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!

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.

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

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.

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...

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.

oh the nostalgia, just need to spend 2 hours tuning the RF into your black and white portable TV

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

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

Ah, my first computer. I remember playing Monster Maze for the first time....and I actually remember it scaring me when the T-rex popped up. :)

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

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

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.

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

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

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.

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.

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.

Though I never had the zx81 (I went for the powerful 48K Spectrum... Not sure what to do with ALL that ram) but mates did have the 81 that did trigger my initial interest in home computer.. so many good memories triggered by this video. Thanks 👍

There's a way to get 32K RAM working in the ZX81. A fellow British KZfaq channel, The Retro Shack, posted a video on this back in September of 2021. He provided links for the information as well in that video's description.

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

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

I got a package from Germany last year and the packing peanuts were giant Wotsits (but without the cheesy coating)! I suppose they're quite environmentally friendly compared to the normal polystyrene sort, being biodegradable.

Nice detective work!

There are ZON X-81 sound generator and Chroma 81 interface for use with ZX81. Great video as always.

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.

Nicely done and another ZX81 lives to fight another day! Now, try using it stock with a 16K Ram Pack for the ultimate, infuruating experience!!!

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...

Somebody made the classic mistake on that rubber mains plug: they forgot to slide the hood over the cable before screwing the conductor wires, and rather than undo them, they chose to cut the plug housing instead.

My old Speccy PSU was 15 Volts also. Even if the Regulators can handle it, the more volts dropped = more heat ? Yes ?

The good ol' ZX81 was the first "real" PC that I put together as a kit WAY back in 1981 I believe! I even eventually got the 16K expansion pack that was a pain in the @ss to keep connected sometimes if you jiggled the ZX around too much. There was something I did to fix this but I just can't remember anymore. I even cut out a much larger aluminum heatsink for the 5 volt voltage regulator as the small one supplied with it could often overheat and shut down especially if it was a hot summer day! I loved that PC so much that I just wore out the "cheapie" membrane keyboard eventually. This was something well known to happen back then. I was getting ready to possibly buy a better "real key" style keyboard made just for it, but I got a FAR superior Commodore 64 instead! I still have that ZX81 buried somewhere in my home along with all of the software, books, and personal basic programs I wrote for it. Good times they were! 👍👍 PS: Damn, I just remembered that all of the tape software I have for it is probably in my storage closet that get EXTREMELY warm and humid in the summer! No bueno! I have to find it and see if it even works anymore.....😞 PSS: Nice videos you have here! I am subscribed! 👍👍

It's time to spare more Z80s! :)

I enjoyed that - glad it was the CPU and not the ULA. Out of interest, where do you get your basic components? For the likes of Digikey to make sense I've got to spend a whole lot ...

"tip" = tape, "pen" = pin, "bahd" = bad, "kiss" = case, "wackee" = wiki - scottish is so much fun!

now to find which kaleidoscope iteration gives QR code for some ZX81 site.

I find with tape player apps (I use Tapdancer) I get best results if I put my phone into aeroplane mode. I suspect it cuts out any interference caused by the WiFi, Bluetooth, RFID, etc.

nice time spending Sir

I think that 'resistor' was actually a capacitor. You'll see them in the ZX Spectrum too de-coupling the RAMs.

Hi there. Did you get round to using the Oscilloscope with any 16 bits? Was it still viable for the Amiga?

hi I have a zx81 with lots of games on cassette tape ,I also own a zx80 which is white ,I have had them stored away in my loft which 4 other early computers from my childhood,I kept everything including the boxes ,zx80 was my first computer I was 8 ,Then I got a zx81,followed by the zx spectrums then the spectrum 48k then the 128k version. Then I went to comadore then to the amiga 500 and the the Amiga 1200 .After that I started getting windows PCs. I have them all in great condition with there original boxes plus alot of games for them all kept in great condition,I don't know how I kept them in great condition after I used them alot .I left out my Atari with the card slot on top I think it's a 800 model.I also had a lot of copied games on cassette too I remember spending hour tweaking the cassette head and tuning the volume level just right so I could copy my mates games.I have the 16k memory upgrade block that pushed into the back of the ZX81 plus I have chest cartridges the plugged into my other systems it was called a action replay adapter .I would love to donate them to a computer museum. I will never play them again and my kids are to old to play with them ,I kept it all in my mum's house where I grew up ,She still lives there 54 years on,there are still boxes there full of my old toys my big track was a programmable space buggy with 6 wheels you could program it to move forward 8 feet then turn right 90 degrees them forward 2 feet and it would follow you commands.I think I will look into rehoming them ,I also have 3 brothers with there stuff there too but unfortunately one of my brothers died from cancer in 2018,he has 2 daughters who's not interested in his matchbox cars and his action man collection. If I sell ,And if anyone is interested in my computer or early toys contact me

Interested to see a test of the oscilloscope! If it says "120 Mhz" that's supposed to be the bandwidth right?

red poska pain pen to restore that logo.

It seems you removed my remark on ZX-Wespi, which is simple storage for your ZX81. This was not Spam. I was just trying to help you.

My issue one had resistors where caps were needed even in the clock circuit

for a 7805 you'll need to power it at least 1volt above output voltage, when loading the "tape" it probably just resets. due to starvaation, unless you set it already to a higher voltage... which in that case i missed

Happy my father bought the C128D, and later Amiga 2000, i loved the Amiga 2000 the most when i was a kid, Amiga computers were fantastic back then.