

[miked]

your scope worked great!! thats surprising but hey youll take it! so would i!great job on the spectrum as well!

[RudysRetroIntel]

Very nice!! I have a Timex Sinclair 1000, which is the ZX 81, but sold in North America. I made my own board for the composite mod. Thanks for sharing

[Leos-World]

I enjoyed watching this episode. The ZX81 was my first ever computer I've owned.

[tenminutetokyo]

There is an eBay seller in Canada that sells a tiny composite board for $11. You can remove the rf box and solder on a gold-plated RCA jack to the board.

[asaprocky]

Nice troubleshooting. You have the patience of a saint. Kudos for getting it to work.

[Vanders]

Back in the day the official advice was to paint a bit of Tipex onto the volume & tone dial(s) once you'd found the exact settings that would actually work. I'm not sure how you do that on a mobile 'phone.

[Curt_Sampson]

It's worth putting a load on the output of old power supplies to see what they really supply when under load; I use 100 Ω 50 W resistor for this, which should draw about 150 mA at 15 V and correspondingly less at lesser voltages. 15 V doesn't seem particularly high for an unloaded nominally 9 VDC unregulated linear PSU.

Regardless, even 15 V into a 7805 is fine for a short time (as you noted later, the datasheet max Vin is 25-35 V) during which you can check the voltage when the ZX81 is running, especially when as well heat-sinked as that one, and fine for an even longer time if not a lot of current is being drawn from it.

One thing you did that you generally _don't_ want to do is apply a significantly higher voltage to the output of the regular than is on the input. Regulators are not designed to deal with being reversed-biased and can be damaged by this. The solution is simple: apply the regulated power from your external PSU to _both_ the input and output of the regulator, and at least it's not reverse-biased. Or in a case like this, you might just as well apply 8-9 V from the bench supply to just the input of the regulator, and let it do its job.

A couple of other points:

• That clock signal looked terrible. Terrible enough that I had a closer look at the 'scope screen and it looks as if you're using the probe in 1× mode. (The screen says "1X" and the VPP was around 5 V, which is the expected voltage level, so the probe must bet set to 1× as well.) That could be what's making the clock signal look so bad; it would definitely be worth trying in 10× mode. If the probe doesn't have a 1×/10× switch, that's something worth knowing as well, and a good reason to buy a different cheap 'scope.

• 4.6 V would be low for CMOS, but is a quite high "1" signal for TTL, where the minimum output level for "1" is generally 2.7 V, and you'll typically expect 3.3 V or so from many TTL chips. And the reason it's at 4.6 V is because, as you see moments later when probing the Vcc pin, your voltage there is 4.7 V, which is a wee bit low, though apparently not so low that things don't work (at least with the other CPU). Looks like something somewhere is dragging down the voltage from your PSU; it's definitely worth checking at the connection to the board to see if it's the cables and/or connection between the PSU and board. (Or it could also be related to the reverse-biased regulator sinking current; that's not something I've ever tried.) Or it might just have been the bad Z80 itself.

[grotsoftsadventuresinelect]

Just watching you go over the RAM. I bought the kit version of this back when it was released, and I as about 15. I tried to buy a 2K RAM chip, but my local supplier wanted £75!!

Also turned out that the thing didn't work. There was a trace missing to supply power to the ROM. Adding a jumper got it going. The kit was £20 cheaper than premade and Sinclair charged £10 to fix kit issues. Can't help thinking the kits were a faulty batch of motherboards...