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ZX81 video isues

D

Deksor

Member
Joined
Nov 13, 2016
Messages
35
Location
France
Hello !

Today I tried to make a composite mod to my father's ZX81 that he bought second hand in the late 80's because I couldn't use it on any of our TVs. The mod is the ZX2020 mod which replaces entirely the RF modulator.

The mod works for the most part, but there's a major incovenience and I have no idea what could cause this. I have never been able to get a stable picture before, but I suspect that this issue is actually unrelated to the mod.
The best "picture" I could get before was something where I was capable of seeing the letters of the text, but it was out of sync and really snowy. It was unusable.

After the composite mod, the picture is now fully stable and recognisable, but there's a major issue, check this out :

So the mod seems to work but there's this terrible black bar that passes in the background as you can see.
I don't think the composite mod is causing this, because in all the mess that I could barely see in the RF signal, but I think I remember this being in the patterns of the ultra messy RF signal.


Here's the machine before the mod :


It looks like an early revision of the machine, all date codes are from mid/late 1981.

RF modulator removed :


For some reason instead of a straight wire getting into the RF modulator, there was this odd contraption connecting to it :


Here's the board installed :


Of course I tried to change the settings. Being an early revision, my machine doesn't generate a back porch by default, so I enabled this on the mod, which didn't change anything. I tried to change the potentiometers' values and that didn't do much either.

What could be causing this issue ? have anyone experienced this before with their ZX81 ?


Edit : oh an by the way the TV set isn't the cause, I had the exact same issue on my CRT.
 
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The power supply you are using, is it an original ZX81 power supply or some other unit? If your power supply has either the +ve or -ve of the DC output connected through to mains earth inside the PSU this could be causing a 'hum loop'. The original power supply has a completely isolated output, neither +ve or -ve are connected to mains earth.
 
It is the original power supply. It had some bad capacitors (one of the two caps inside started to leak) so I changed them before I even made this mod.
 
Your TV / display, does it have a two-core mains cable (only live and neutral) or a three-core mains cable (live, neutral and earth)? If it has an earth wire in the flex temporarily disconnect it and tie it safely out of the way and try it with the earth wire disconnected. Does this clear up the 'bars' problem? If the TV / display only has two cores in the flex, disregard this.

Other things to try: I know you said you changed the capacitors in the original supply but it could be defective in other ways - for example one or more of the bridge rectifier diodes could be reverse leaking and causing a high level of ripple and lower than expected voltage on the DC output to the computer, this would make it difficult for the onboard regulator to do its job of providing a perfectly smooth 5V supply. The easiest way to check this out is to try powering the ZX from another PSU, ideally a bench / lab power supply if you have one, observing correct voltage and polarity of course. If you don't have a lab PSU then maybe a 'universal' PSU set to the correct voltage and polarity, but it will need to be able to supply at least as much current as the original ZX PSU.

If you have a meter, measure the DC voltage going from the original PSU to the ZX while it is powering the computer. If you have a scope, look at the DC supply going from the PSU to the ZX, again while it is actually powering the computer.
 
By the way, that 'contraption' which was hung on the outside of the original modulator looks like a composite mod - someone had already done the job for you, but your way is neater in any case.
 
Thanks for the tips. I'll check that tomorrow :)

My CRT TV has no ground though, so that's already checked out.

I don't have any bench power supply, but I could perhaps hack something using a random 9V psu I have laying around or take an old USB cable and temporarily solder it where the output of the DC/DC converter is.

I have a scope so I can try to check this out, though I'm not very experienced with them yet.

As for the contraption, I tried the computer with a composite cable before and I didn't get anything. Also there were no signs of rework on the back, so it makes me wonder if this might be original. Being an earlier revision it could be some sort of patch. I am french by the way and since the PCB has markings specifically made for France, I suspect this to be some sort of fix for whatever could be different to accommodate with french TVs. The signal pin was clearly connected to the "FR2/USA" hole.
This or maybe something else broke this very old mod in some way :unsure:
 
Maybe so, of course I have never seen a French ZX81.

Your 'hum bar' frequency looks to have a direct relationship to the frequency of the mains / TV frame rate so my suspicion is that maybe the low 'point' of the ripple on the DC supply input is going below the minimum input voltage which the ZX81's regulator needs to regulate properly.

Any other '9V' supply should do as long as it has the right polarity and can supply at least as much current as the original ZX81 supply. Looking at my own original PSU I see that it is rated 0.7A (700mA) at 9V so any other supply you try will need to be able to supply at least 0.7A.

A USB supply would be 5.0V, too low for the input of the ZX81 regulator which needs an absolute minimum input voltage of about 7.0V - 7.5V to work properly and it won't usually supply more than 0.5A unless the item you are trying to power with it engages in a bit of digital negotiation to ask the PSU for more current: Of course the ZX81 will not do this.

Better just to find an old fashioned 'heavy' 9V power power supply containing only a transformer, capacitors and rectifier diodes and make connections to the pins of the ZX81's DC power socket to try it, just to see if it clears that problem.
 
So I tested with a cheap noname 9V PSU ... and it works ! Thanks !!
So the PSU is indeed the problem here.

I'd like to keep the original look of the PSU though. What would you suggest ? Fix the old PSU (and how ?), or just retrofit the new PSU inside ?

The old PSU is very simple and not very safe I believe. Plus it's heavy and generates a lot of heat.
The new PSU is very light, and probably much more efficient even thought it's a cheap chinese PSU.
 
As a 'purist' I would say repair the original PSU... because it is the original PSU and it has managed to stay together with the computer for 40 years, so it would be a pity to separate them now.

Luckily for you there is not much inside one of those old linear unregulated power supplies. You have said you changed a leaky capacitor - forgive me for asking this but you do know that most electrolytic capacitors have a 'right' and a 'wrong' way around that they have to go? If you fitted the replacement capacitor the wrong way around, the problem you were having is what I would expect (although the capacitor would blow up eventually).

Assuming the replacement capacitor is the right way around, and assuming you don't have a meter, the next thing to try is replacement of the rectifier diodes - there are probably four - replace all of them together - they don't cost very much. As with the capacitors, the diodes also only have one correct way around - take a few good digital photos before you remove the original ones so that you have a record of which way around they fit. The PCB is probably marked with how they should be fitted but sometimes, manufacturers to make mistakes with the printing on their PCBs.

Obtain replacement diodes first and then you can make the job of replacement easier by cutting the leads off the original diodes next to the diode bodies. You can then desolder and remove each diode lead individually, which will cause less stress to the PCB.

Because the ZX81 PSU is easy to take apart, you could crack open your Chinese PSU and replace the original internals of the ZX PSU with the internal PCB from the Chinese PSU but you would need to do this in a safe way, especially, you would need to make sure the wiring from the mains input to the PSU was kept completely separate from the low voltage side with no possibility that the PCB can move around and connect to anything it should not.

I really think the safest / best option for now is to repair the original PSU.
 
Well I did the opposite before you answered.

I took the chinese PSU appart, which is made out of one single PCB, I soldered the wires of the ZX psu to it and I put the PCB in the ZX psu case. I didn't take the effort of attaching it for now, there's nothing metallic in there, the wires are well attached to the plastic so there's no way I can rip them off by accident. I could perhaps attach it with some zip ties, but that's the best I can do, there's not much room left in there and I don't feel like cutting the insides just in case I want to revert this.


I definitely took care of the polarity of the caps. There are indeed four rectifier diodes, but the annoying thing is that half of them are underneath the transformer, which considering its size must be really hard to desolder from there :/

And yeah that PCB isn't labeled in any way. Good thing I took a picture of the caps before removing them otherwise it'd have been hard to troubleshoot.
I also wrote where the + side of the output is just in case I change my mind and want to fix it.


I've now made the internal RAM mod of the ZX81 which definitely seems to work (I say definitely because, as expected, the keyboard died, so now I am waiting for a replacement membrane)
 
I respect your decision to put the modern PSU electronics inside the ZX PSU - I have a tendency to try to keep everything which is original together and working. My main concern is whether the Chinese PCB can move around and whether parts of the PCB, especially on the low voltage side, can hit anything they should not, but I am sure you have that covered.

So I've just said that I like to keep things original but ZX81 memory expansion is an exception to my 'rule' because it makes much better sense to replace the internal 1K or 2K of RAM with an internal 62256 RAM when they are so widely available and cheap and, the power consumption of a single 62256 SRAM is much lower than a whole bank of older style RAMs.

Fitting the memory internally preserves the nice shape of the basic machine while making it usable, which it really isn't with the standard 1K / 2K. Most of the official and third party add on memory packs looked terrible and they were very prone to wobbling about so you could be entering code for an hour or more and suddenly something would bump the machine and all your work would be gone. With the memory fitted internally the operation of the machine is almost bulletproof.

The problem with the key membranes is the way the ribbon cable is laid, it ends up lying close to the heatsink which of course runs very warm, so over time the ribbon material becomes more and more brittle and sometimes they are fine until you take the machine apart and remove the PCB for some reason, but then when you do, the ribbon cable just cracks into pieces. The modern remanufactured keyboard membranes are made from much stronger material.
 
So today I received and installed the new membrane.

The computer works fine and I could play with it a little.

... Or is it ?

I kept it turned on for a few hours because I wanted to find a way to save the program I wrote in. However when I came back the computer was off despite the PSU being turned on. The computer was cold so I knew there was something wrong related to the power again.

I checked the psu : it still outputs 9v as usual.

So I opened the computer and checked the 7805 : 9v on the input as expected ... And 0.5v on the output. Just to make sure I checked if there was a short on the circuit with my ohm meter but nope.
So there's a new issue (I'm starting to feel like this computer is falling apart :/) the regulator is dead.

I've seen people use some kind of new regulator that's much more efficient but I haven't seen what's their reference yet.

Now I also wonder if the hum bar wasn't a symptom of a failing regulator.
Is this possible or is it just a coincidence ?

Edit : found the reference https://www.sinclairzxworld.com/viewtopic.php?f=7&t=2168
 
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As you have a meter, power up the PSU and unplug it from the ZX and measure the AC (not DC) voltage between the inner contact on the DC power plug and a known mains earth, and between the outer contact on the DC power plug and a known mains earth. What do you see, any significant voltage? Like maybe 110V AC, 115V AC?

The modules you mention are little SMPSU (switch mode power supply) modules which take the place of the original linear regulator. One advantage they should have is that they will run much cooler so there will be less heat cooking everything inside the computer. As you already know I prefer to keep things as original as possible and I would fit the original type of linear regulator, but of course it is up to you.
 
So I measured and found about 82v AC :(

This is obviously bad but what does this imply exactly ?
 
Your power supply should be double insulated, so you should read nothing. Does your power supply have a symbol on it that looks like one square within another (https://www.icgbuilds.com/electrical-cord-safety/double-insulated-symbol/)?

If you have an electrician friend, ask him/her to perform a Portable Appliance Test on it. I recon it will fail. If it does, throw it away quickly!

Is it possible that the 7805 regulator got too hot and shut itself down?

Dave
 
I don't have an electrician friend but I guess I'll try to fix the old PSU then.

The 7805 might have gotten too hot, but it didn't just shut itself down, it failed as now it doesn't work anymore (it was stone cold when I came back)
 
Normally switching off and back on would have recovered the regulator if it had overheated.

I just read back a little. Does your Chinese PSU have capacitors, or is there a transformer in it?

Some ‘fake’ power supply units just have resistors and capacitors internally rather than a transformer - or are otherwise sub-standard - and are quite dangerous. They wouldn’t pass a Portable Appliance Test for example. These units drop the voltage down, but do not isolate the mains input from the output. Any ‘spikes’ on the mains input will make their way through to the output - and be seen by the ZX.

Dave
 
Most of the cheap SMPSUs around don't have an earth connection, which creates problems when it comes to trying to suppress the radio interference which most such PSUs are prone to generating - to get around this there is usually a high voltage, low value capacitor between one side of the DC output (usually the 0V side) and the 'GND' reference rail of the high voltage side. This capacitor is intended to let unwanted high frequencies pass but block low AC frequencies (like the mains frequency).

In practice it acts more like a high value resistor to low frequencies so what you end up with is high AC voltage, typically around half mains voltage, superimposed on the 0V rail of the DC output. It is not dangerous to humans because the low value of the capacitor limits the current being passed at 50Hz or 60Hz to a safe low value. Even so, you can sometimes feel an unpleasant tingle when you touch the outer barrel of the DC plug on that sort of power supply.

However, 85 Volts or 110 Volts is still 85 Volts or 110 Volts, and if the item you are connecting the PSU to is connected to something else in turn (like a TV) then at the moment you plug the DC plug into the item being powered, there is that voltage difference between the two items as they connect. As you can imagine, this has the potential (!) to cause trouble.

It is for that reason that I never power old retro equipment from un-earthed modern SMPSUs, and another reason why I would have tried to fix the original supply.
 
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This is why you need to identify (from an electrical safety perspective) whether the equipment is Class I or Class II.

Class I equipment has a mains earth (and should, therefore, only be operated with the safety earth connected). There should be greater than 20 MOhms insulation between the input live and neutral and the output connections (and earth) and less than 0.1 Ohms from the mains earth pin (of the plug) and the exposed metal case (assuming no long connecting cable of course).

Class II equipment doesn't require (nor have) an earth connection. Most 'wall warts' should be Class II and wouldn't (therefore) have an earth connection.

Dave
 
But to clarify, most Class II (un-earthed) SMPSUs of the type Deksor has used will have this 'Phantom' high voltage, low current AC superimposed on the DC output, for the reason explained in #18. I wasn't surprised when he checked his, and found exactly what I had expected. I just don't use them at all with any kind of retro equipment.
 
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