Sharp MZ-821 small mods

Started by Midori, October 22, 2009, 05:47:18 AM

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I've aquired a Sharp MZ-800, the European version of the MZ-1500. Modell 821. Lord knows why... I decided to mod it a little to make it more user friendly for me.

There was little info on this machine. There is the great site located here: of course. But there are few users and boards usually can contain a lot of good info. I found this service manual though: that helped me a lot.

Mod 1 was to trick the computer that a Tape Recorder was connected and to route the read and write pins from the internal tape connector to the external read and write 3,5 mm connectors that are located on the computers back so that I can use a PC or similar device to load tape game audio files from.

The mod was simple once you knew how to do. The service manual explains how. Locate the internal tape connector called T-5 and short the sense pin to ground, the route the read pin to the external read pin and do the same with write. The computer needs a quite strong audio signal to load the games though, my PSP for example needs a amp for the MZ to read the signal properly.

The computer has an interal speaker, I can't say I was very fond of it and wanted to have a line level sound output instead. The sound chip is a 76489AN, the same as a Master System uses for example. On pin 7 it outputs audio, from what I could understand from the datasheet it was already line level and could be routed to a TV or an amp. So I dissconnected it from the internal amp and also dissconnected the internal speaker so that no load would be on the amp. Then I routed a cable from pin 7 on the 76489. Worked flawless, quite loud signal though. The datasheet sugested a capacitor on the line, thats probably wise. Sad thing is that I ain't very wise...

To separate the amp was easy, just remove or lift one of the legs on the resistors R5 and R6, easy to reverse later if one would change his/her mind.

The resistors lifted and insulated:

The computer has a RGB socket, but it is a RGBI socket. Without composite sync, 5V or the audio I have routed from the 76489. So I decided to mount a new jack with the signals I wanted.

All the signals I wanted except the audio was easy to grab from the RF modulator which also produced the composite video output which I use as sync. I could probably have tied the H and V sync together but it was easy to grab the composite video. The modulators PCB has clear markings of which signals that are which, its on the other side of the PCB though so you have to remove it to read. Or do painful work which includes a mirror...

The image signal cables attached:

I used a DIN-8 socket since it was hard to mount a DB-9 connector in the case.

Mounted although without screws:

The RGB signals are a bit to strong so I used 68 ohm resistors on each one to bring the color levels down. The resulting image is great, good colors and a very sharp picture. My TV really likes it too and scales it close to perfectly, no ugly pixels here!

Pic of proof, not very good quality though since its hard to photograph the TV :-)

Now, what am I going to do with this piece of junk...


After reading a little bit more in the service manual I found this line:

"83 - NTPL - I - NTSC/PAL selection(PAL = "L") - GND"

It was on page 11 and it discussed the pins on one of the main chips. I figured that L was low and if so then the pin set to high should switch it to NTSC. So I decided to try and see what happened if it was connected to 5V instead.

I needed to cut a trace to do this, however it was easy. I found a good trace to break on the backside of the PCB.

Here in the red box you see where I cut the trace and soldered a wire:

Unfortunately things decided to be hard for me. This didn't work, or at least it isn't enough to make this work.

The good things are that the computer could boot and give image when 5V was routed to the pin. So it probably isn't totally wrong. Although there the joy ended. The image was not stable and rolled in a... odd way, it was not viewable. You could se the prompt but it wasn't a usable image.

This could be because I use the composite video as sync. The composite video is created by the RF modulator which is a separate module in the computer. Perhaps the problem can be solved if I use a different sync source instead. Or perhaps something else is wrong... I routed the wire to ground instead and it now produces a stable 50 Hz image just like before again.

If the wire is connected to neither ground or 5V the computer doesn't boot, at least not with image. Just random flickering.


I'm guessing that the chip in question is an encoder? Given the age of the machine, I would wager that this is a two-part modification. Not only do you have to change the input on that pin, but you probably have to replace a crystal somewhere in the machine. If it's got its own RF modulator, I'd feel pretty confident in that assessment.


I'll quote the manuals words about the chip:

4-2 Custom LSI
The custom LSI is a 100-pin singel chip LSI on which the MZ-800 memory controller (I/O controller) and CRT controller, etc. are contained

So it isn't really an encoder :-) It outputs RGB signals though.

I think the RF modulator is the culprint to this problem. The RF modulator is a quite big module that has RGB, H and V sync and Intensity bit input and outputs the composite and RF signal through RCA connectors, it also offers an RGBI pass-through connector. It probably has a crystal inside it(I have not checked since it is very tightly sealed and the Service manual mentions nothing about it).

If I grabbed the H and V sync signals before the RF modulator and created composite sync from those perhaps the problem could be solved.

There is of course another crystal on the board too near the LSI chip, that could perhaps also be a culprint but it seems to be the clock input for the LSI and the LSI clock should be the same regardless of it the system is running at 50 or 60 Hz, or am I wrong?


Well let me try to explain to you why they used different crystals for different video systems:

First, a read on this great article will give you a great insight on the video systems:

Then, after you read the article, take in consideration that these systems usually run at the same or slightly lower frequency than the color carrier of the video system intended for them.
So it makes sense they employ a single clock crystal that is a multiple of the color carrier frequency.

A good example is the NES/Famicom which uses a clock of six times the color carrier that in NTSC it's 3.5759545 which results on a crystal frequency of 21.455727mhz.
Because PAL frequency is much higher they had to modify both the CPU and PPU of the console and it's using a clock crystal of 26.601708mhz which never became much of a standard.

On the SNES they decided to keep the same CPU/PPU chips so they instead used a 17.734472mhz crystal and a prescaler chip that from that generates a frequency close to the 21mhz of NTSC for the CPU and PPU while generating the required 4.433618mhz frequency required for PAL. This is just a example.

So knowing that you can make a NTSC SNES generate 50hz RGB video just by "flipping" a pair of configuration bits on the PPU chips (1 leg on each chip) one can easily figure out that the clock does not have much to do with the video refresh rate. 

Of course it depends on the system design.  Now if your RF modulator take RGB straight from the CRT controller, it should have a encoder inside. And again if it has a crystal inside it will be the color carrier one.

But again like I mentioned on my example it's something most manufacturers would try to avoid because it's cheaper/cleaner/nicer to use a single crystal to clock everything and keep things synchronous.


I see! Thank you for the lecture, I always seem to have more to learn :-)

The MZ-800 uses a 17.734472 mhz crystal that is routed to the LSI chip, just like the snes then. A multiple of 4 times the PAL frequency used around here. But that shouldn't have much to do with the refresh rate as you say, which makes a bit more sense now...

I do wonder what is the problem... Will have to experiment some more... Not that I need to have the system running at 60 Hz but I see it as a challenge and I just can't back from one of those :-)


Oh well, now I tried not using the RF modulators composite video and instead joined the H and V sync and used as composite sync. This didn't help, the system still can't produce a good image when set to 60 Hz. Don't really know what the problem is. Good stuff though. The joined H and V sync actually improved the image a bit, at least that's something :-)

What I would need is a service manual for the MZ-1500. That could help me understand why a few games can't show sprites aswell...

Edit.2 Just noticed, the image has now got a slight ripple which is noticeable on graphics. I connected the composite video as sync again and there was no ripple anymore. If I do a proper join of H and V sync instead of just soldering them together I probably won't have this problem but for the moment I'll be satisfied with using composite video as sync.


The proper way of making composite sync out of H and V is a circuit with XOR ports (74LS86).

While the purpose of that circuit is different, the part that makes the C-SYNC might work for you.
Oh and yes definitely a service manual would make your life much easier. Take it easy with the machine and be careful to not damage it if you don't have a spare.