About 10 or 12 years ago, I visited a car boot sale, on the look out for some vintage TV a friend of mine had seen there the previous week. I found the TV, it wasn't in great condition, and the seller wanted far too much money for it, I walked away*
This did, however, follow me home. I think it cost me £5. The seller didn't know whether it worked or not. It's missing a key on the right hand side...
It's a Sharp MZ-700 personal computer. I remember these from my youth, but had never had a chance to use one. It dates from around 1983.
I rushed it back home, plugged it into the living room TV, and eagerly switched it on. BEEP! it booted up, followed by a bang, and acrid smoke billowing out, much to the delight of the (not quite Mrs back then) Mrs Doz.
"I'll look at that later..."
... much, much later ...
First off, let's get into the machine.
This is the rear of the machine, there are 5 screws to remove here. You can also see the composite video output, RF output, RGB output, cassette input and output jacks, the "figure-of-eight" (Telefunken) connector for the mains input, an on off rocker switch, a volume knob, a reset button, and various little covers over some ports. There's also a huge earth binding post (got to love a big binding post!)
It appears to be of modular construction. Under the centre section is a port marked "printer" , and to the left (in the photo) is the in-built cassette drive, which just lifts out, after disconnecting it's robbin connector. The right-hand most cover (in the photo) doesn't move...
There's a couple of screws which appear to attach the top to the case below.
... and finally three underneath across the front.
Hmmm, the badge underneath states it's an MZ-711... OK it's time to consult google.
The MZ-700 is just a range of computers. The MZ-711 is the base model. It came out of the factory as a bare-bones thing. No in-built tape or printer. The tape was obviously an add on (at £50 in 1983!) for this machine.
There's seems to be quite an active forum on the web too, at
sharpmz.org I've applied to join.
So, off with the lid...
Nicely built I must say....
Z80 at 3.5 MHz, 64K of RAM.... Not much in the way of ROM.
Apparently the ROM only contains a boot loader and monitor... which means you needed to load basic from a tape... and google shows me there are lots of different basic's and many other languages too... interesting.
Zilog licensed the Z80 to sharp, in this case it's an LH0080A. It's pin for pin compatible.
... anyway , let's not get sidetracked ...
As this machine went bang, and is utterly dead, it's out with the power supply...
It's a switched-mode affair, made by Sharp. Single output, presumably 5V.
I quickly spot the culprit....
Big bang, lots of smoke, acrid smell... it's our old friend the Rifa suppression capacitor!
It's duly replaced with a suitably rated X-class capacitor. 47nF for future reference (or should that be rifarence 😆). One of the two 33uF 400V reservoir capacitors has a slightly domed top. I change them both out. Testing of the other caps shows excellent ESR and values within tolerance, so are left in place. The mains fuse is open, unsurprisingly. It's a T500mA. A new HRC one is fitted...
...and the supply seems to work well.
With the supply installed back into the machine, the supply was switched on, and the machine beeped... RF was coupled into the tiny workshop TV, and ....
... well, it's a bit of a disappointment. The display is interacting with the keyboard, but the characters are corrupted. Resetting the machine changes the pattern of corruption.
The keyboard is VERY carefully removed... go easy with the ribbon connector, I got away with mine, only "folding out" two of the contacts, and managed to repair the socket.
All the socketed chips (the monitor and character roms, and the Z80) are removed, the sockets cleaned and refitted. No change.
The main board is removed from the case, and there's a rusty spot on the metal screening tray beneath..
.. with an accompanying mark on the PCB itself...
... it's right by this IC, a 74S157, a non-inverting quad 1 to 2 line data selector chip. It's removed from the board and tested..
... and it's bad, which is a pity, as I don't have a '157 , but a '158 (which is the same, but inverting). A replacement is duly ordered.
.. and fitted, and makes a slight change to the display. Last time, hitting reset changed the characters on the display, now, at least it's constantly corrupt!
Now the wonderful people on the sharpmz.org forum had suggested the CG (character generator) EPROM may be corrupted. They only had a design life of 10 or so years, and we're way outside that now. The good news is, if it is corrupted, all I need to do is erase it (with UV light), and re-programme it. I plug it into the TL866 programmer and read the contents of the ROM out. This is compared with a known good .bin file, and the bad news is the ROM is fine ...
Back to basics. Check the supply rails to the IC's... I scoped up the 5V rail, and it gets horribly noisy around the MB8264-15 memory IC's. Ugh, there's more than 500mV of hash on the rail...
There's a couple of electrolytics there , C82 & C86 ... swap them out ... and bingo !
For the sake of it, all 9 electrolytics are changed.
For future reference, these are :
2 x 100uF 10V
2 x 10uF 16V
4 x 1uF 50v
1 x 22uF 16V
Be careful when changing the caps, as the PCB quality isn't the greatest, and it's double sided. Were a track is soldered component side, it's best to warm that side up, and pull the cap out. There's also a couple of sold tantalum capacitors in there. I don't have any, so they can stay for a while, although they do have a reputation for failing short circuit with no warning, and going pop like little firecrackers!
Note there are several components in the top right of the picture, which are lying down flat against the board, so to clear the modulator. This includes one of our new ones.
With the machine partially reassembled, the keyboard is tested... and much to my surprise works perfectly! I expected a disassembly and clean up of the contacts, but no. The missing key turns our to be a "blank" key on this UK machine. On the Japanese version, it has a character on it. One of the users on the MZ forum has said he has a spare key from a spares machine I can have. What a super person!
Onward with the machine.. I'm keen to get something loaded. The cassette deck is mounted, and L (for
load) followed by CR (carriage return) switches the motors to the cassette deck on, and it actually appears to work. It's cleaned up.
Now, a couple of years ago, I obtained a wav file of "Z80 Basic", which my good friend, Nathan, duly recorded it to a cassette for me. I put the cassette in a safe place.... and even remembered where that was !
So the tape is inserted, and, there's no take up. I quickly stop the tape before we get into a mess...
Out with the cassette deck, and remove the four screws holding the back on... the one on the bottom right is longer...
I notice there's some debris on the back panel...
Now the belt is shot, and is replaced, but it's not
that shot, because it managed to turn the capstan, and rewind is working well... out with the deck itself...
Undo the slotted pillar thing (there's probably a technical name for that) , there's three in total, all of
differing lengths.
Lift the mechanism carefully from the case. More debris evident ...
... and look at the state of the supply reel table rubber, it's got chunks out of it! Despite that it's still
working (albeit a bit "lumpy" !)
The take up is reel is removed, and the reel rubber, is badly cracked, and beyond refurbishment... The Lidl box of plumbing washers is pressed into service and a suitable new tyre located!
The unit is reassembled, and we attempt again....
Bingo! Sadly this is as far as I can get.
The deck most likely needs checking for speed and azimuth.
After an alignment of the cassette drive, and a lot of cussing, it was determined that the cassette I'd used to align the deck was of poor quality. I asked my friend Nathan to record a new one for me, which he duly did, and after aligning with that, all was well ...
The ubiquitous test program is run ...
Except all this tedious messing around this cassettes is all very well, but ... my ageing MP3 player works really well at putting data into the Spectrum ... can this work here?
Now the MZ-700 was also sold without a tape-drive. It's equipped with 3.5mm jacks for reading and writing data to any tape deck, so let's use those... except it doesn't work.
This is the relevant part of the circuit diagram. It can be seen that the external plugs (P-8 read and P-7 write) are not jumpered across P-12, which is where the cassette deck is connected.
Making some jumpers up out of dupont connectors, and grounding pin5 (sense) allows the machine to load perfectly from my MP3 player via the external connector. Perfect, and easily reversible if I ever wish to use cassettes again!
Intrigued with the "PETSCII" style characters on the keyboard, I conjured up an infinite maze programme... not as elegant as the C64's one-liner, but gives a pleasing result.
Another saved from landfill!
*the set was there the following week, now with the tell-tale mark on the tube face that the neck had been knocked off the back, and the tube was now shot. He still wanted silly money. I still walked away.
