Attic 81's - My collection and attempts to repair

Discussions about Sinclair ZX80 and ZX81 Hardware
sheddyian
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Attic 81's - My collection and attempts to repair

Post by sheddyian »

If it's of interest, and if I might ask for help along the way, I thought I'd share the details of my ZX81 "collection" - 3 ZX81s that had sat in a very hot / very cold attic at my late Father's house for several decades.

I've recently started looking at them and trying to see what works. In researching this, I found this forum resource, which I'm very grateful for.

Here they are, as I found them :

ZX81 #1
=======
IMG_20201114_194035.jpg
Issue 1

In case with original keyboard. Probably my original ZX81 that I bought in 1982, possibly not original case/keyboard.

ULA 2C184E 8151 in socket
ROM MOSTEK MK36809N-IRL 8206 in socket
CPU ZILOG Z8400A Z80A 8152 in socket
RAM 24 pin MOSTEK 8203 MK4801AN MK4118AN-41 soldered wonkily to board
Modulator 8205
heatsink is larger but thinner piece of alu from old clock face fitted by Grandfather


ZX81 #2
=======
IMG_20201114_194105.jpg
Issue 1
ULA Ferranti 2C210E 8242 in socket
ROM Sinclair Research P21669 D2364C 649 (c) 1981 in socket
CPU Zilog Z8400A Z80A 8147 W in socket
RAM 18 pin NEC R1Z2IY-204 PD2114LC x2 soldered
Modulator 8208

ZX81 #3
=======
IMG_20201114_194117.jpg
Issue 1
ULA Ferranti ULA2C184E 8209 in socket
ROM Motorola? ZCM38818P 8244 in socket
CPU NEC P24488-218 D780C-1 in socket
RAM Motorola? MCM21L14P30 8225 in sockets MCM21L14P45 8223 Ram was inserted upside down!
Modulator 8217
no heatsink


So, all boards had different varieties of chips. 2 x older ULAs and 1 with the backporch fix.

I tried all 3 of them in turn, and all had the same fault - white screen, no cursor.

I know that my original ZX81, the one with the home made heatsink, HAD been working when it was put away, but that was several decades ago. The other two I think I'd got from a boot sale and a junk shop respectively. They may not have been working when I got them. I know I paid £1 at the boot fair, I may also have paid £1 at the junk shop, both back in early 1990s.

I swapped some chips around to see what I could achieve, of course not knowing which ones worked and which didn't. Starting with ZX81 #1, I fitted this with the newer ULA, initially without success. But after leaving it for a few hours and coming back to try again, it worked first time - a cursor! Picture wasn't great, but I had a cursor. Power off and on and the cursor had gone again.

I found it would work occasionally but not often. And when it did work, tapping the board made it fail.

I took the socketed chips out, sprayed contact cleaner in the sockets and gently cleaned and bent the legs before putting them back.

After a lot more fiddling around, I got the 81 to work reliably. I was delighted! Though I found the first power on from cold would give a white screen, subsequent power ons would work OK. I replaced the two electrolytic caps, but this didn't help. I thought I could live with this quirk though.

But when I tried to load a 1K game from a laptop via the EAR cable, I got error code 4. Out of memory. I tried a few 1K games, all did the same thing.

PEEKing RAMTOP showed a stupidly low value - I didn't write it down and forget what it was now. But it wasn't right.


So I tried a different board, and settled on ZX81 #3, as it had socketed RAM.

This didn't work, and I more or less randomly tried swapping in different ICs without success. But when I removed the ROM I found the legs very dirty, so removed all ICs and cleaned as I'd already done on ZX81 #1. On reseating all the ICs, it worked!

RAMTOP showed a 1K ZX81, and I was able to load a few 1K games from the laptop. Success!

What I hadn't spotted until many days later when reviewing the photos for this post, was that the socketed RAM was upside down until I reseated all the ICs. I had naturally put them back the right way round without second thought.

This is what is currently working for me :
IMG_20201120_145815.jpg
It's not been perfectly reliable though. I left it randomly plotting dots to slowly turn the screen black, and when I came back I found it had frozen. This may be the power supply I was using. It's a switch mode one from a Linksys router. Rated 9 volts, 0.6A. I'd previously put a 3.5mm jack on it for an Atari 2600 which it powers fine. Maybe it's noisy, maybe it's under rated, maybe the 81 doesn't like it?

I've also tried a 12 volt transformer PSU, which the 81 seems happier with (possibly) but I've not left this running it for long as I didn't want the 81 to overheat.

I've some 7809 regulators on order to make a better PSU for the 81. I'd welcome advice as to what might be the best PSU for long term use and reliability.

I've already had a splurge on ebay and ordered various parts
IMG_20201120_150317.jpg
Some sockets, a longer mono 3.5mm lead to play progams into the ZX81 from the laptop more conveniently, drive belts for the cassette recorder I'm trying to repair, a heatsink for the ULA and a 32K SRAM that I'm hoping is as compatible as was claimed in the listing.


My general plan is to try to get at least two of the ZX81s working. One will be broadly original in it's case. It'll have 16K usable onboard, a backporch ULA and maybe a composite mod.

The other ZX81 will sit on a piece of plywood that's got a ZX81 keyboard stuck to it - something I did back in the 1980s. This 81 can have the 10% speed upgrade I've read about here, more RAM (32K?) maybe a modern ULA replacement, maybe CPU upgrade... basically a tinkering ZX81.


Lets see what I can get working reliably first....

I'd welcome thoughts, comments, suggestions, things to avoid.. anything really.



Thanks


Ian
Last edited by sheddyian on Sun Nov 22, 2020 3:18 pm, edited 1 time in total.
Lardo Boffin
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Re: Attic 81's - My collection and attempts to repair

Post by Lardo Boffin »

Good luck with the builds! :D

If you need any Sinclair heatsinks I have quite a few! Mostly because I replace the regulators with these murata ones:

https://retroleum.co.uk/spectrum-repairs-mods

I’ve bought a few PSUs from here:

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

Ask for the 2600 version.

ZX8-CCBs make for a great composite mod, from sell my retro.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Toastrack
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

Lardo Boffin wrote: Fri Nov 20, 2020 9:07 pm Good luck with the builds! :D

If you need any Sinclair heatsinks I have quite a few! Mostly because I replace the regulators with these murata ones:

https://retroleum.co.uk/spectrum-repairs-mods

I’ve bought a few PSUs from here:

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

Ask for the 2600 version.

ZX8-CCBs make for a great composite mod, from sell my retro.
Thank you for the advice. I may stick with the homemade clock face heatsink since my late Grandfather fitted it in the 1980s, so it's a little sentimental. But I may also upgrade to switch mode 7805 replacements. Not sure yet.

I was looking at the ZX8-CCB circuit yesterday, and was delighted to see they fix the back porch issue, so it's likely I'll fit them to some or all of the ZX81s once I know how many I've got working reliably.

The "working" ZX81 turned out to be far less working than I thought - I tried to get some of Dr Beep's 1K hires games to load and mostly it failed, or crashed quite soon after loading. But it's early days yet!

Thanks

Ian
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

Last night I started desoldering the sockets from ZX81 #2. I've got some 2K SRAM chips to try, as well as 1 x 32K chip for the 16K mod. But I want to replace all the sockets on all the 81s, and fit sockets where there are none to make it easy to swap things around to see what works.

I don't have a desolder station, and my funds won't stretch to one at the moment.

But I've years of soldering and tinkering behind me, a manual desolder pump and desolder braid. How hard can it be?
IMG_20201121_132754.jpg
Harder than I expected, it turns out. This is the result after over an hour's work.

I tried desoldering the socket intact, but got nowhere with that. So I snipped at it with cutting pliers, and it quickly broke into pieces, leaving me with the contacts which could be grabbed with pliers while I desoldered from the rear. That bit wasn't too tricky, but clearing the holes of solder has proved a lot harder. I used the solder sucker and desolder braid, repeatedly from both sides.

As you can see, few of the holes are perfect, and some are still blocked with solder.

Does anyone have any advice or hints as to how to do this ?

The soldering iron is a 15 watt Antex with 2.3mm tip (I thought I had a smaller tip to use but it turns out also to be 2.3mm :( )

Thanks

Ian
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

It is also worthwhile looking at the date codes printed on modulator labels (81xx or 82xx where 81 or 82 is the year and xx is the week number).

A lot of the vertically fitted resistors on your boards look rather drunk. True some may never have been fitted straight in the first place. But do check to make sure none are shorting out to any of their neighbours, or indeed to the top tracks/traces on the PCB.

Some of the ceramic disc capacitors are also flat or at low angles rather than being vertical. This is less of an issue, unless they or their leads have been damaged.

The pins on some types/makes of chips used metals that tarnish (silver). I use a fibreglass cleaning pen/pencil on these.

I don’t routinely replace sockets unless I spot any damage, or I suspect any of the contacts. But when I do, I chop them up so that they are far easier to desolder. I recommend that if you don’t want a soldered in chip, that you cut each pin as near to the package as possible, so that again each pin can be removed individually.

For small pads/tracks, your iron should be fine. But for larger pads/tracks, like used for the 0V/GND or the supply rails, a 15W iron may not have enough power and heat reserve. Antex irons (if it is anything like mine) have a tip with relatively large mass, so have a reasonable heat reserve. So you will have to work quickly once the solder melts. Actually a 2.3mm tip is not unreasonable. For work on larger pads/tracks, I actually use a tip that is bigger than this so that the heat from the iron can flow into the solder joint quicker.

When desoldering, you need three things. The first is liquid flux, the second is new solder and the third is enough heat to flow through the plated-through hole. The flux helps the solder to flow. The application of new solder (and the flux in it) helps the heat from the iron to flow to all of the joint and to ‘freshen up’ the old solder. Only when the solder on both sides of the board (and therefore in the plated-through hole) has melted, is it time to use the solder sucker / solder pump.

You can also try using the iron on the underside with the solder sucker / solder pump on the top side. Try to get as good an air seal as possible between the tip of the solder sucker / solder pump and the board.

In terms of power supply, if you are NOT using a 16k RAM pack, any DC PSU that produces an on load voltage of 7.5V to 12V can be used. The Sinclair UK700 PSU (supplied with most ZX81 computers) is rated at 700mA. But a ZX81 only needs around 350mA. I recommend a PSU with a minimum current rating of 500mA (0.5A) so that there is some headroom for expansions. A higher current rating is fine. The PSU must be a DC type with the correct polarity. It needs to be a “smoothed” or “filtered” (also known as an unregulated) type or a regulated type. But a regulated type is not essential. The Sinclair UK700, UK1200 and UK1400 PSU are all unregulated types.

If you are keeping the 7805 series pass voltage regulator, then there are two advantages to using a regulated PSU with a on load output voltage of between 7.5V and 9V. The first is less heat produced by the 7805 regulator and it’s heatsink. Less heat inside a cased Zeddy is good. The second reason is it’s more electrically efficient.

However, if you are going to replace the 7805 voltage regulator with a modern DC-DC converter, then there is no significant advantage in either having a regulated mains PSU or having a lower input voltage. Modern DC-DC converters (that are pin compatible with 7805 voltage regulators) are so efficient that they don’t even get warm.

If the value of RAMTOP is wrong, then it’s likely that a/the SRAM chip is faulty or there is a poor connection somewhere. Either between the RAM chip and socket, pin and PCB track (poor or dry solder joint, broken leg, cracked PCB track/trace) or the same with the Z80 or it’s socket.

A Zeddy with a white screen is also often (but not always) due to RAM chip problems.

Mark
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

Thank you for this detailed response, it's very welcome!

Individual responses below :
1024MAK wrote: Sat Nov 21, 2020 4:17 pm It is also worthwhile looking at the date codes printed on modulator labels (81xx or 82xx where 81 or 82 is the year and xx is the week number).
I've amended my original post and added the modulator date codes, which are ZX81 #1 : 8205 #2 : 8208 #3:8217 Which pretty much confirms my original thought that #1 was my original ZX81 that I bought in early March 1982. Curious that #2 had the later ULA whereas #3 is the newest board. But #3 had it's ram in upside down, so who knows the history of that one.
A lot of the vertically fitted resistors on your boards look rather drunk. True some may never have been fitted straight in the first place. But do check to make sure none are shorting out to any of their neighbours, or indeed to the top tracks/traces on the PCB.

Some of the ceramic disc capacitors are also flat or at low angles rather than being vertical. This is less of an issue, unless they or their leads have been damaged.
The boards have been rattling around in an open topped box with other junk for 2+ decades, and had a fair bit of handling, so it's unsurprising. I did check them for shorts before powering them up, but I also intended to tidy them all up overall once I get some/all of the 81s working.
The pins on some types/makes of chips used metals that tarnish (silver). I use a fibreglass cleaning pen/pencil on these.

I don’t routinely replace sockets unless I spot any damage, or I suspect any of the contacts. But when I do, I chop them up so that they are far easier to desolder. I recommend that if you don’t want a soldered in chip, that you cut each pin as near to the package as possible, so that again each pin can be removed individually.
The first point was what prompted me to do the socket replacement. One of the boards that was briefly working would fail when it was tapped. I reworked a great number of solder joints, none of which looked suspicious, but the problem remained. Reseating and spraying with contact cleaner possibly improved things, but still intermittent. So swapping the sockets seemed the logical next step. I'm now wondering if I should have got better quality sockets, the cheap ones I got from ebay have flimsy metalwork - the Sinclair ones seemed tougher when trying to get them out!

I continued removing the old sockets and SRAM yesterday, and made better progress, thanks for your advice - Applying the iron to one side and the sucker to the other did indeed clear the blocked holes.

I've got lots of switch mode 12 volt power supplies, and a few transformer-based unregulated 12 volt supplies as well. I was blaming the PSU for the instability of one of the 81s but I'm less sure now. That said, I thought for the time being I'd stick a 7809 on /in the 12 volt PSU (that gives close to 16v unloaded) with a smoothing cap, maybe even the diode trick to drop the voltage further, so that the 81 gets 8-9 volt maximum.

Aside from keeping the 7805 happy and not generating too much heat, is there theoretically any problem with powering a ZX81 with too higher voltage? Say 15 volt? Looking at the board, am I right in thinking that the 81 only uses the 5 volt it regulates for itself, but the unregulated nominal 9 volt does get passed out to the edge connector? (If so, what for?)


Thanks

Ian
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

These are the replacement SRAM chips I got recently from ebay :
IMG_20201122_145229.jpg
Which I believe to be 2K SRAM (x5 in the plastic tube) and 1x 32K SRAM (on the antistatic foam).

Before I pop them in the sockets later and fry something, could anyone confirm that they're compatible with the ZX81?

(I realise that the 32K will need mods done or legs bent and wiring work to allow it to work, and will give me 16Kbyte, that's fine.)

Regarding the 2K SRAM, if I just plug it into the socket, will it give me 1K by default? And if I solder link L2 I get 2K? Is that right?

Thanks

Ian
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Re: Attic 81's - My collection and attempts to repair

Post by Aurelbois »

The M55117P is a 2K X 8 static ram.
Datasheet at https://datasheetspdf.com/pdf-file/1110 ... M5M5117P/1
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1024MAK
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Re: Attic 81's - My collection and attempts to repair

Post by 1024MAK »

sheddyian wrote: Sun Nov 22, 2020 3:49 pm I've amended my original post and added the modulator date codes, which are ZX81 #1 : 8205 #2 : 8208 #3:8217 Which pretty much confirms my original thought that #1 was my original ZX81 that I bought in early March 1982. Curious that #2 had the later ULA whereas #3 is the newest board. But #3 had it's ram in upside down, so who knows the history of that one.
Yes, I was thinking that some of the socketed chips may have been swapped. I once bought some ZX81s that had been in a college, on one board the ROM had been fitted in the middle of the socket instead of at the bottom (leaving the top four pins empty).

I buy my sockets from Rapid Electronics in tubes.
sheddyian wrote: Sun Nov 22, 2020 3:49 pmI've got lots of switch mode 12 volt power supplies, and a few transformer-based unregulated 12 volt supplies as well. I was blaming the PSU for the instability of one of the 81s but I'm less sure now. That said, I thought for the time being I'd stick a 7809 on /in the 12 volt PSU (that gives close to 16v unloaded) with a smoothing cap, maybe even the diode trick to drop the voltage further, so that the 81 gets 8-9 volt maximum.

Aside from keeping the 7805 happy and not generating too much heat, is there theoretically any problem with powering a ZX81 with too higher voltage? Say 15 volt? Looking at the board, am I right in thinking that the 81 only uses the 5 volt it regulates for itself, but the unregulated nominal 9 volt does get passed out to the edge connector? (If so, what for?)
The 7805 voltage regulator has a maximum input voltage of either 30V or 35V depending on the manufacturer. However there is an electrolytic capacitor (issue 1 board) or sometimes two (some issue 3 boards) also connected to the +V input line. Some of these are rated at 16V. Hence the maximum input voltage is 16V.

Note that the unregulated Sinclair UK700 and UK1200 PSUs often have no load voltages of between 14V and 16V. And on load voltages of between 10V and 12V.

All the semiconductors and the modulator on a ZX81 board run off the +5V from the 7805 voltage regulator. The +V input (which is +9V nominal) is only used by the 7805. Yes it also feeds to a contact on the edge connector. Various expansions use it. What for depends on the expansion. Most expansions use it to feed their own 7805 or equivalent voltage regulators. Non-Sinclair 16K RAM packs use it for the +12V rail to the 4116 (or equivalent) DRAM chips. The Sinclair 16k RAM pack uses it to power a transistor based DC-DC converter circuit. The ZX Printer uses it for the motor.
sheddyian wrote: Sun Nov 22, 2020 4:02 pm These are the replacement SRAM chips I got recently from ebay...
sheddyian wrote: Sun Nov 22, 2020 4:02 pmWhich I believe to be 2K SRAM (x5 in the plastic tube) and 1x 32K SRAM (on the antistatic foam).
sheddyian wrote: Sun Nov 22, 2020 4:02 pmRegarding the 2K SRAM, if I just plug it into the socket, will it give me 1K by default? And if I solder link L2 I get 2K? Is that right?

Yes the M5M5117 chips are 2k byte SRAM chips. If you fit one without changing LK1/LK2, the ZX81 will work as a 1k byte machine. If you remove LK1 and fit LK2, it will work as a 2k byte machine (like a TS1000).

The CY62256 is a 32k byte SRAM chip and will require modifications if you want to use it as an internal 16k byte upgrade or a 32k byte upgrade. It is also possible to use it before fully modifying the board (or bending all the required pins out) if required. But pin 1 does need isolating from the Z80 /REFSH signal and pin 23 needs isolating from the /WR signal. Both need connecting to address pins. Details here.

Mark
sheddyian
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Re: Attic 81's - My collection and attempts to repair

Post by sheddyian »

1024MAK wrote: Sun Nov 22, 2020 8:57 pm
[...]
Yes the M5M5117 chips are 2k byte SRAM chips. If you fit one without changing LK1/LK2, the ZX81 will work as a 1k byte machine. If you remove LK1 and fit LK2, it will work as a 2k byte machine (like a TS1000).

The CY62256 is a 32k byte SRAM chip and will require modifications if you want to use it as an internal 16k byte upgrade or a 32k byte upgrade. It is also possible to use it before fully modifying the board (or bending all the required pins out) if required. But pin 1 does need isolating from the Z80 /REFSH signal and pin 23 needs isolating from the /WR signal. Both need connecting to address pins. Details here.

Mark
Thanks for the detailed reply, it's very useful. I hadn't realised I had to remove LK1 if I fit LK2. thank you for clarifying that.

I've not made much progress yet, as I've been waiting for some stuff to arrive.

I've made a regulated linear PSU giving 9 volt from a 7809 - I got a kit from ebay to make this which I attached to a 12volt power plug that outputs 16v unloaded. In putting the kit together I realised that my antex iron bit is too chunky for fine work, and I don't want to mess up the ZX81 repairs, so I've ordered a finer (1mm) bit and some finer solder as well. So waiting on that arriving before I start fitting sockets and new SRAM chips to the 81's to see what I can get going.
IMG_20201202_130541.jpg
I stuck a volt&ammeter on with epoxy, the board enjoys fresh air on top. Though the 7809 gets very hot dropping the 16V down to 9, so I might have to improve the heatsink. I may get a proper PSU one day...
IMG_20201202_131059.jpg
Meanwhile, ZX81 #3, the least worst of the 3 as it at least gives me a cursor, isn't as well as I'd thought. It powered up OK, and I typed in a program which ran, but after 10 minutes or so it locks up.

Seems to be heat related, as letting it rest for a while then powering up again will allow it to run again. Powering up too soon will generate a noisy screen with a cursor then it locks.

As it's currently in a case I've not properly checked if anything is getting excessively hot, but I only noticed warmth from it before.

If it's a faulty ULA this will be a shame as it's the only one with a later backporch fix, the other two make a very dark screen on the colour TV.

I hope you enjoy the quality of this Bratz TV set - I found it dumped by the roadside, changed the mains filter cap and degaussed the very splotchy screen with a tape-head demagnetizer wand! If I get this all working I might stick a picture of Sir Clive Sinclair over one of the Bratz characters, but who should go on the other side?


Thanks

Ian
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