Hardware Tips

Some MicroAce Hardware Peculiarities

Problem 1: Keyboard Entry

If your MicroAce will not accept keyboard entries or will accept some, but not all, the most likely hardware problem is a defective IC U11, a 74LS365.

The solution is to replace it. You must know that the replacement part (either a new 74LS365 original part or a substitute 74LS367) is a good working part, i.e., it has been tried in a working MicroAce and it works perfectly. I had ordered six 74LS365s from one source, and none worked properly. Even the replacements did not work. Then I ordered from Jameco three types (3-SN74365N, 3-74LS365, and 3-DM9085N which are equivalent made by National Semiconductor). All of these worked fine.

Sometimes IC19, a 74LS04, would not allow the computer to accept the shift key entries of + ( – ) >. Changing the IC19, the 74LS04, seemed to correct the problem.

Problem 2: Cursor Flicker

If your K cursor flickers or if the LS display flickers when the LET command is keyed in, the problem may be a defective IC U24; a 74LS74. The C12, a 47PF capacitor, could also be at fault. A good substitute for the latter is a 100PF capacitor.

Spare Chips

Since the LS series of chips used in the MicroAce is not expensive, I suggest having a spare set of known good chips on hand. Though you might never have to use them, the assurance of being able to plug in a known good chip balances off the possible frustration of time delay and expense in sending the instrument out for repair.

Here is a list of the chips to have on hand: 1-74LS00, 1-74LS04, 1-74LS05, 1-74LS10, 1-74LS32, 1-74LS74, 1-74LS86, 1-74LS93, 1-74LS157, 1-74LS165, 2-74LS365 or 74LS367 or plain 74365, 1-74LS373.

Note: be very careful to plug in the chips properly. Pin 1 of the chip must correspond to socket 1. Also be sure to prevent static discharges of your body to the computer.

References for Troubleshooting

The following articles on general hardware background and on hardware problems may be of further help.

SYNC

• “Adding an LED Load Monitor to the ZX80 (MicroAce),” (1:1, p. 38).
• “Four Tips for MicroAce Owners,” (1:2, p. 5).
• “Using the MicroAce with Sinclair Accessories,” (1:2, p. 10).
• “First Aid for Your Keyboard,” (1:3, p. 3).
• “Key Click Generator,” (1:3, p. 40).
• “The ZX80 Keyboard,” (1:3, p. 42).
• “A Parallel Interface for the ZX80/MicroAce Computer,” (1:4, p. 38).
• “Thick Black Bars,” (1:6, p. 2).
• “Cecil Bridges LED Load Monitor,” (1:6, p. 2).
• “Experiments in Memory and I/O Expansion,” (1:6, p. 20).
• “Hardware Helpers,” (2:1, p. 28).
• “4K/8K ROMs in One ZX80,” (2:2, p. 40).
• “Getting Loaded,” (2:2, p. 41).
• “Two-Switch Human Interface for the Communicative Impaired,” (2:3, p. 23).
• “Keyboard/System Conversion: The First 40/1 Keys,” (2:3, p. 28).

Syntax

• “8K ROM Modification,” (2:4, p. 2).
• “ZZZap! NS74LS365,” (2:4, p. 6).
• “Erratic Keyboard Entries,” (2:4, p. 7).
• “All Purpose Beeper,” (2:6, p. 4).
• “Pull-up Resistors—Keyboard,” (2:8, p. 8).
• “ZX80 Logic States of IC’s,” (2:9, p. 2).
• “Video Sync Information and Tape Recording Loading Solutions,” (2:9, p. 11).
• “Trouble Shooting—The Effect of Removal of Each Chip from the Computer (incl. chart of equivalent ZX80/MicroAce IC numbers),” (2:10, p. 3).
• “Power Plug Blinking Characters,” (2:10, p. 9).
• “SABRE Info on EPROM,” (2:11, p. 1).
• “Improve Video Driver,” (2:12, p. 3).

Byte Magazine

• “Memory Expansion for the ZX80,” (Jan. 1982, p. 216).
• “Add Full Size Keyboard,” (Mar. 1982, p. 256).
• “Adding a Reset Key,” (Mar. 1982, p. 256).