This section ranges from a simple idea to give better access to the tape heads, through external security, all the way up to memory expansion.

Some computers - but not the MZ-80K have a RESET switch which is useful when a program "hangs up". Without such a switch the only solution is to turn off the power, so here is the alternative.

1. Switch the computer off, and disconnect from the mains supply.
   
   
2. Carefully turn the computer upside down, and remove the four screws that secure the bottom plate.
   
   
3. Turn the computer the right way up, and lift the top. ( Keyboard, picture tube, and cassette ).
   
   
4. Lock the support hinge and locate the RESET switch terminals, that look like this:
   
   
   I've inserted numbers 1 and 2. They originally do not exist on the PCB.
   
   
5. Solder, or otherwise properly connect, a pair of wires to terminals 1 and 2.
   
   
6. Bring these wires out to the side or back of the computer, where you can drill a hole and fit a suitable switch.
   
   
7. The switch can be of any type, ( e.g. push-button ), that will temporarily connect the two wires together.
   
   Having fitted this switch, you can take several actions after pressing it:

• Type GOTO$1200 for a fresh start with BASIC, ( Program lost ).
• Type GOTO$1260 for a "WARMSTART". ( Program retained ).
• Type GOTO$1050 to SAVE, or GOTO$1060 to VERIFY a machine code copy program.
• Type GOTO$2000 ( usually ! ) to restart a machine code program.
  

This switch can be used to BREAK out of machine code programs, but it will not help in certain other situations, ( if you have been experimenting with certain POKEs or USRs for instance ).

This can be a very useful facility, enabling your video picture to be shown on one or more remote TV screens around a room, ( especially useful for teachers ).

This should NOT be attempted, however, by anyone with no electronics / electrical knowledge, as only the basic principles are dealt with, as follows:

1. What you need, basically, is a simple UHF modulator, of the type used in TV games. This is available quite cheaply from most electronics or TV component stockists.
   
   The output from this can be taken to one or more coaxial sockets, mounted on your MZ-80K casing, and can be "tuned into", ( probably around channel 36 ) by one or more domestic TVs.
   
   
2. This kind of modulator requires two inputs; a power supply, and the picture signal.
   
   
3. The power supply, usually 6 - 9V DC, may be taken off the MZ-80K 12V supply, using a "dropping" resistor to reduce the voltage, ( probably a few hundred ohms ), and a Zener diode to regulate it.
   
   
4. The picture signals are taken off the five-pin connector CN2, on the printed circuit board inside the MZ-80K. You need two connections:
   
   One to pin 2 for the video sync pulses, and one to pin 3 for the video signals. ( Pin 1 is clearly marked on the PCB ).
   
   
5. If both these are taken off via two preset resistors, ( of a few K ohms ), then their levels can be adjusted for a good picture.
   
   
6. These signals are then combined, and fed into the video input of the modulator via a small current-limiting resistor, and a larger, ( multi-K ohm ) resistor to Ground.
   
   
7. If you don't understand most of this, then don't do it! ( Use mirrors instead ;-)
   

If you have a 20K, 24K, or 26K machine you can upgrade its memory size to a maximum of 48K by adding and / or changing the dynamic RAM chips and making a few extra socket connections. This should not be attempted, however, unless you have the appropriate knowledge or experience.

The RAM chips ( types 4116 and 4027 ) are located in three lines of RAM sockets labeled RAM1 - RAM 3. Their configurations are governed by the connections made on the two small connector sockets CS1 and CS2 which have 16 connector pins ( 8 on each side ) numbered 1 - 8 and 9 - 16.

You can upgrade to any size by adopting one of the arrangements shown below.

For example, to upgrade a 24K machine to 36K you need to change the 4K of RAM in the bank RAM2 for 16K ( 4116 ) and change / add the pin connectors on CS1 and CS2 as shown.

The present Sharp monitor is held in a 2332 chip which can easily be changed to a 2732 by connecting pin 18 of the socket to GND, taking the track which is connected to pin 18 and connecting it instead to pin 21 and cut the existing track from pin 21. To alter the existing monitor, all you have to do is to use an Eprommer to copy the monitor to RAM, alter it to suit yourself, and then program it into a 2732 to put in the socket.

Alternatively you can use a 2532 EPROM. Using this type of EPROM you can exchange the monitor ROM by this chip without any hardware modifications.