ZX-81 composite video mod and various upgrades (Pimp My Eight, episode 4, part 2)

A composite video mod including one transistor and two resistors is applied to the ZX-81. Several upgrades are then performed: 3.5 mm jacks are replaced with gold-plated ones, the linear voltage regulator is replaced with a switch-mode DC-to-DC converter, the CPU is replaced with a modern CMOS equivalent sitting on a gold-plated socket, and finally the original RAM chips are replaced with a 32KB modern SRAM, which requires modifications to the board.
CHECK OUT www.thebyteatt... FOR MORE.
USED IN THIS EPISODE:
Modern CMOS Zilog Z80: Z84C0010PEG, Distrelec Article Number 173-75-439.
Flush-mounted, gold-plated 3.5mm jack socket : CL13845G, Distrelec Article Number 301-04-287.
Gold-plated, precision DIP-40 IC socket: DIL 40 EG, Distrelec Article Number 148-15-658.
Switch-mode on-board DC-to-DC converter: Traco Power TSR 1-2450, Distrelec Article Number 169-53-484.
32KB CMOS SRAM chip: Alliance AS6C62256, Distrelec Article Number 301-51-432.
MY KIT:
Weller WE1010 soldering station
ZD-915 digital desoldering station
Owon SDS1102 oscilloscope
Brymen BM235 multimeter
ZD-129A desk lamp with magnifying glass and 80 LEDs
Kontakt IPA spray can 600ml
Contact cleaner Kontakt S (step 1) and Kontakt U (step 2)
Aoyue 486 solder fume extractor
EP9993501 USB microscope
Ecobra 760 300 fiberglass pen
RS Pro (251-3637) no-clean flux pen
Jokaru Super 4 Plus wire stripper

[@halfacanuck]

I had no idea such unholy hackery was going on inside my innocent-looking ZX-81! I too started on it age 7, and have had my nose buried in software ever since, so I completely understand your desire to add all the bling-though I'm a little surprised you didn't also put in a golden keyboard. Next time!

[@jklein17]

On the TS-1000 model there is a channel select switch on the bottom of the machine. If you do the composite mod, shown here, this switch is no longer needed for selecting channels 2 or 3. I reused this switch as a power switch. The power switch mod requires cutting three traces and adding a wire. The switch is a two position switch with three pins. The pin close to the voltage regulator has a trace that must be cut and this will end up isolating it and making it the "off" position for the switch. Solder a wire to the center pin and to the negative side of the 1/8" power jack. The 1/8" power jack supplies the negative side to the board via a trace that runs directly out of the back side of the jack. This trace connects to the negative rail right above the voltage regulator and a resistor. You need to cut this trace so that the negative can only come through the switch via the wire that you installed between the jack and the center pin on the switch. One last trace needs to be cut but this time it's on the bottom of the board and connects to the center pin of the switch. If done correctly you will now have a permanent power switch to turn your computer on and off. ;) Keep in mind that this mod is reversible but since you are cutting traces it will be harder to reverse this mod easily.

[@richardburt4769]

The US ZX81 also has the 2 - 3 select switch.

[@data5634]

Thanks for sharing. I did the composite mod too on my TS-1000. I left the switch in place and use it as a reset button since it turns off the 5 volts but I may try your approach which would be even better since currently the 9 volts is still flowing. I also added a little led light to the switch so I can see that it's on.

[@vanhetgoor]

Ah, finally COLOUR! That became time the ZX81 got capabilities for games and fun! Now the ZX81 is no longer only for bookkeeping and mathematics.

[@john-do1cw]

Absolutely awesome video! So thorough and precise. I was lucky to recently acquire an '81 boxed... for free... and, according to the elderly gentleman that gave it to me, used maybe a couple of times from new (~May 1982) then put away for the next 40 years. I couldn't believe my luck! There was a crack across 2 strips on the keyboard connector (repaired by cutting off the bad section and removal of oxidation), some light corrosion on the expansion port contacts and sticky residue on the cables where they were touching the polystyrene box. All fixed with scissors, an eraser and lighter fuel. Apart from the box, it looks absolutely brand new. Crazy! 😮 Considering doing the composite mod, maybe the RAM mod too but not sure I want to go that far or just buy a 16k pack. What would folks here advise?

[@richardburt4769]

Thank you for this video and for your help, I now have my ZX81 Pimped and working.

[@thebyteattic]

Great to hear it!

[@robertdutcher8081]

You videos are excellent. Subbed!

[@thebyteattic]

Awesome, thank you!

[@markfarrugia8226]

One of the best ZX81 upgrade videos around. I wonder if there is any way you could do a similar job on my 2 ZX-81 machines. Any way i can get in touch with you ?

[@thebyteattic]

Thanks for the kind words! But this is just a hobby for me, I don't do this for payment; the day I did that, would be the day I lost a hobby. But I'm sure you can do it, the ZX81 is very, very simple to work on.

[@richardburt4769]

Great video, just assume. I live in Canada and I am having some dificulties locating the memory chip and the Z80 upgrade CPU. The numbers that I can source are AS6C62256-55PCN for the SRAM, and for the Z80A I can get Z84C2008PEG both from Digi-Key out of the US. The only BC337 that I could locate is from Amazon and the number is BC337B, any idea what the B is? Do you think these numbers will work? I have two ZX81 and a TS1000 that I want to get updated and working, but I want them to look as they are just NOS. Keep up the great videos. How about doing one on the TS2068, and not turn it into a ZX Spectrum.

[@thebyteattic]

Yes, the SRAM chip is the Alliance AS6C62256. The 55ns version you mentioned will work, as timing is not at all critical in a ZX81. But the Z84C2008PEG is NOT good; it's a PIO chip, not a CPU. You may use a Z84C0010PEG, a Z84C0008PEG, or a Z84C0006PEG; any of these will work, as they are capable of at least 6MHz clock, which is much more than a ZX81 requires.

[@thebyteattic]

Here is a list of transistors more or less equivalent to a BC337-40: alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=0.36&ucb=50&uce=45&ueb=5&ic=0.8&tj=150&ft=60&cc=20&hfe=250&caps=TO92 (the ones mentioned on the top of the list are closer to the BC337-40). It is important that you use a transistor capable of at least 200 or 300mA current between collector and emitter. The BC337-40 can tolerate up to 800mA. You should NOT use small signal transistors, such as a BC547.

[@MichaelDoornbos]

This is the best buffer explanation I've ever seen. Awesome job.

[@thebyteattic]

Glad it was helpful!

[@tenminutetokyo2643]

Can you do one on repairing the TS1000 power stage on the board?

[@richardburt4769]

Another question. I finally got the gold 3.5 mm jacks and noticed that they have four pins, but there is only 3 pin holes for each jack in the PCB. The jacks that you have, are they 3 or pins? Is it OK to just flush cut the one pin off? Take care. Cheers. Richard.

[@thebyteattic]

It depends on the pinout of the particular jack you have. Mine did have 4 pins and I cut one of them flush. I did know, in advance, that I could do that, given the internal wiring of my jacks. I'd advise you to look it up on the datasheet of the jack you have.

[@spinnetti]

Thanks, really great tutorial on the circuit!. So, this mod doesn't address the "back porch" issue? is it not needed? Funny you have donkey kong at the end of your vid. Me and a buddy just finishing writing our version for the TS1000/zx81 now lol.

[@thebyteattic]

Cool! The ULA I was using generates the back porch. So I didn't need to add circuitry to do it.

[@redhawk668]

I’ve done it with a NE555 timer chip to create the backporch, there is a very nice guide on zx.zigg.net. I also made a little PCB that goes into the modulator housing.

[@pdr0663]

Hi there, do you have a source for the heatsink on the ULA?

[@pdr0663]

Sorry wrong vid....

[@thebyteattic]

www.distrelec.nl/en/heat-sink-dil-40-15-fischer-elektronik-ick-40/p/17560244

[@eugenekiselev]

Hello mr. Kastrup, thanks a lot for the ZX81 video series! They inspired me on making the video mod for my Timex Sinclair 1000. The RF signal out if it was barely visible so I couldn't use it anyway. So I made something similar to what you did except that I used 2N3904 transistor. Also I installed 100 Ohm potentiometer between emitter and video output and empirically set it to somewhere around 30 Ohm because that was giving the most crystal clear pic. (I also repurposed the Ch2/Ch3 switch to Pal/NTSC switch via R30). It looks working more or less fine. However I can see some video artifacts at the last few lines: it looks like some garbage around letters. I made a 25seconds video for you in case you have time to have a look at this: kzbin.info/www/bejne/mJalf555qq-Le9U . Maybe you have an idea what could cause this effect ? Looks like some sort of interference (worth mentioning I didn't recap anything, didn't change the 5V regulator and I didn't clean neither CPU nor ULA contacts, not sure cleaning ULA contacts would help, but I have lack of experience with TS). Thank you :)

[@thebyteattic]

Not sure why you are getting those reflections/echos. Doing what you suggested at the end sounds like the indispensable first step before you even start thinking further about it! Note also that the 2N3904 can only pass 0.2A, while the transistor I recommended can pass 0.8A, 4x more. That doesn't explain your problem, but you are operating with very little margin here, and your transistor may expire at some point.

[@eugenekiselev]

@@thebyteattic Thanks a lot for responding! yeah gonna clean/recap and change the regulator. will see if that helps. Also thanks a lot for the transistor advise. Yeah it's kinda on a brink I agree I just didn't have anything else at that moment but wanted to try if the mod works. Gonna order BC337-40. Thank you ! you're the best !

[@eugenekiselev]

@@thebyteattic Finally reworked mine with use of BC337, replaced DC-2-DC converter, installed Zilog Z80 and 32x8 RAM and made the same memory mod you did. Unfortunately nothing of that fixed the video artifact I had. Then I ordered vLA81 replacement and that fixed it finally :). Also I managed to fix my RAM 16K Expansion (by just re-caping :) I couldn't believe that is gonna be that simple). Worth mentioning the keyboard replacement which consisted of two parts: the blank membrane and the TS1000 sticker ("Delete" instead of "RUBOUT" etc) Got fully functional TimexSinclair 1000 :) Thanks to your very useful Z81 series !

[@Alex_Valentine]

Is there a way to get these pieces shipped to the US? I'd love to give this a try on my TS 1000.

[@thebyteattic]

All components you see me use can be sourced in the US, from places like digikey.com and mouser.com.

[@Alex_Valentine]

@@thebyteattic thank you!

[@redhawk668]

Fun fact, those two RAM chips on the ZX81 are in fact 2114 SRAM chips, the Commodore 64 used one of those as the color RAM. Makes sense getting them off intact and put them in the stash with spares.

[@thebyteattic]

Interesting!

[@vinced3353]

Hi, thanks for this video ! I'm trying the composite mod on a zx81. Do you think a 100mA transistor (BC547) would work ?

[@thebyteattic]

That would be on the edge. If the monitor has a 75 Ohm equivalent impedance, and you use the resistors I used in this video, you would get about 100mA peak current flowing out the emitter. So I think you should use a higher-current transistor to be on the safe side. They cost cents and you can order them from practically any electronics shop.

[@vinced3353]

@@thebyteattic Thanks for your fast answer. The advantage of the BC547 is that a have some already =D But I think I'll wait for new ones instead of frying my ZX...

[@redhawk668]

I’ve tried BC547, it does work. But, as said here it’s on the edge. Better use a BC337.

[@maxsessini619]

Love your channel. Great work! Subbed

[@thebyteattic]

Awesome, thank you!