ZX Spectrum 48K Mainboard Issue 4S - Help!

mikelovell

ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Hi,

Firstly I'd like to say that this is my first post other than my introduction, so please bear with me. I was recently given a ZX Spectrum + which was in a terrible state, but as I've been thinking about buying one from a well-known auction site I thought this would be a 'free-hit', meaning it might not cost too much to get into a working state.

The case, keys, membrane were all completely ruined so my thinking was to remove the mainboard, see if it works and then get some 'rubber key' replica/custom parts to fit it into. Upon initial inspection, there are some pretty major issues with it and one in particular is completely baffling, so I'll show you that first.

1. Z80 CPU missing

I've never seen this before. Whoever previously owned this appears to have desoldered the Z80 from the board and installed some naked pins in its place, badly. I'm looking for some advice on what to do next with this (if anything).

Image

Here is the rear of the board. Hopefully you can see what I mean here, I've boxed the pins that have been soldered in place of the CPU.

Image

2. ULA missing

I'm told that the ULAs for this board aren't commercially produced anymore, so I'm looking for some advice.

3. Ceramic capacitors bent/flattened

Looks as if the previous owner squashed some of these during their repair attempts. Any advice on whether these are still likely to work would be great.

So, in a nutshell my guess is that this project could cost me a bundle just to get into a testable state. Any advice welcome and certainly regarding the CPU. I've never heard of a removable/changeable Z80 CPU that would cradle onto pins in that way, so I'm totally baffled by it. Is it even worth trying to get this board into a working state or am I better off buying one from auction?

Cheers,
Mike
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1024MAK
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by 1024MAK »

Note, I’ve moved this topic into the ZX Spectrum Hardware area.
mikelovell wrote: Tue Aug 24, 2021 6:03 pm 1. Z80 CPU missing

I've never seen this before. Whoever previously owned this appears to have desoldered the Z80 from the board and installed some naked pins in its place, badly. I'm looking for some advice on what to do next with this (if anything).
That looks more like the previous owner has cut off the Z80A plastic case from it’s pins, leaving just the pins behind. An angled side view photo would confirm this.

Note that reasonable size photos and pictures (not the super silly sized four zillion giga pixels type mind) can be attached to your post. Either use the ‘attachments’ tab below the text edit box or if using a PC, drag the image file to the text edit box.

The old pins need desoldering and removing. Then the PCB needs cleaning. Visually inspecting for damage, then maybe testing using a multimeter. Then, a nice new DIL/DIP socket can be soldered in.

But read on first…
mikelovell wrote: Tue Aug 24, 2021 6:03 pm 2. ULA missing

I'm told that the ULAs for this board aren't commercially produced anymore, so I'm looking for some advice.
Correct, the ULA has not been manufactured for rather a long time. That’s the bad news. The good news is that there are two people in the world who make ULA replacement modules. I’ll add links to the relevant sites later. But keep in mind that COVID19 has messed up production and even before then, they sell out just as quick and the guys can make them.
mikelovell wrote: Tue Aug 24, 2021 6:03 pm 3. Ceramic capacitors bent/flattened

Looks as if the previous owner squashed some of these during their repair attempts. Any advice on whether these are still likely to work would be great.
The forgotten heroes of digital electronics. The capacitors that is. If they are only bent over, but not physically damaged in any other way, they should be fine. Visually inspect each one. If you want to, you can carefully bend them to a less silly angle.

If however, there is a crack, break, or a bit missing, then they may fail short circuit. So they should be replaced. However, some have a wax like coating. No need to worry about replacing any where it’s just the wax like coating that is discoloured or uneven.
mikelovell wrote: Tue Aug 24, 2021 6:03 pm So, in a nutshell my guess is that this project could cost me a bundle just to get into a testable state. Any advice welcome and certainly regarding the CPU. I've never heard of a removable/changeable Z80 CPU that would cradle onto pins in that way, so I'm totally baffled by it. Is it even worth trying to get this board into a working state or am I better off buying one from auction?
Only you can really answer that. As long as the PCB has not been too badly damaged, it can be brought back to life. Brand new CMOS Z80 chips are still available. Or there is a almost endless supply of NMOS Z80s on eBay (but don’t rush off and buy one yet).

Before anything else, it would be good to do some other checks and tests first. Do you own or have access to a digital multimeter? What about any other electronic tools and equipment?

And can you solder?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by Lardo Boffin »

One possible source of a replacement NebULA (when bought with a repair):

https://www.mutant-caterpillar.co.uk/sh ... uaqq1ag584

The original site for them (sold out):

http://retroleum.co.uk/zx-spectrum-chips

There is also the vLA82 (sold out everywhere I’ve looked), e.g.

https://zxrenew.co.uk/ZX-Spectrum-repla ... p151287223
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Thanks for the responses.

I do have a solder workstation and access to a multimeter. Why would anyone cut a Z80 away from its pins?! The mind boggles.

Anyhow, I found this - would this work as a replacement Z80?
https://www.dataserve-retro.co.uk/conte ... p1850.html

Cheers,
Mike.
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by Lardo Boffin »

Hi Mike

Not sure - I have used the Z80 from Retroleum in a few ZX81s and Spectrums. You can also get a high efficiency murata regulator from Retroleum (http://retroleum.co.uk/spectrum-repairs-mods) and ditch the heat sink.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Many thanks for the reply.

I've managed to desolder the old Z80 pins from the board and cleaned the through-holes as best I can. I'm going to order a NEC D780C-1 CPU which runs at 4Mhz, so slightly faster than the original 3.5Mhz Zilog or whatever was in there. People report success using them in earlier issue boards and Mutant Caterpillar use them in their repairs, so hopefully...

https://www.dataserve-retro.co.uk/conte ... p1850.html

As for the ULA, I'll keep looking.

Many thanks for all the help, I'll be back sooner than you think.

Cheers,
Mike.
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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by 1024MAK »

mikelovell wrote: Tue Aug 24, 2021 8:10 pm I do have a solder workstation and access to a multimeter. Why would anyone cut a Z80 away from its pins?! The mind boggles.

Anyhow, I found this - would this work as a replacement Z80?
https://www.dataserve-retro.co.uk/conte ... p1850.html
Unless you have the right equipment and are experienced with desoldering large DIL chips from through hole PCBs, cutting the legs and removing the chip, then desoldering each pin one at a time (with the help of fine tipped long nose pliers) is far easier. Plus it means less likelihood of damage to the PCB tracks.

Yes, that Z80A is compatible (the Sinclair produced machines are more likely to have that NEC version than a Zilog Z80A). However, that’s not cheap for the NMOS version.

Have a look here. This is also the site that sells the Nebula - A Spectrum 16/48 ULA Replacement Module.

And as mentioned above, the other ULA replacement is the vLA82.

But first, in my sig below, there is a link to ZX81 Chip Pin-outs. In this, there is a pin-out for the 4116 (or equivalent) DRAM chip. This is the type fitted in positions IC6 to IC13. All at the front left of the board. After checking that there are no short circuits around the cut-off pins of the Z80A, measure using the 20k ohms resistance range between pin 1 (any one of the eight chips will do) and any convenient 0V/GND point (the heatsink tab of the 7805 voltage regulator or the TV/RF/UHF output connector outer for example). Swap the leads around and retest. If the figure is low enough switch down a range. Note the results.

Then do the same for pin 8 (again, you only need to do one chip).
Then same again for pin 9.

Report back your results.

I assume you have a suitable DC PSU that can output between 8V and 11.9V, at 800mA or greater? Even better if you have a bench PSU with an adjustable current limit.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by 1024MAK »

mikelovell wrote: Tue Aug 24, 2021 10:44 pmI'm going to order a NEC D780C-1 CPU which runs at 4Mhz, so slightly faster than the original 3.5Mhz Zilog or whatever was in there.
Err, no. If you fit 150MPH tyres to an old banger with a 998cc engine, do you think it will go up to 150MPH?

All ZX Spectrum computers have a 4MHz Z80A (well, the occasional one may have a faster rated chip, but it’s rare).

The speed the Z80A runs at is determined by the clock signal from the ULA. Which is appropriately 3.5MHz regardless of the speed rating of the chip in position IC2…

The NEC µPD780C-1 is the equivalent to a Zilog Z80A. The datasheet is here. They are both rated at 4MHz maximum clock speed.
See also here.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
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mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

Hi Mark,

Thanks for your detailed reply.

As mentioned, this project was sort of a 'free hit' - meaning if I bugger it up, no real harm done. I've managed to desolder the old CPU pins from the board and cleaned the through-holes best I can - see the image below, board was held up to a backlight so you can see the through-holes - bit swarfy but might be okay ;) :

zx-spectrum-4s-front-depinned.jpg

I've decided to go for the NEC D780C-1 because it's a relatively inexpensive failure, should that be the case. I've got access to a multimeter and lab bench PSU, so I'll take a crack at installing the CPU and then follow your detailed instructions to make sure I haven't messed anything up along the way, report back and go from there.

PSU has been ordered and should arrive in the next week or so.

Many thanks again for your patience and help!

Cheers,
Mike.
mikelovell

Re: ZX Spectrum 48K Mainboard Issue 4S - Help!

Post by mikelovell »

1024MAK wrote: Tue Aug 24, 2021 11:12 pm Err, no. If you fit 150MPH tyres to an old banger with a 998cc engine, do you think it will go up to 150MPH?
No, I don't think that. I was merely pointing out that the D780C-1 clock speed is adequately above and close enough to the original (4Mhz > 3.5Mhz), so no point buying a D780C-2 (6Mhz > 3.5Mhz) or higher.
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