Sparkly pixels

Discussions about Sinclair ZX80 and ZX81 Hardware
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy »

Fwapp wrote:If it still "sparkles" after the fitting the recom, try applying something suitable metallic to draw heat out of the ULA. If that cures the sparkles, it'd be worth fitting a DIP40 heatsink which might prolong the ULA's life a bit.

If that is the case, PM me your address and I'll send one over, return the favour for the sideways ROM a bit ;)

Sorry to be the nay sayer but fitting an heat sink to a chip in an enclosed case with no air flow will do no good what so ever.
It will only serve to reflect heat back into the chip at worst or reach thermal equilibrium at best as there is no convection/conduction at all inside a closed system like the ZX81.

I base this on 45 years of building audio amplifiers and their respective cooling systems. when ever a customer said "hide the heat sink inside the case please" it always ended badly when they were insistent.

The heat has to go somewhere and inside a closed system like the zeddy there ain't nowhere for it to go.


The biggest culprit is the regulator so fit a switching jobby and remove the biggest heat source in one go or as karl says use a 9v switching power pack and at least reduce any heat the reg might give off.


Another thing to remember is that the chips inside a ZX81 do not create argon arc furnace temperatures , they are relatively warm at worst and people do tend to panic a little where the ULA is concerned.

Of all the ULA's I've knackered in the last thirty odd years it has always been down to me messing about with peripherals IE removing them when powered up etc, I've never buggered one up through heat problems and don't know of any one else who can PROVE they have either, wether or not your 81 has been used 10 times or 10,000 times in the past your ULA has survived a third of a century and is more hardy than most theorists seem to think.


As a final note, Lardo, should you or anyone on the forum be able to prove your ULA has been totalled due to excess heat I will happily dismantle one of my zeddies and send you the most up to date replacement F.O.C.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Sparkly pixels

Post by 1024MAK »

With respect to semiconductor temperatures, the thing that matters is the core temperature of the semiconductor chip. These are normally rated for a maximum core temperature of between 100 degrees C and 125 degrees C.
The encapsulation / case of a chip can therefore be hot to the touch without the chip in question being in distress.
I don't have a datasheet for the ULA, but this SGS Z80 data sheet shows that a normal grade plastic DIL can be used up to 70 degrees C (see the ordering information on the last page).

Pictures of my ZX81 fitted with a RECOM 5V Regulator

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 :!:
Looking forward to summer later in the year.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy »

Excellent post as ever Mark.

The fact that the ULA chip whose casing is rated at 70 degrees Celsius does not turn into a melted mass inside the zeddy when in use means the core is running well under its 100-125 degree maximum therefore not likely to implode any time soon unless you decide to pull the ram pack off when powered up.

It really is a more hardy chip and doesn't run as hot as some people think it does.
User avatar
PokeMon
Posts: 2264
Joined: Sat Sep 17, 2011 6:48 pm

Re: Sparkly pixels

Post by PokeMon »

Yes - chips are often stated to operate in environments up to 70 degrees Celsius.
But the data sheets do not state how long it operates or how the influence of heat is for aging of semiconductor parts.

The die size of the Z80 cpu for example is about 25 square mm.
The thermal resistance of a dip 40 package as junction-ambient is give with about 40-45 K/W.
If a part has a power dissipation of 5V/100mA this 0.5W and will increase temperature with 20K.
If the case has for example 50 degrees as the ULA (never measured but feels like 50 :mrgreen: ) then the junction or internal silicon has minimum 70 degrees. I think it could be higher due to bad quality when pressing the chips in the 80ies on old manufacturing machines.
Bildschirmfoto 2017-01-21 um 15.16.35.png
Bildschirmfoto 2017-01-21 um 15.16.35.png (39.15 KiB) Viewed 3832 times
But there is indeed another effect, this is an average. There could be (and will be) parts of the silicon producing more and less power. So it is likely that there are partly structures operating at 90 or 100 degrees in the ULA. And this will take lifetime. When Texas Instruments talk about 10 years lifetime with junction temperature of 105 degrees Celsius there might be smaller lifetime on 80ies chips due to older production technologies.

And last but not least, lifetime is also measured as an average. The failure rate increases but there are parts with longer and shorter lifetime as well. We don't know too much about the ULA internals but we all know that it is getting quite hot and that hot temperatures in general are not good for lifetime. It is always a kind of stress. Overheat, over voltage, over current.

By the way - a typical candidate of failure is pin 16 of the ULA (video out) as this delivers all the current for the modulator input and operates not digital but linear. Showing a black screen is quite harder stress for the ULA as there is a voltage difference to take while delivering current for a very small part of the internal silicon. And as we all know Sinclair, they never hesitated to check out the limits of products and parts. ;)

These documents give quite good information on thermal resistance and lifetime operation:
http://www.ti.com/lit/an/sprabx4/sprabx4.pdf
http://www.nxp.com/documents/thermal_de ... PTER_6.pdf

And if someone wants to get deeper into this (interesting) stuff about thermal calculations and effects:
http://www.infineon.com/dgdl/smdpack.pd ... 05ea1d4599
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy »

Great post Karl that adds to my knowledge. :D

There is a micrograph on the net which I used to have but have now lost of the ZX81 ULA I will try to seek it out.

The ONLY point I am trying to get across is that in an enclosed unventilated system heat sinks serve little purpose without a through put of air like a PC for example.
My own Zeddies are mounted on chassis with no case and YES I attach small heat sink and fan cool the whole board with a PC case fan but would never dream of heat sinking a cased 81 and the documentation you have provided re-enforces the fact that heat causes life reduction of the chip, that's why mine are cooled the way they are in the first place, but I think my point still stands that an heat-sink is next to useless in a cased ZX81.

There is an advertisement for ULA heatsinks here...

http://www.thesinclairshop.com/docs/ula.pdf

You will notice that the test is done with the board out of the case with free air circulating to cool the chip and the regulator plate. I would love to see the test results of a fully enclosed 81 and the temp taken at the heat sink/chip junction if possible, somehow I think the readings would be very different after a few hours use, especially as the regulator is now free to turn the internals into a mini oven, though I am willing to be corrected.

I still await for provable evidence of any ULA destroyed not life reduced by heat as opposed to a clumsy fool like me removing a peripheral when powered up which I suspect is the main cause of ULA failure. :oops:

Changing the reg to a switching device I believe would cure most of the heat related problems associated with our Zeddies not heat sinks.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Sparkly pixels

Post by 1024MAK »

A while ago there was this interesting thread on World of Spectrum forums about the temperatures of ZX Spectrums...

It's debatable which causes more life reduction, cold/heat/cold cycles, switch on surges, or running for long periods of time at elevated temperatures... None of these is desirable.

Of course, where possible, it is wise to run electronics at lower temperatures (but not below 5 degrees C, and not to the point that condensation forms), or to provide a forced airflow so that the heat generated can be easily and quickly removed from the electronics ;)

And of course, if the air surrounding a semiconductor is hot, the power rating of the device should be derated.

One thing that is known about the ULA chips used in Sinclair computers, is that the main logic operates at a much lower voltage than the +5V supply voltage. So the ULA chip includes multiple on chip internal voltage regulators. I suspect these are a significant source of a lot of the heat. They are arranged around the chip, so the heat should be relatively even across the chip die.

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 :!:
Looking forward to summer later in the year.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy »

1024MAK wrote:
One thing that is known about the ULA chips used in Sinclair computers, is that the main logic operates at a much lower voltage than the +5V supply voltage. So the ULA chip includes multiple on chip internal voltage regulators. I suspect these are a significant source of a lot of the heat. They are arranged around the chip, so the heat should be relatively even across the chip die.

Mark

Wow I'm learning more and more each post!! :o

If the main logic is say for an hypothetical example running at 3 volts would feeding the ULA 3 volts work or do other parts of the ULA need 5 volts?

My point being that just as lowering the voltage to the Reg allows it to run cooler could the same thing apply with the ULA? If so I might risk rigging up a lower voltage supply to the ULA though knowing my luck I'd blow the bloody house up! :lol:
User avatar
Fwapp
Posts: 152
Joined: Mon Jun 01, 2015 10:32 pm

Re: Sparkly pixels

Post by Fwapp »

Very interesting indeed!

I've got a stunt ULA I use for testing (works apart from not reading the keyboard) - think I might also try supplying the ULA with 3v to see what happens :)
ZX80, ZX81 - 2C158C Bugged Rom iss.1, 2C158C "kludge fix" iss1, 2C184E iss1, 2C210E iss3, US NTSC iss1 "straight traces", US NTSC iss3 TS1000
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Sparkly pixels

Post by 1024MAK »

The parts of the ULA (peripheral interface cells) that interface with the pins, and therefore the outside world need +5V.

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 :!:
Looking forward to summer later in the year.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Sparkly pixels

Post by Moggy »

1024MAK wrote:The parts of the ULA (peripheral interface cells) that interface with the pins, and therefore the outside world need +5V.

Mark
Bugger!

Thanks for the info Mark.
Post Reply