Anatomy of an 8-Bit Over-Voltage

[Bryce]

Hi All,
     after many discussions with Blurredman regarding his unfortunate over-voltage incidents with several CPC's, it got me wondering exactly how much damage and what gets damaged when this happens. Obviously, I wasn't going to destroy a good CPC to find out, but then a friend of mine gave me a ZX Spectrum 48K (Rev. 4B for the hardcore Speccy enthusiasts among us) which had died in exactly the same manner. When I asked how many volts, he answered "Can't remember, it happened in 1985". Apparently he went on holidays with it and forgot the PSU, so he tried another one they had with them, with the obvious consequences. From the damage done, I'd guess it was around 24V.

Although both the CPC and the ZX are both Z80 machines and have many similarities, from a hardware point of view there are important differences. The CPC gets a regulated 5V supply, all done in the monitor. In the keyboard there are no regulators or any protection devices at all, so every component gets the full whack of whatever you send it. The ZX has a 9V unregulated supply, which gets converted inside the keyboard to +5V, -5V, +12V and +12VA, with a very interesting method which I'm not going to go into here.
Because of this, in the ZX there are several parts which might fail first and save everything further down. The reason for this mixture of voltages is because they used bipolar RAM ICs (4116), that need +12V, +5V and -5V and the 12VA is an extra 12V line just for the Modulator, isolated from the RAM 12V to stop RAM noise from interfering with the TV Signal.

So what got destroyed? Well if you don't want to read the entire post, the quick answer is: F@ckin everything!

But here's the detailed answer. When I turned it on, there was absolutely no life / picture, it just turned the TV screen black. The supply told me it was pulling almost 3A, so there was obviously short-circuits inside. The 5V line read 9V, so the regulator had shorted through. After replacing the 7805 all other voltages seemed to be fine, but it was still trying to pull around 3A (The 7805 can only supply 1A) and no signals (MREQ, Refresh etc) were being produced, although the CPUs clock signal was present. It was time to start removing things. The ULA being socketed went first, the Amps stayed at 3  Now I had to start cutting pins. The physically closest part to the supply is the ROM, so it went next and so on until the board was nearly empty. Each time I had isolated something, I tested it individually, to see if it still worked. In the end it turned out that the following were damaged:

ROM - wasn't shorted, but wasn't answering requests either
CPU - Fried, shorted, no signs of life.
Logic - (5x 74HCT157, 1x 74HCT32 and 1x 74LS00) - All shorted inside.
Lower RAM - (4116) At least 3 damaged, stop checking after that and just replaced them all.
Upper RAM - Suprisingly all fine (these are type 4532 and only require +5V)
ULA - Also strangely survived.

All the Modulator and TV stuff (LM1889) was fine. All capacitors, transistors, diodes and resistors were fine as suspected (they can usually take quite a bit). The transistor to the speaker was damaged, but I suspect this happened before the short circuit, I replaced it with a BC337, the ZTX450 was dis-continued years ago. Below you can see the repaired ZX. Everything that's socketed had to be replaced (except for the ULA). The strange piggy-back IC's in the middle and the extra blue wires are a mod I needed to do to get a modern EPROM to work, the Hitachi and NEC ROMs used back then had different signals and pinouts than today's EPROMs.

After everything had been replaced, the ZX sprang to life after 26 years Hibernation

Hmmm, turns out the keyboard membrane is also damaged (from age, not from the over-voltage), but I have a new one here, so I'll swap that out too.

Bryce.

[robcfg]

Wow!


That was quite a nice repair job!


I find it also super the nice explanation, you never know when are you going to fry a ZX (not on purpose, that is  ).


Thanks, mister!

[MacDeath]

24V ? good old junk got what it deserved...

did the rubber keyboard melt ?

(Wait, was it even the rubber version ?)
And the guy still got this ?
Well, this must look good put on a wall...

[Bryce]

Yes, it's the rubber key version. I swapped the membrane afterwards and everything (I can test) works fine. I wrote a short basic program to test the memory and there were no errors. Unfortunately, other than a ZX Power-supply, I don't own any other Spectrum hardware or software, so I can't test much more at the moment. If I have time at the weekend, I might build a simple ROMBoard and run a few programs on it.

The reason the owner still had it, is because he has the same situation as me. I left home (and the country) back in 1991, when I still had a good collection of hardware and components. Everything was put in a box in my parents attic and hadn't been touched since then. Which is also the reason why I still had a brand new ZX membrane and quite a few of the ICs required to fix it

Bryce.

[redbox]

Great post, really interesting and I managed to understand the gist of it (mostly).


Thanks for this and nice to hear you got it work again.


Also interesting to see the modulator looks like the one in my GX4000 

[steve]

The only good spectrum is a dead spectrum

[Bryce]

If we had a "Dislike" button, you would have just got one from me 

Every 8-Bit is worth saving![nb]Except C64 of course [/nb]

Bryce.

[steve]

Only joking, I have a spectrum 2 which I bought somewhere, never even found out if it works or not.

I may cannibalize it for parts one day , maybe put a c64 inside it .

[Bryce]

Well the only socketed ICs in a Spectrum +2 are the Z80 and the ROM, everything else was soldered. The Z80 is obviously a good spare to have, but the ROM (Amstrad 40055 as far as I can remember) isn't useful for much else other than fixing another Spectrum +2. The AY is useful but soldered, as is the RAM (4164 as used in CPC too).

Bryce.

@MacDeath: You get a "Dislike" from me too, for "Liking" steves comment

[steve]

"methinks he protesteth too much", admit it bryce you liked my comment too.

[Gryzor]

Of course every machine is worth saving, but then again there are more important things than saving a 48k; taking a piss, for instance. Or staring at your wife for the equivalent of time spent on it (added bonus: she's flattered or she thinks you're crazy and leaves you at that for the rest of the evening. Win/win).

[Bryce]

It took me 3 days to fix. If I stared at the wife for 3 days she would think I was mad / creepy / dead or all of the above. I doubt she would be flattered, more worried

Bryce.

[Blurredman]

Interesting topic. I'll defo be sending you those two blown boards, and possibly my very special 464 (same problem as one of the 6128's) 


Naturally, the more motherboards types you ressurect, a better archive on how other people may fix such machines, if they have the means to


Really interesting.

[Bryce]

Cool, I will do similar reports on them too then

Bryce.

[MaV]

Guys, remember: The Spectrum was successful in the 80s because it was cheap. And: you shouldn't blame the computer for the Speccy ports, it's the software companies who are to blame.

[sermon mode]
Now I want you to remember that each time in your future when you have a choice of two items, one cheap and the other expensive, and you buy the cheap one, that you would have been a potential buyer of a ZX Spectrum.

Also you, MacDeath, my little black sheep: I recall a picture of your collection with a ZX Spectrum. If you still have it, what's your excuse?

Every little 8-bitter is worth rescuing from silicon hell.

Amen!
[/sermon mode]

[MacDeath]

Also you, MacDeath, my little black sheep: I recall a picture of your collection with a ZX Spectrum. If you still have it, what's your excuse?

Schizos are never alone...

Anyway it was my brothers one actually... I mistook it for a grey 464 (proto/german ones ?)

[Gryzor]

It took me 3 days to fix. If I stared at the wife for 3 days she would think I was mad / creepy / dead or all of the above. I doubt she would be flattered, more worried

If you spend three days on a 48k then *everyone* would think you're mad/creepy/a zombie or all of the above. And we're all worried for you, mate.

[Bryce]

I appreciate your concern, but I doubt it's curable, so we'll just have to live with it.

Bryce.

[Womble]

Good repair - but the cause of the destruction is probably not over voltage , it is likely to have been reverse polarity DC. The power jacks used on the speccy are the inverse of what has become the defacto standard of having the centre pin +ve. Many other 8 bits of the era actually have the polarity the opposite of the speccy, such as the Oric, and easy to find 9V PSUs these days are almost always pin +ve.

Giving a Vreg DC of the wrong polarity can cause it to become a dead short and suddenly you have -9V across all the components that can only deal with +5.

Ironically the cause of death for many Orics is that they were used with a Speccy PSU as they were far more common than Oric PSUs, the results are much the same.

[Bryce]

There was definitely over-voltage involved, but it may have been the wrong polarity aswell

Eitherway it's working again fine and I've even loaded up a few games lately, just to remind myself how shit they were.

Bryce.

[Badstarr]

Speaking of crap Spectrum games, has anyone seen or played the Speccy version of Metrocross? Yikes! Here's one I'm glad we didn't get a Speccy port of ! No catchy music and colour clash galore, it's giving me nightmares. Horrible, just horrible.

[Bryce]

No catchy music and colour clash galore, it's giving me nightmares. Horrible, just horrible.

So it's more or less like ALL speccy games then?

Bryce.

[endangermice]

Excellent post, really interesting! I have a c64 that I accidentally plugged 12V into instead of 9 several years ago. Since I have two other working replacements I have't really thought about doing anything to it. There are certainly several chips that have shorted (get very hot to the touch) which seem to be the memory and some support logic. Since the SID runs on 9V I suspect that might be fried along with probably everything else!


I may take a look at it one day but will need a lot of spare parts to replace!

[Gryzor]

Thanks for resurrecting this post. I did not remember it and it's a great read.

[MacDeath]

I have a c64 that I accidentally plugged 12V into instead of 9 several years ago.

haha, inferior system.

CPC is supplied with monitor giving the perfect power supply.

yeay, me trolling again.