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

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!

John Earland 14th Mar 2020 11:40 pm

Re: Zx81
 
Quote:

Originally Posted by Slothie (Post 1225563)
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 keyboard 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.

Fantastic!

John Earland 14th Mar 2020 11:44 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1225543)
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.

That sounds perfect! All neatly hidden away inside. Probably need to insulate the exposed pins too.

John Earland 15th Mar 2020 12:10 am

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1225543)
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.

Would there be any need to cut the +5v wire? Could I just solder into it?

SiriusHardware 15th Mar 2020 12:14 am

Re: Zx81
 
It is not essential to cut the +5V but if you do not it will leave the modulator circuitry powered unnecessarily.

John Earland 15th Mar 2020 12:31 am

Re: Zx81
 
Ah-I see - yes thank you. Lovely neat solution you gave me. I shall do it when the transistor arrives!

On a slightly different note, I obtained another ZX81 today-checked inside, unfortunately it has the early ULA you mentioned. Tried it on my modern TV and it gave a black screen-my original ZX81 does work on the modern tv though picture is a bit wobbly but it does work through the aerial socket. Will try the early ZX81 on the crt tomorrow.

Interestingly for my original ZX81, which I will modify, when I plug in a 16k RAM pack the picture quality is significantly reduced on my lcd tv. Very grainy and stripey-Looks like an awful lot of interference. Maybe the mod will help!!

Refugee 15th Mar 2020 2:35 am

Re: Zx81
 
Quote:

Originally Posted by Slothie (Post 1225563)
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.

My mate did better.
He bought a replacement keyboard sticker and chopped all the legends out and stuck them to the key caps. He bought several if I remember rightly as they were pretty cheap. There was a sacrificial keyboard for stripping and tracing.
The keyboard was a big chunky item made out of folded aluminum with enough space for the ZX81 and the guts of the mains adapter easily.
The computer the donor keyboard came from had a CPU that took up a whole room.
The screens were so heavy that hardly any got pulled out of the skip.

SiriusHardware 15th Mar 2020 8:44 am

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1225605)
when I plug in a 16k RAM pack the picture quality is significantly reduced on my lcd tv. Very grainy and stripey-Looks like an awful lot of interference. Maybe the mod will help!!

It may certainly be very revealing. If the picture is still not much improved after the mod and gets noticeably worse when you have a RAM pack plugged in that would suggest that there may be problems with the onboard PSU circuit, or, as others suggested earlier, the external mains PSU.

If the 'new' ZX81 also came with an original PSU, try running the 'old' ZX81 on the 'new' PSU to see if that makes any difference to your video quality.

John Earland 15th Mar 2020 9:23 am

Re: Zx81
 
That’s a great idea-I will try it. I do actually have another 9v dc psu too. It powers an old Binatone Mk IV game console from the 1970s and has the same connection. The Binatone works well on the crt-picture a little wobbly but very usable. I could try that one too.

Slothie 15th Mar 2020 9:40 am

Re: Zx81
 
Inside the RAM pack there should be a 22uF electrolytic across the supply rails. If this has gone bad then it wont be supressing all the noise generated by the 12v inverter circuit used to supply the DRAM chips. This is likely the source of your interference.

Schematic & info:
https://www.myprius.co.za/ZX81.htm

John Earland 15th Mar 2020 9:54 am

Re: Zx81
 
Thank you for that-I shall check.

SiriusHardware 15th Mar 2020 10:24 am

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1225647)
I do actually have another 9v dc psu too. It powers an old Binatone Mk IV game console from the 1970s and hasn the same connection. The Binatone works well on the crt-picture a little wobbly but very usable

If two completely different RF connected devices give you a rough / wobbly picture on the same TV then I can not help but wonder if it is the TV rather than the devices which has the problem.

When swapping PSUs between different devices, beware of polarity - roughly 50% of PSUs from that era had positive on the tip of the plug, the other 50% negative on the tip of the plug. Use your meter to make sure the polarity of the Binatone PSU is the same before trying to run either of the ZX81s on it. I seem to remember the ZX PSUs are tip=negative, but don't take my word for it, check.

John Earland 15th Mar 2020 3:24 pm

Re: Zx81
 
Ah-right! I will double check!

julie_m 15th Mar 2020 3:50 pm

Re: Zx81
 
The ZX81 supply is positive tip, and -- by happy accident or cunning design -- will not cause any lasting damage if plugged into the EAR or MIC socket for long enough to notice something is amiss. (Applying negative voltages to inputs of MOS ICs with protection diodes, even when unpowered, is a bad idea. Sinclair would probably have known that when choosing the wiring and reckoned accordingly, selecting the option least likely to be damaged by misconnection without adding expensive protection diodes, resistors or fuses!)

The Spectrum supply used a "barrel" connector with negative inside, so no possibility of misconnection. I don't know if the change was for better reliability (only one sprung contact) or user-proofing.

John Earland 15th Mar 2020 5:49 pm

Re: Zx81
 
Thank you-that’s really interesting. There’s so much I don’t know! There’s so much I don’t know that I don’t know too!

SiriusHardware 15th Mar 2020 7:22 pm

Re: Zx81
 
Just hauled out my original ZX81 'Ski Jump' power supply and yes, Julie is right, tip positive on the ZX81, not tip negative as per my (obviously faulty) memory.

John Earland 15th Mar 2020 8:19 pm

Re: Zx81
 
Okay - an update! I have two ZX81s now, both with 16k RAM packs. Both with their own PSUs and both with their own TV cables.

My original ZX81 (the subject of this thread) works on the RF of both the portable crt TV and the LCD - though poor picture quality in both cases. I will be modifying the ZX81 to hopefully improve this. It works on about channel 36 but with wobbly picture as discussed on both the LCD and CRT TVs.

I have tried powering this ZX81 with the two different PSUs without any discernible difference in the quality of the video output on either of the two TVs.

I have also tested the ZX81 with the Sinclair RAM pack and the MEMOPACK RAM pack. Using either of these RAM packs results in the same very distorted video output which can’t be corrected with fine adjustments to either of the two TVs. I’ve tested these RAM packs on the second ZX81 and its video output remains steady and is totally unaffected by them. This ZX81 is an early model unfortunately which generates a dark screen. However, from what I can see, the picture is quite stable no matter which PSU or RAM pack I use with it. If the picture was white it would be perfect!

My assumption that the ZX81 would not save or load programs was (potentially) false because I have been using stereo cables (mono ones ordered) so I do think that this will be okay.

So, I can deduce that:
1. There is nothing wrong with either of the PSUs.
2. There is nothing wrong with either of the RAM packs.
3. There is nothing wrong with the RF leads.
4. There remains a problem with the original ZX81. Not only is the video output poor it is affected by the attachment of the RAM packs which it shouldn’t be of course.

I will of course be testing this when I have undertaken the modification. I would be grateful for any comments, advice and guidance.

Thank you all so much for your input thus far. I really do appreciate you taking the time to help me.

SiriusHardware 15th Mar 2020 9:07 pm

Re: Zx81
 
Nice methodical approach, made possible by having two of everything. For the 'dark picture' problem with the second ZX81 you basically have a number of options:-

-Re-sell it. It will eventually end up in the hands of someone who has a TV which is retro enough to work with it.

-Fit an additional bit of electronic circuitry which reinstates the missing portion of the video signal. This is not a particularly simple circuit so it would be best to buy it as a kit, but looking around, such kits can cost more than a replacement 2C210 ULA and are more complicated to fit which makes no sense, so the next option is :-

-Obtain and fit a genuine replacement 2C210 ULA. Unfortunately these are not particularly cheap - often more expensive than a whole ZX81 which already has the 2C210 ULA.

- At various times enterprising people have produced cheaper reverse-engineered modern versions of the ULA - typically a PCB with the 40-pin footprint of the original IC, but with one or more surface-mounted ICs on it. Unfortunately these tend to be made in fairly small batches and the two examples I found are currently sold out, although more may be made available in future.

For the original ZX81 I am now inclined towards thinking there is a problem with noise on the internal power supplies, the most likely cause of which would be tired capacitors - the more so if the machine still has poor / unstable video output when modified for composite video output.

John Earland 15th Mar 2020 9:17 pm

Re: Zx81
 
Thank you - that’s really useful to know. I have found someone online who sells something called Chroma-81 which attaches to the ZX81 which generates SCART RGB signals - however, it costs £72 for the basic model but you do get other interfaces and 16k RAM - there are other things too I think so may be the best thing to do- other than resell it. Which I might!

John Earland 15th Mar 2020 9:19 pm

Re: Zx81
 
With regards the noise on internal power supplies on my original ZX81, I think I’d like to try to replace those capacitors. Do you think the video modification would help though too or should I replace the capacitors anyway?

John Earland 15th Mar 2020 9:34 pm

Re: Zx81
 
Hi, I found this which seems promising? https://www.sellmyretro.com/offer/details/3712

Slothie 15th Mar 2020 10:07 pm

Re: Zx81
 
I would recommend replacing the capacitors smoothing the power supply because they are likely to be worn out and it will be hard to diagnose other problems. Also if they finally give up the could cause a fire!
I also noticed that the page i linked to in my previous post shows a diode in the video output which is supposed to improve working with modern tv's - might be worth a try.

John Earland 15th Mar 2020 10:55 pm

Re: Zx81
 
Yes-I noticed the diode. I’m willing to try anything!

SiriusHardware 15th Mar 2020 10:59 pm

Re: Zx81
 
Quote:

Originally Posted by John Earland (Post 1225819)
Hi, I found this which seems promising? https://www.sellmyretro.com/offer/details/3712

I had a look at that, apparently not too bad on price and it appears to do what needs to be done, it provides the missing 'back porch' portion of the video signal.

I would prefer a tidier, but more expensive approach like this

https://retrorevivalshop.co.uk/index...roducts_id=560

Here you just remove the original ULA and replace it with that modern reverse-engineered version - that particular one is quite expensive though, they seem to have been available for about half that price in the recent past.

If you consider the current price to be reasonable, check with the seller because that item is sourced from New Zealand - if the UK seller has local stock, fine, but if it has to come from New Zealand there could be considerable delays due to current events.

John Earland 16th Mar 2020 8:23 am

Re: Zx81
 
This looks great. I would have to get my friend to do the replacement for me if it’s soldered in. Would this be okay on my original ZX81 with the distorted video output too?

SiriusHardware 16th Mar 2020 10:01 am

Re: Zx81
 
In theory yes, but hold that thought until you have tried the composite mod on ZX81(1). If it still has buckled / wobbly video-out even then, then the ULA is one possible cause for the fault.

If doing the composite mod produces good, steady video out from ZX81(1) then it may be that the modulator was actually at fault. No sense changing the ULA without due cause.

John Earland 16th Mar 2020 10:43 am

Re: Zx81
 
I agree. I shall try the mod first. Did you see the post from Soothie which included a link which shows this modification but with an additional diode? What do you think?

SiriusHardware 16th Mar 2020 11:09 am

Re: Zx81
 
I've seen variants where there is both an additional diode and an additional resistor in series with the diode. Try the simplest version first, then add the other components if it proves necessary.

John Earland 16th Mar 2020 11:21 am

Re: Zx81
 
Thank you, will do.

John Earland 16th Mar 2020 2:27 pm

Re: Zx81
 
Quote:

Originally Posted by SiriusHardware (Post 1225543)
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.

Does it matter which way round the resistor goes?

Dave Moll 16th Mar 2020 2:49 pm

Re: Zx81
 
No, a resistor is non-polar (i.e. it works the same either way round).

John Earland 16th Mar 2020 2:51 pm

Re: Zx81
 
Thank you for confirming. I've just found 'Electronics for Dummies' online which said the same thing - coincidence. I may invest in a copy! I need it.
Thanks again!
J

John Earland 16th Mar 2020 11:44 pm

Re: Zx81
 
3 Attachment(s)
I thought I had posted this resolution but I canít find it.

I have completed the mod of the ZX81 and the image is strong and stable! Though the screen is slightly darker than it was before it really is a good crisp image. I will try it on my lcd but on the crt, I am happy. Image 1 is showing the ZX81 without a memory pack, the second is one with the memory pack (tv screen has very slight waves which Iím not bothered about) and the third image is the modification.

Thank you everyone for helping me with this project! Really appreciate it.

SiriusHardware 17th Mar 2020 12:41 am

Re: Zx81
 
Well done, not bad at all for someone who keeps saying he doesn't know how to solder. Quite difficult to work in that confined space as well.

It would appear, then, that the real problem may have been a faulty modulator (possibly intermittent, as you did manage to get it to look better occasionally by putting pressure on it). That presumably doesn't matter if you plan to keep it modified this way.

John Earland 17th Mar 2020 11:06 am

Re: Zx81
 
Yes-I’m really pleased! Thank you for all your advice and guidance!

Slothie 17th Mar 2020 1:22 pm

Re: Zx81
 
Very neat, especially considering your experience as Sirius said. I prefer it when mods like this are "hidden", it would have been easy to make this messy which would have been a shame with a machine that was actually quite elegant for its time.
I loved the old ZX80/81 though i never had one of my own i borrowed a friends for more time than he was happy with!!

John Earland 17th Mar 2020 2:00 pm

Re: Zx81
 
The first computer that I owned was the Spectrum! I never had a ZX81 either. I was going to have one for Christmas (I was 13) then the Spectrum came out instead! The first computer I ever used was the Commodore PET, so I am fond of them all!

emeritus 17th Mar 2020 4:59 pm

Re: Zx81
 
Mine was a Z81 too. The modulator failed and as I already had an Amstrad PCW8256 by then I converted it to a dummy laptop for my daughter's school play by making a flip-up hinged lid for it from black card. I printed a screen dump of the windows desktop from my work PC, covered it with shiny self adhesive plastic film, and stuck it on the inside of the lid. From a distance it was quite realistic! Probably sill buried in the dephs of the loft. I well remember spending ages typing in games programmes in BASIC from magazine listings.

Dave Moll 17th Mar 2020 5:27 pm

Re: Zx81
 
Quote:

Originally Posted by emeritus (Post 1226291)
I well remember spending ages typing in games programmes in BASIC from magazine listings.

... then spending further ages downloading them to cassette tape.

In my case, though, it was a BBC micro. It was only more recently that I acquired examples of the ZX80, ZX81 and Spectrum.

SiriusHardware 18th Mar 2020 12:35 am

Re: Zx81
 
I have two ZX81s, one is my original machine, built from a kit and still in working order.

The other had been in an auction lot bought by a fellow forum member here and I acquired it from him, it is housed in a 'Dktronics' full sized enclosure / keyboard which is in almost as-new condition - clearly very little used by its original owner because when they were used regularly, the glue which secured the stick-on key legends would soften and the key legends would start skating all over the key tops. ( I know this because my original Spectrum spent most of its life in the Spectrum version of the same Dktronics enclosure / keyboard).

Both were updated to 2C210 ULAs when those parts were still readily available and not too expensive, and both are internally fitted with 16K RAM because, despite what I have said elsewhere about originality being key, the original 16K RAM pack is a dreadful abomination which neither looks good nor works well, due to the inherent unreliability of the connections (AKA 'Ram Pack Wobble').

It is a relatively straightforward mod to fit a 62256 SRAM in place of the original 1K RAM and you then get a machine which retains Rick Dickinson's classic design looks combined with a respectable 16K RAM, and nothing dangling off the back. Because the RAM is internally fitted, the connections are secure and solid and the machine can withstand a thump or three without the memory contents getting scrambled. The mod can also be done in such a manner that it can be completely reversed if required.

(You may point out that the 62256 is a 32K RAM, and you'd be right, but the most basic form of the mod only maps 16K of the 32K - there is arguably no point in mapping more of it because there is no mainstream software from that period which needs more than 16K.)

John Earland 18th Mar 2020 9:46 am

Re: Zx81
 
That sounds like a great idea. I have, as I mention, a 16k RAM pack but I like the idea of fitting the RAM inside.


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