1982 Atari 400 Repair & Restoration

Believe it or not, I’m known around my workplace as a bit of a geek, primarily because of the tinkering around with vintage electronics that I do in my spare time – as such, every now and again I get a request from a colleague to take a look at some hardware for them.

This time, it was a very lovely 1982 Atari 400, with its original box and accessories – apparently, it was completely dead. The owner had got this computer brand new, so was understandably keen to have it working again.

The owner suspected the original power supply was at fault – these PSUs are very simple, just a step-down transformer with an unregulated 9Vac output @ 3A, but they are fused so can require repair.

I tested the output voltage of the PSU, and sure enough, it was dead.

Someone had already drilled out the rivets, so the PSU was already open and was easy to inspect – I tested the primary and secondary windings of the mains transformer, and they were both open-circuit.

The primary had a 117C-rated thermal fuse (3M D115-002), which had failed open-circuit – I replaced this with a 5mm x 20mm 0.25A 250V slow-blow cartridge fuse, soldered into place across the terminals.

The secondary had a sand-filled glass cartridge fuse, which had seemingly been blown apart – I replaced this with a 6mm x 30mm 3A 250V fast-blow cartridge fuse, soldered into place across the terminals.

With the fuses replaced, the PSU was now outputting approximately 12Vac (as to be expected for an unregulated supply).

I took this opportunity to fit a new 3A-fused UK mains plug (a must for safety purposes), then rebuilt the case using nuts, bolts, and washers.

I tried powering on the computer using the rebuilt PSU, but I didn’t have an analogue tuner to properly test it using the RF video output – the power LED lit up and the internal speaker beeped whenever a key was pressed, so it was safe to assume that the computer was booting up OK at this point.

In order to test the computer properly, and to also improve both picture quality and usability with a modern display, I decided to install a composite video modification – this meant replacing the original RF modulator and RF cable with a composite video buffer board and composite cable.

The first step was to disassemble the unit, which is just held together with four cross-head screws on the underside.

The keyboard membrane is plugged into the mainboard, and can be carefully removed; the RF cable can be unplugged from the modulator; then, the entire chassis (including the power supply board, mainboard, and shield can) lifts out of the lower case.

The power supply board is simply held onto the mainboard with a single pin header, which pulls out of its socket. The power board was where I started the restoration and modifications.

The power board would require the most intensive modifications to make space for the composite video board, primarily the removal of the redundant video components and the large regulator heatsink.

This process is covered in detail on the excellent Tynemouth Software blog, from the creators of the composite video kit.

The redundant video components included:

• The RF modulator and standoffs, which are held into place with screws; the RF cable can also be removed from the case.
• The video pin header, which needs to be desoldered.
• The channel select switch (not used on PAL machines), which needs to be desoldered – the existing hole in the case will be used for the new removable composite video cable.

In order to remove the heatsink, the original linear voltage regulators (a 7805 +5Vdc @1A regulator and a 78M12 +12Vdc @0.5A regulator) need to be removed and replaced with modern switch-mode equivalents – these are far more efficient than the original regulators, and run cold. With switch-mode parts fitted, the large power resistor at R202 can also be removed.

The composite video board can then be fitted using double-sided tape, and its connector cable soldered to the footprint for the modulator header; a pin header was added to the footprint for J203, which allows audio from the mainboard to be directed into the modulator header.

The new voltage regulators can then be installed – I used a a TSR 1-2450 in place of the 7805, and a TSR 1-24120 in place of the 7812.

The mainboard also required modifications to work with the composite video board, and needed to be removed from the shield can – this is held closed with eight cross-head screws.

The Atari 400 digital circuitry comprises of the mainboard (with ROM and I/O), and two daughtercards – a CPU card (based around the 6502 8-bit CPU) and a 16KB RAM card.

It’s possible to upgrade the Atari 400 from 8KB or 16KB RAM to 48KB RAM, like the Atari 800 – this requires installing an aftermarket 48KB RAM card and making some modifications to the mainboard.

I took this opportunity to clean all the boards with an anti-static brush, the ports/switches and IC sockets with contact cleaner, and the edge connectors with a rubber eraser.

The mainboard requires minor, reversible modifications to split the audio and video signals going to the power board – the design is similar to other Atari systems of the era, such as the Atari 2600.

This starts with removing the audio subcarrier oscillator transistor Q105, tuning inductor L101, and audio/video coupling capacitor C183.

I then added a cable with a Dupont-style female connector to the emitter footprint of Q105, allowing audio to be passed to the power board.

With everything reassembled, the computer seemed to work well, and the new composite video output looked great on a modern display.

All that was left to do was to give everything a thorough clean, pack it all back up, and return it to its owner.

Along with the computer, I provided the redundant components to the owner in case they ever wanted to return the computer to stock condition.