Changing stock CPU for a fast version.

By Underscore

Rookie (22)

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12-05-2019, 08:53


I am thinking about replacing the stock 4MHz Z80 from my Toshiba HX-20 for a less power consuming version of it to deal with overheating problem. But, I cannot find any with 4MHz version that can be shipped to my location at a reasonable price. Everything I can find is 6, 8 or 10 MHz.

In either case, and for general programming, what could this increase affect? I read it changes the sound tempo/pitch. Since I want learning and general programming with BASIC, this is not a problem.

Other than that, what other important things could be affected?

Thank you.


By macca

Supporter (2)

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12-05-2019, 09:01

The CPU speed stated on the labels (for example Z84C0010 which is a 10MHz unit) is the maximum certified speed at which it can run, not the speed at which it actually runs, that is the clock frequency from the target machine. If you just change the CPU you should not have any problem.

By rderooy

Paladin (686)

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12-05-2019, 10:22

As macca basically said, changing the CPU with a faster one should not cause any speed change unless you also change the crystal on the systemboard. It is this crystal that actually sets the speed.


Enlighted (6093)


12-05-2019, 10:56

All important has been said already. CPUs do not select the speed by them self. They have maximum speed they can run, but no minimum speed. Even your "4MHz CPU" is not actually running on 4MHz... It is running 3.5795MHz speed... but even that is not the real speed that could be compared to other Z80 computers. Due to memory refresh the actual effective speed is slowed down more close to 3MHz than 4MHz.

By Underscore

Rookie (22)

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12-05-2019, 14:45

I see, that is good news. Thank you for the info.

By sd_snatcher

Prophet (3675)

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12-05-2019, 16:31

The PSG sound pitch will be affected only on machines that have the PSG integrated into the chipset (aka MSX-Engine). On a machine that has a discrete PSG, all you have to do is to lift the pin of the clock divider chip (external for the AY-3-8910 and internal for the YM2149) and connect it directly to the 3.57MHz clock source on the PCB (usually the VDP).

MSX1 machines usually don't have pull-up resistors on the databus, so if you replace the CPU with a CMOS version, be sure to check if the pull-up resistors are present (easy to check with a multimeter) and if they're not, solder two 5-pin 4K7 SIP resistor-networks as described below:

a) On the underside of the PCB, solder the two resistor-networks as described below. Pay attention because the pin-1 is marked on the resistor-network and has a specific function. If you solder this pin-1 incorrectly the CPU will behave erratically.

b) The pin-1 of the 1st resistor-network must be carefully connected to the pin-11 of the Z80 socket, and the pins-2 to 5 must be connected to the pins 12 to 15 of the Z80 socket respectively. But don't solder its pin-1 just it yet. Solder only the pins 2 to 5 of this resistor network for now. Make sure that the respective Z80 socket pins also get properly soldered to the PCB now, of course.

c) Now, carefully put the 2nd resistor-network in its place. Its pin-1 also must be connected to the pin-11 of the Z80 socket, and the pins-2 to 5 must be connected to the pins 10 to 7 respectively. Now you can solder all the pins of this resistor-network, and the pin-1 of the 1st resistor network too, since both pins-1 must be interconnected to the Z80 socket pin-11. Again, make sure that the respective Z80 socket pins are also properly soldered to the PCB.

After the PSG mod and pull-up adjustment, it's easy to swap the CPU and install a SuperTurbo 3.x kit on your machine. You'll have to cut the trace where the original clock source came from (usually the VDP).

On NTSC machines, after that trace is cut you'll also have to solder a wire to send the 3.57MHz clock to the TV-Encoder.

By Underscore

Rookie (22)

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14-05-2019, 14:36

Thank you for the advice. I will definitely look into it.

But as for now, since I don't need sound output, I will leave it as-is.


I forgot. Is there any way to know if a Toshiba TMPZ84C00AP CPU is fake one or not? How much they typically cost?

By RetroTechie

Paragon (1563)

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14-05-2019, 16:42

@sd_snatcher, I think Underscore is not looking for a turbo upgrade. But rather for a cooler running CPU @ stock speed. Simply replacing NMOS Z80 with a CMOS one would do that (and not require any other change). NMOS Z80 running at 4 MHz: typical 90 mA. CMOS Z80 running at same speed: typical 10 mA or so. That's 0.4W (+ associated losses in power supply) less heat in the machine. More efficient or external power supply would also make a big difference.

@Underscore: note that eg. eBay has lots of cheap '20 MHz' Z80 on offer. For many of those, what you'll get is a fake: a Z80 that's been relabeled to appear a higher-speed version. But... if the rebranded part happened to be a CMOS part as well, that would be exactly what you need! Shocked! Regardless of speed the part was originally rated for. Only problem is if an NMOS part was relabeled as a CMOS one.

In other words: for this particular purpose, a probably-fake '20 MHz' Z80 from eBay might be just what you need! Might be worth the gamble @ a few sellers.

By RetroTechie

Paragon (1563)

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14-05-2019, 17:10

Underscore wrote:

Is there any way to know if a Toshiba TMPZ84C00AP CPU is fake one or not? How much they typically cost?

TEST! There is no -reliable- other way. If you can't do that, don't buy from eBay & co. What tests you run (plug in, "works or not", run test software, hang scope on some pins, measure current consumption, X-ray the chip, decap & compare silicon die with original LOL! ) is up to you.

Toshiba Z80's are uncommon enough (but not special enough to be sought after) that I think few Chinese will bother to fake these. If any. So if it works, better chance you have a genuine part than in the case of Zilogs. Cost.. maybe around $4 or so for a New Old Stock part? Perhaps less if unsoldered or "pulled".

If you don't like gambling, buy from respected distributors like Farnell, Mouser, Digi-Key etc. Z80's are still being manufactured & sold BRAND NEW today. Cool

By Pentarou

Hero (563)

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14-05-2019, 20:04

sd_snatcher wrote:

The PSG sound pitch will be affected only on machines that have the PSG integrated into the chipset (aka MSX-Engine).

I had read many times that one of the downsides of having a turbo CPU was the PSG, but then sometime ago I built my turbo on a machine with a MSX engine (Philips 8235) and had no problem of that kind.
I kept the machine motherboard running at the stock clock, and only fed the turbo clock to the Z80.
As a result I got a situation similar to the Turbo R (*): CPU running at high speed, sound normal, and problems with FM sound (missing notes).
With your patched games the FM sound problems are fixed, and there are no video problems as I'm using a 9958 with the interrupt line connected and the BIOS patched to enable it.
So, I wonder, why this isn't done more often? Why people always talk about the problems and nobody mentioned this solution?

(*) Never used a real Turbo R, but It's something that I've read. TurboR fixes work on my machine too.

BTW. with SDMapper at 8MHz I'm getting ~3x performance increase in reads, I'd like to test the mod that you mentioned in your 'WIP' thread, to see what more can be squeezed out of it.

By sd_snatcher

Prophet (3675)

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14-05-2019, 23:59


Ah, ok! When I read "faster version", I thought it was a turbo. Smile


The problem isn't the PSG, but in fact the DRAM controller that is also included in the MSX-Engine. This is why the MSX-Engine usually was sped up too, then the PSG gets high-pitched as an unwanted side effect.

It probably worked in your machine because you replaced the DRAMs with SRAMs? That's the best solution, since in this case you won't need to speed up the engine.