Appraisal of my ZX81's worthiness?

[Monty]

Hey all! I scored this ZX81 from the local classifieds for £15 and before I proceed any further, I'll need the wisdom of the experienced.





The case is loose due to at some point, a couple of screws being removed and subsequently not replaced - which made me immediately conclude that someone has been inside at some stage. I don't know the history of the machine in this respect: the seller is a teacher and received the Zeddy as a gift from a student whose son had tragically passed away - but it had then remained in a cupboard for years. It's missing the PSU so I haven't been able to even see it if works yet.

Here's some shots of the motherboard. I didn't venture any further than what you see here, especially as I was wary of damaging the keyboard - as I've seen highlighted in so many YouTube videos. The underneath of the edge connector appears to be quite badly worn (the top side however, is pristine). Will this prevent a ZXpand from working and if so, can it be easily rectified? I'm out of practice with soldering but I can re-learn and I already have a Weller PU-2D on hand.



What do you think of the general condition of the motherboard? I think there's signs of dry joint.









Will I have to carry out some soldering work to the board before I even power up the Zeddy?

On that note, I have a number of PSU's that output 12V at 1.5a - could these be used a stopgap for testing whilst I check for/solve problems? I wouldn't want to cause any/further damage to the computer!

I'd appreciate your advice on this.

[Lardo Boffin]

Looks nice and £15 is a bargain! If you use another PSU make sure the tip polarity is correct. I have bought PSUs from here before:

https://www.oldskoolconsoles.com/produc ... wer-supply

They are the same as Atari 2600 PSUs.

Dry joints are not uncommon on Sinclair stuff. I have seen some joints on 16K RAM packs so dry there is no solder!

This is a RAM pack and amazingly it worked before I resoldered it.

The edge connector may clean up a bit - it looks quite tarnished. I use an eraser (pencil but apparently you can use pens erasers as well) and IPA as it removes a lot of murk without being too abrasive. I would be surprised if it didn’t work after a clean but it may require careful positioning or at least plugging in and unplugging a few times.

You can buy case screws on Sell My Retro / eBay or you can try and find some that fit.

Zeddies are fairly robust in my experience (I have bought one or two) and to be honest aside from checking for obvious physical damage I tend to just plug them in and switch them on.

I do check the PSU before using them though - out in the garage, on the concrete floor, switch on and test with a multi-meter before it goes anywhere near a zeddy.

[1024MAK]

Apart from the edge-connector, it looks about normal for a Zeddy.

The ZX81 (without a RAM pack) will run on any smoothed DC voltage between 8V and 15V as long as the polarity is correct. The tip is positive, the ring is negative.

However it’s not recommended to use it continuously (more than half an hour) at voltages above 10.5V as the heatsink on the 7805 voltage regulator will get very hot.

The only check that is normally needed before powering up, is to check that the M3 machine screw holding the 7805 voltage regulator and heatsink to the board is nice and tight.

Mark

[Lardo Boffin]

Mark makes a very valid point about the voltage regulator heat sink screw. I always replace the voltage regulator and don’t leave them switch on for long in their original state.

[Monty]

Thanks for the replies!

I'm going to order this PSU https://www.amazon.co.uk/gp/product/B07 ... _qh_dp_hza- it supplies up to 1000mA, which will be more than sufficient if I decide to add peripherals and the multi-tip and multi-voltage option seems like a great bonus for £10.49. In the meanwhile, I'll scour through the local pound shops for a coaxial cable in order to sort out the RF connection.

Hopefully the Zeddy actually works when the PSU arrives!

[Monty]

The PSU arrived on Thursday and after taking great care to select the correct voltage and polarity, I plugged it into the Zeddy, hooked up an RF cable to my TV and for the first time in many years, manually scanned for an analogue signal...

Hovering around Channel 36, I glimpsed the faint signs of the iconic K and a white screen underneath a distorted and unstable image. I think it's safe to say that my Panasonic plasma cannot lock onto the Zeddy's signal. My Polaroid LED TV fared even worse and failed to detect anything. My remaining TVs to try are a 2000 20" Samsung CRT and a Cello 4:3 LCD portable. I started with the latter and fiddled with the settings.

Using the manual fine tuning option garnered success: I located the Zeddy's UHF frequency!



Next, I tested the keyboard and remarkably for a machine of this age, it appears to be working fine.



The TV struggled occasionally to maintain the signal but for RF quality, it was better than I expected. At some point I'll tackle the composite video mod and if there are any test programs or commands that I should run, please let me know. Also, what would you recommend for upgrades? The ZXpand etc? I'm quite excited about seeing what fun I can have with the Zeddy!

[1024MAK]

Auto tune is often mostly harmless but also completely useless for getting a modern TV tuned in. Where possible use manual tune and fine tune. Unfortunately some TVs will only lock on to signals that are at, or very near to one of the ‘correct’ TV channel frequencies. Because the modulator in a ZX81 is completely analogue and does not use a crystal oscillator, the frequency can be in-between the official allocated channel frequencies...

Well, there’s not many test programs for a 1Kbyte machine. You can type in this to discover how much RAM your Zeddy thinks it has:

PRINT PEEK 16388+256*PEEK 16389

Result
17408 = 1k byte of RAM
18432 = 2k bytes of RAM
32768 = 16k bytes of RAM

A normal European ZX81 should report 17408.

Then, if you can sort out a suitable audio lead (mono 3.5mm jack plug at the ZX81 end, plug into the EAR socket), a suitable plug for your audio source at the other end (a stereo 3.5mm jack plug {see note} if your audio source is stereo for example), you can load some of dr beep’s 1K games to play

Notes: modern audio devices are nearly always two channel stereo. But the ZX81 is mono. Any leads or adaptors should only use one of the stereo audio channels (by convention the left channel). If the leads or adaptor just connects the left and right channels together, this often messes up the signal (and the ZX81 won’t load anything) and in some cases may damage your audio device. A cable like this is what you want (3.5mm STEREO Jack Plug to 2 x MONO Jack Plugs SPLITTER Converter Cable Lead 1m).

The ZX81 was designed for use with shoebox mono cassette tape decks, which can output a loud signal with a relatively large amplitude. With the vast majority of modern audio devices, a lack of signal amplitude will result in the ZX81 acting as if it is completely deaf. So you may need to use an audio power amplifier. You need one with a headphone output. Cheap mains powered PC speakers are often okay for this. Or if you can wait for a cheap module from China, a LM386 audio amplifier from eBay (e.g, like this one).

Mark

[Lardo Boffin]

If you are just looking to load games reliably and fairly cheaply try one of these:

https://www.ebay.co.uk/itm/254080943285

Doubtless there are others but I have bought a number of these (and bundled them with refurbished zeddies) and found them to work well and the seller is very helpful.

It is no faster than tape but very reliable and you can download and run ‘most’ games on it. I haven’t quite figured out how multi-part loading games work on it yet but these are rare-ish.

[Moggy]

Hi Monty.

The composite mod is a must I feel, as is the Zxpand+ which is more than just a storage/memory device.

Things I have done with my ZXpand+ include AY composition done with a tracker program and software for playing AY (3 channel built in) add on daughter board for 6 Channel, GM MIDI file player,PC text sent via serial/usb connection to to ZXpand+ with a connected speech box(Cheetah) for a speech program without having to type allophones manually.
Also the ability to save/load to and from memory blocks make it a very useful device I find.

[mrtinb]

That's a very nice tv picture for the RF-signal. It looks as nice as composite signal.