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-   -   Zx81 (https://www.vintage-radio.net/forum/showthread.php?t=164648)

John Earland 11th Mar 2020 1:43 pm

Re: Zx81
 
3 Attachment(s)
Iíve just switched it back on and placed the ref cable on half way-first two images-fairly decent. However after 10 mins on-3rd picture! 😱

SiriusHardware 11th Mar 2020 2:22 pm

Re: Zx81
 
The thing to realise about the mini transmitters (modulators) used in tune-in tech from this era (like the ZX81) is that the little signal generator inside is quite crude and simple. The transmitter / channel frequency is determined by the adjustment of a coil, nothing more, and as they warm up the frequency tends to drift a little.

TV tuners of the time were also quite prone to drift so they incorporated a feature called AFC (Automatic Frequency Control) which would automatically keep the TV's tuner centred on any signal it was tuned to, so, as the frequency of the incoming signal slowly drifted, the TV would self-retune to keep itself centred on the signal.

Modern TVs have incredibly stable tuners and they expect any incoming signal to which they are tuned to be the same, so they don't track drifting signals the way older TVs do.
They stay precisely tuned to whatever frequency you tune them to.

The upshot of all this is that if you tune a modern TV to the ZX81's output while it is cold you may have to retune the TV in little steps from time to time as the ZX81 warms up.

That's why an older TV with a simple analogue tuning knob on the front is the best type to use for the ZX81. Up to a point the AFC will try to maintain a lock on the signal, and then if it drifts out too far it's just a simple matter to reach out and retweak the tuning for best picture.

You could bypass all of this by converting the output to composite video , as you did on your Spectrum. Only you will know whether you feel confident enough to do that.

John Earland 11th Mar 2020 8:25 pm

Re: Zx81
 
Hi, yes I could give that a go. It looks trickier when I've looked on YouTube that it was to convert the Spectrum - any tips? It may be the best solution!

SiriusHardware 11th Mar 2020 9:29 pm

Re: Zx81
 
In theory you could do it in the same way as on the Spectrum, by disconnecting the input and output wires from the modulator inside the housing and wiring the video-in signal straight through to the centre pin of the UHF-out terminal.

The only problem with that is that it directly exposes the output of the ULA (an exotic, expensive IC) to the outside world. It might be better to build a simple buffer circuit to go between the ULA and the outside world into the modulator housing, as in this article:-

https://www.classic-computers.org.nz...o-for-zx81.htm

That shows the basic circuit and general principle of a buffered composite video modification, although I would hope you would try to make yours look neater than the end product in that article.

Dave Moll 11th Mar 2020 11:04 pm

Re: Zx81
 
As an aside, it also passes on a useful tip about that notoriously fragile ribbon cable between the keyboard and the motherboard.

SiriusHardware 11th Mar 2020 11:35 pm

Re: Zx81
 
Let's take one more punt at trying to get it to work as built.

John, can you flip the lid off the modulator and check that the wire going to the lug on the centre terminal of the UHF output socket has not come loose? When it works, it appears to work quite well.

John Earland 12th Mar 2020 8:54 pm

Re: Zx81
 
Yes - that's a great idea - will do and I shall report back

John Earland 13th Mar 2020 10:57 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1224641)
In theory you could do it in the same way as on the Spectrum, by disconnecting the input and output wires from the modulator inside the housing and wiring the video-in signal straight through to the centre pin of the UHF-out terminal.

The only problem with that is that it directly exposes the output of the ULA (an exotic, expensive IC) to the outside world. It might be better to build a simple buffer circuit to go between the ULA and the outside world into the modulator housing, as in this article:-

https://www.classic-computers.org.nz...o-for-zx81.htm

That shows the basic circuit and general principle of a buffered composite video modification, although I would hope you would try to make yours look neater than the end product in that article.

Iíve just noticed in the first picture in this article, showing the set up, the cassette recorder is the one I have, a Sony!

Iíve read this article and it involves using a resistor and a transistor. I have a 100R resistor and Iíve ordered a transistor so will attempt the mod. However, I did this on the Spectrum without using either of these. Would this approach work here do you think?

SiriusHardware 13th Mar 2020 11:56 pm

Re: Zx81
 
I think the best way to answer your question is to suggest you Google around and see how many different ways people have done the composite mod on the ZX81. If you see enough articles independently suggesting that it be done the same way as you did it on the Spectrum then it probably works OK.

If the general consensus is that you need to use a buffer transistor, then I would go with that option. As I suggested before, using a buffer provides a degree of protection to the exotic and difficult to obtain ULA IC by sitting between the ULA and the outside world - if something untoward happens, it hopefully takes out a twelve pence transistor instead of the ULA IC.

I would favour the buffer method, and in particular this version of the buffer circuit which comes already made on a little PCB which is small enough to fit inside the modulator without removing any of the original modulator parts. All you have to do is add four wires, Power, 0V, Video in, Video out.

https://www.thefuturewas8bit.com/sho...video-mod.html

Although the headline describes as being for Atari it also works with the ZX81.

However, I would not discourage you from trying either the 'simple' video pass through mod like on your Spectrum or the homebrew transistor buffer version - that's how you learn.

John Earland 14th Mar 2020 12:11 am

Re: Zx81
 
Thank you for this. Food for thought.

John Earland 14th Mar 2020 11:02 am

Re: Zx81
 
1 Attachment(s)
Quote:

Originally Posted by SiriusHardware (Post 1224683)
John, can you flip the lid off the modulator and check that the wire going to the lug on the centre terminal of the UHF output socket has not come loose? When it works, it appears to work quite well.

All seems okay to me.

John Earland 14th Mar 2020 12:15 pm

Re: Zx81
 
I think I have decided to try the mod described in the blog but could I ask for some clarity?

According to the blog:
"I decided the best approach was to snip that video line from the ULA and divert the signal originally destined for the RF box to the base of my transistor instead. I then wired up the collector to the 5V line and add the resistor to the emitter, the other end of which I earthed on the RF box. I then used a wire to grab the signal from the emitter (prior to the resistor) and threaded this through the spare hole. It was then a matter of unsoldering the RF output wire from the RCA socket and soldering on my composite video wire (coming through the spare hole). Figure 9 shows the finished result. Not pretty but it works!

Okay, so I have a 100R Ohm resistor and I'm waiting for the 2n3904 transisitor to arrive. So, in my language (and I thinking this through in my head) am I correct with the following:

1. Cut the RF line and attach to the transistor's middle pin.
2. Solder the 'collector' side of the transistor to the 5v wire (never seen a transistor so I hope I know which pin is the collector when it arrives!)
3. Solder the resistor to the 'emitter' pin of the transistor and solder the other end to an Earth
4. Solder a wire to the middle section of the TV socket (RCA?) pass this out through the spare hole
5. Solder this wire to the transistor's emitter pin but before the resistor

Could you explain how the position of the resistor in this examples saves the IC?

Refugee 14th Mar 2020 12:21 pm

Re: Zx81
 
Back in the 1980s a mate of mine took the PCB out of a ZX81 and housed it in a keyboard from an old computer system that was being removed from a commercial premises.
He did the pass thru mod and hid the power supply in the keyboard as well.
He had to peel the keypad to look at the tracks in order to copy them onto the back of the key-switches in his new keyboard.
It worked fine.

John Earland 14th Mar 2020 1:29 pm

Re: Zx81
 
Wow! Peeling back the keypad and copying tracks is a bit too much for me!

SiriusHardware 14th Mar 2020 6:04 pm

Re: Zx81
 
In any case, your membrane keyboard seems to be working. If it isn't broken, don't fix it.

In the case mentioned above it would have been possible to use a meter to ascertain which keyboard connections join together whenever a particular key is pressed, without (potentially) destroying the original membrane keypad.

John: Tell us which transistor you have ordered and we will tell you which of its leads are the base, emitter and collector.

If done correctly, the video output from the ULA goes to the 'base' of the transistor, then the buffered video-out signal is taken from the 'emitter' of the transistor. The transistor therefore sits between the ULA IC output and the video-out connection on the socket and is first in line if any damaging signal or voltage is inadvertently connected to the video-out socket.

The transistor also assumes the role of video output driver, relieving the ULA of that load. The ULA now just has to generate the video signal: The transistor toughens the video signal up a bit before passing it on to the monitor / display.

To be slightly more technical about it, most composite video inputs on TVs and monitors have quite a low input impedance, 75 Ohms. The ULA IC, originally intended to drive the input of the RF modulator, may not be intended to, and may not like, being asked to drive such a low resistance load directly.

SiriusHardware 14th Mar 2020 7:24 pm

Re: Zx81
 
Too late to edit the above. Here is an online illustration of the 2n3904 showing its e, b, c connections.

http://www.electronicecircuits.com/w...-Amplifier.jpg

SiriusHardware 14th Mar 2020 7:59 pm

Re: Zx81
 
1 Attachment(s)
Turning the transistor pinout into a practical layout, this is how to physically wire everything up. (See attached). I've laid it out so that the incoming +5V and the incoming video-in arrive from roughly the right place and direction.

When you cut the incoming video-in and +5V wires do it about 5-7mm inside the modulator housing, the transistor's base and collector leads can then be soldered directly to the ends of the incoming wires.

The resistor can be soldered between the transistor's emitter and the inside of the modulator case which is a handy 0V point - if you do that, do it low enough down the inside so the modulator lid can still be fitted on afterwards.

Slothie 14th Mar 2020 9:11 pm

Re: Zx81
 
Quote:

Originally Posted by Refugee (Post 1225401)
Back in the 1980s a mate of mine took the PCB out of a ZX81 and housed it in a keyboard from an old computer system that was being removed from a commercial premises.
He did the pass thru mod and hid the power supply in the keyboard as well.
He had to peel the keypad to look at the tracks in order to copy them onto the back of the key-switches in his new keyboard.
It worked fine.

Oddly enough I did the same thing at the same time for my chicklet keyboard commodore PET 2001. i got a DEC VT52 keyboard from a computer scrap place in Maldon, Essex, cut all the tracks on the PCB and rewired it with single core wire from phone cable my dad liberated from a skip during a refit of the office he worked in. I made a 2-way adapter from veroboard and header connectors so you could use both keyboards as the mood took me (original keyboard for graphics, nrw one for actual text). Then we made a case for the keybosrd from plywood painted to match the PET connecting to the adapter by a rainbow ribbon cable. Labelling the keys that needed it was done by writing on bits of paper and sellotaping onto the keys which was crude but worked! Twenty years of being in the loft has meant most of them have dropped off, but they lasted for the 8 or so years it was my main computer and was a joy to use.

John Earland 14th Mar 2020 11:34 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1225500)
In any case, your membrane keyboard seems to be working. If it isn't broken, don't fix it.

In the case mentioned above it would have been possible to use a meter to ascertain which keyboard connections join together whenever a particular key is pressed, without (potentially) destroying the original membrane keypad.

John: Tell us which transistor you have ordered and we will tell you which of its leads are the base, emitter and collector.

If done correctly, the video output from the ULA goes to the 'base' of the transistor, then the buffered video-out signal is taken from the 'emitter' of the transistor. The transistor therefore sits between the ULA IC output and the video-out connection on the socket and is first in line if any damaging signal or voltage is inadvertently connected to the video-out socket.

The transistor also assumes the role of video output driver, relieving the ULA of that load. The ULA now just has to generate the video signal: The transistor toughens the video signal up a bit before passing it on to the monitor / display.

To be slightly more technical about it, most composite video inputs on TVs and monitors have quite a low input impedance, 75 Ohms. The ULA IC, originally intended to drive the input of the RF modulator, may not be intended to, and may not like, being asked to drive such a low resistance load directly.

Hi, itís the 2n3904 one used in that blog. I was hoping they would arrive today-alas no!

John Earland 14th Mar 2020 11:37 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1225532)
Too late to edit the above. Here is an online illustration of the 2n3904 showing its e, b, c connections.

http://www.electronicecircuits.com/w...-Amplifier.jpg

Brilliant! Thank you for that! Just what I needed!


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