8MHz Z80 on Sell My Retro

[Lardo Boffin]

I've noticed that Pokemon is selling replacement Z80 chips on Sell My Retro (http://www.sellmyretro.com/offer/detail ... ck%29-3471) and that they are rated at 8MHz. Obviously this additional speed won't be utilised (without some additional serious changes I suspect) but a comment on the 'for sale' page was that it ran cooler than the original.

I was wondering if this was the case? Given the small internal volume of the zeddy anything that can be done to reduce heat creation must be a good thing.

Presumably they also make rather cheap spare CPUs in case of failure.

Lardo

[Paul]

Yes. They run cooler. Replacing makes no sense until you replace the 7805 with a traco or equivalent replacement because that is zeddys main heat source.
Once you replaced the 7805 next steps would be replacing the CPU, ROM and RAM in this order.
That will maximise the life of the ULA due to heat reduction.

Pokemon sells these CPUs because they are required for ZXMore which is able to run at twice the original speed.
kind regards Paul

[Lardo Boffin]

Yes. They run cooler. Replacing makes no sense until you replace the 7805 with a traco or equivalent replacement because that is zeddys main heat source.
Once you replaced the 7805 next steps would be replacing the CPU, ROM and RAM in this order.
That will maximise the life of the ULA due to heat reduction.

Pokemon sells these CPUs because they are required for ZXMore which is able to run at twice the original speed.
kind regards Paul

Thanks Paul. The fact that I don't know what a 7805 is (other than being a power regulator of some sort) suggests I had better leave this alone. Certainly until I have learned more about my zeddy!

Lardo

[PokeMon]

I don't think that the 4 MHz Z80 is running with more Energy than the 8 MHz version. As for all CMOS devices, power dissipation depends mainly on device type and frequency operated. There could be a difference to the very old 4 MHz NMOS versions which are not available anymore, I guess. The point is, that the price of the 8 MHz was the same as lower frequency versions. I think the 10 MHz version was getting down last times but the 20 MHz version is still expensive. Anyway you can not operate a standard ZX81 reliable at higher frequencies as the internal RAM and ROM are not very fast from the 80ies.

And yes, I use them for the ZXmore, Paul is right in this point.
And it is by the way original Zilog hardware, bought from a well known distributor - no chinese copy of Z80s like sold mainly via eBay or Alibaba.

[RetroTechie]

Yes. They run cooler. Replacing makes no sense until you replace the 7805 with a traco or equivalent (..)

Not true. Replacing the 5V regulator with a switching equivalent, and replacing internal components like a NMOS -> CMOS Z80 swap, comes down to attacking the same problem from 2 different angles. Either will give improvement. For best results, do both.

because that is zeddys main heat source.

(the 7805) That is only a problem as long as power consumption of the rest of the machine is such that the 7805 heats up a lot. When component replacements elsewhere on the board reduce power consumption to -let's say- half, then the losses in 7805 are also halved (and thus, much less a problem).

Another option is replace not with a switching drop-in like the Tracopower or similar, but with a low-drop 7805 equivalent. And then feed a lower DC voltage into the machine. For example not 10V DC into a 7805 with ~50% losses, but 6V DC into a low-drop regulator with ~17% losses.

Like said, you can attack the problem from multiple angles. One of my ZX81's currently runs at ~0.8W (total! ~65 mA @ 12V). That's internal 16K included.

[RetroTechie]

And it is by the way original Zilog hardware, bought from a well known distributor - no chinese copy of Z80s like sold mainly via eBay or Alibaba.

That's good to know. It's hard (if not impossible) to sort OK stuff from fakes these days when sourcing from sites like eBay. Although price is often a good indicator. Original, never used Zilog 20 MHz Z80 for a little over $1 ? And free shipping? Not happening...

Btw product image doesn't match description.

[Paul]

By saying it makes no sense I mean the following:
Replacing NMos CPU against CMOS CPU will reduce the internal temperature of the Zeddy from (guessed) 55 degrees Celsius to (guessed) 53 degrees Celsius.
Replacing the 7805 will reduce to guessed 28 degrees.
So if you want to reduce the temperature there are many things possible. But without replacing the 7805 you won't get remarkable results.
A different way of reducing heat effectively without soldering is to use an adjustable power supply. Adjust it to as little voltage as possible.
This also reduces heat significantly.

[PokeMon]

Original, never used Zilog 20 MHz Z80 for a little over $1 ? And free shipping? Not happening...

I think these chinese companies do not pay for shipping at all. It's probably sponsored by China's authority to hold it exports. When you compare prices from mouser as official Zilog distributor, the price for 8 MHz is about EUR 5.60 on a single piece base including VAT and for the 20 MHz version it is nearly 13 EUR including VAT.

http://www.mouser.de/Search/Refine.aspx?Keyword=Z84C00

About the picture - you are right. I think I was too lazy to update picture when changing 6 MHz to 8 MHz.

[1024MAK]

Replacing a NMOS Z80 CPU (as fitted when the ZX81 was made) with a CMOS Z80 CPU (regardless of it's maximum rated speed) will reduce the temperature inside. Both from the reduced amount of heat from the CPU and the reduced dissapation from the 7805 series voltage regulator (as it has to supply less current, it's own heat output drops). However, the difference when only changing the CPU on it's own is not going to make a huge difference in the internal temperature of the computer.

The same goes for replacing the ROM and the original SRAM chip(s).

Replacing the 7805 voltage regulator with a modern miniature switching type, will make a big difference. As the modern miniature switching type does not produce much heat. The 7805 type works by getting rid of the difference in voltage between the input and the output as heat. So if the input is 10V, the output is regulated to 5V. If the current being drawn is 330mA (a typical amount) the amount of power that the 7805 converts to heat is 1.65 Watts.

If you don't want to change the 7805, an alternative is to use a regulated external switch mode power supply. You want one that has a 7.5V regulated output. The lower input voltage means the 7805 regulator in the computer does not have to dissipate as much heat, so it runs less hot. (7.5-5) X 0.33 = 0.825 Watts.

Note however, that some third party 16k byte RAM packs will not work properly with such a low input voltage. As they take advantage of the Sinclair power supply producing between 10V to 12V.

Mark

[Lardo Boffin]

Thanks all. It certainly sounds like the 7805 has to go!

What is it replaced with? There are too many hits on 7805 on the forum to be able to tell when that was asked before...