I have repaired nine motherboards of the Home Computer Texas Instruments TI-99/4A for a dear friend. Below the description of each repair.

Texas Instruments TI-99/4A Repair (1 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound … and then garbage screen.

Replaced parts:

• 1 x 1501392-27 ROM (U611)
• 2 x 4116 RAM (U102 / U105)
• 1 x 74LS245 OCTAL BUS TRANSCEIVER (U614)
• 1 x 74LS03 QUAD 2-INPUT NAND GATE (U506)

Texas Instruments TI-99/4A Repair (2 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound.

Replaced parts:

• 1 x CD3227A ROM (U611)
• 1 x CD3226A ROM (U610)

Texas Instruments TI-99/4A Repair (3 of 9)

Defect:

• Garbage Screen.

Replaced parts:

• 4 x 4116 RAM (U105 / U106 / U108 / U109)

Texas Instruments TI-99/4A Repair (4 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound.

Replaced parts:

• 1 x MCM 6810P RAM 128×8 (U609)

Texas Instruments TI-99/4A Repair (5 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound.

Replaced parts:

• 1 x MCM 6810P RAM 128×8 (U608)
• 1 x 1501392-27 ROM (U611)
• 1 x 74LS138 1-OF-8 DECODER/DEMULTIPLEXER (U504)

Texas Instruments TI-99/4A Repair (6 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound.

Replaced parts:

• 1 x CD3226A ROM (U610)
• 1 x 74LS138 1-OF-8 DECODER/DEMULTIPLEXER (U504)

Texas Instruments TI-99/4A Repair (7 of 9)

Defect:

• Blue Screen with tiny Vertical lines and deafening sound.

Replaced parts:

• 1 x TMS 9900-NL CPU (U600)

What happened to the other two motherboards ?

• The eighth motherboard was used for spare parts.
• The ninth motherboard has all components burned (Power Supply problem?)

Repairs and calibrations made:

• After a few days of normal use of the Apple /// is died the Keyboard Encoder (341-0035-00). I have replaced the chip with the Keyboard Encoder SMC KR3600-075B1 which fortunately is still available.
• Removed the filter capacitor.
• Replaced the bulb of the power-on that is embedded in the keyboard.
• Replaced all rubber feets.
• Calibration of the Floppy Drive.
• Replaced some switches of the keyboard.

Repairing keyboard Apple ///:

It seems incredible but there is always a button that doesn’t work ;-D

I found on some keyboards of the TI-99/4A the same buttons but with the plunger much smaller. I have used the plunger of the Apple /// button on the TI-99/4a button and i have fixed the problem elegantly ;-D

Thank to my friend Ninetyniner Iuc for the donation of spare parts.

Gallery:

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The Apple III (often rendered as Apple ///) is a business-oriented personal computer produced and released by Apple Computer that was intended as the successor to the Apple II series, but was largely considered a failure in the market. Development work on the Apple III started in late 1978 under the guidance of Dr. Wendell Sander. It had the internal code name of “Sara”, named after Sander’s daughter. The machine was first announced and released on May 19, 1980, but due to serious stability issues that required a design overhaul and a recall of existing machines, it was formally reintroduced the following autumn. Development stopped and the Apple III was discontinued on April 24, 1984, and the III Plus was dropped from the Apple product line in September 1985.

The Apple III could be viewed as an enhanced Apple II – then the newest heir to a line of 8-bit machines dating back to 1976. However, the Apple III was not part of the Apple II line, but rather a close cousin. The key features business users wanted in a personal computer were a true typewriter-style upper/lowercase keyboard (as opposed to the Apple II which was based on a teletype keyboard) and 80 column display. In addition, the machine had to pass U.S. Federal Communications Commission (FCC) Radio Frequency Interference (RFI) qualifications for business equipment. In 1981, International Business Machines unveiled the IBM Personal Computer (IBM PC) – a completely new 16-bit design soon available in a wide range of inexpensive clones. The business market moved rapidly towards the PC DOS/MS-DOS platform, eventually pulling away from the Apple 8-bit computer line.

Despite numerous stability issues and a recall that included the first 14,000 units off the assembly line, Apple was eventually able to produce a reliable and dependable version of the machine. However, damage to the computer’s reputation had already been done and it failed to do well commercially as a direct result. In the end, an estimated 65,000–75,000 Apple III computers were sold. The Apple III Plus brought this up to ~120,000. Apple co-founder Steve Wozniak stated that the primary reason for the Apple III’s failure was that the system was designed by Apple’s marketing department, unlike Apple’s previous engineering-driven projects. The Apple III’s failure led to Apple reevaluating their plan to phase out the Apple II, and eventual continuation of development of the older machine. As a result, later Apple II models incorporated some hardware, such as the Apple Scribe Printer, a thermal printer, and software technologies of the Apple III.

source: wikipedia

Gallery of the repair:

Texas Instruments TI-99/4A Repair (#1)

Defect:

• Blue Screen with tiny Vertical lines.

Replaced parts:

• 1 x MCM6810P (128 x 8-Bit Random-Access Memory TTL/DTL)

Texas Instruments TI-99/4A Repair (#2)

Defect:

• Blue Screen with tiny Vertical lines.
• Wave/Raster Lines.

Replaced parts:

• 2 x IC MCM6810P (128 x 8-Bit Random-Access Memory TTL/DTL)
• 1 x IC CD3236A-NL (ROM)
• 1 x Capacitor 12 pF.

Note:

In one photo you can see the curious change made in this TI-99/4A by inserting a resistor in the pitch where in other motherboards is soldered a leg of a electrolytic capacitor and then one leg of the capacitor is welded and the not the other leg.  Compliments.

Video of the defects:

I have received from a friend a couple of components that can be used as spare parts for the repairs. Thanks Andrea.

Close-up description:

• 1 x Commodore CBM 4040 Dual Floppy Disk Drive PCB
• 1 x Commodore CBM 8050/8250 Dual Floppy Disk Drive PCB
• 2 x Texas Instruments TI-99/4A PCB
• 1 x Main powersupply of Commodore CBM 4040/8050/8250 Dual Floppy Disk Drive.
• 1 x Sinclair ZX Spectrum+
• 1 x Sony HB-10P MSX

I have decided to repair my two ColecoVision consoles bought like non-functioning many years ago. Below a description of the defects and of components being replaced.

Defect: black screen

• 1 x 2114 (VRAM)
• Joystick port # 1 broken (replaced with the joystick port of a Texas Instruments TI-99/4A)

Defect: black screen

• 1 x 2114 (VRAM)
• 3 x 4116 (RAM)

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I have received to repair five TI-99/4A with the following defects:

1. Black Screen
2. Working but with some dirt on the screen.
3. Vertical Stripes moving with a deafening sound.
4. Blue Screen with thin vertical stripes and deafening sound.
5. Blue Screen with thin vertical stripes and deafening sound.

The first two are repaired with the following faults:

Black Screen Fault: replaced TMS9929A followed by ROM CD2156NL
Dirt on the screen: replaced a 4116 RAM.

The third motherboard was used for spare parts.

The fourth and fifth motherboard with the blue screen have been the most annoying.

I spent 6 + hours to make measurements with an oscilloscope comparing one of the two motherboards with a working one to find the fault but the signals/voltages were absolutely identical.

So i have decided to go for exclusion and with a TI-99/4A Tech Data manual on my hand and 39.5 degrees in my Laboratory i have started to desolder and put a socket of the following components:

• 1 x TMS9901NL (20 +20 PIN)
• 1 x TMS9900NL (32 +32 PIN)
• 1 x TIM9904NL (10 +10 PIN)
• 1 x TMS 4732/2532 ROM U610 (12 +12 PIN)
• 1 x TMS 4732/2532 ROM U611 (12 +12 PIN)
• 1 x 74LS244 (10 +10 PIN)
• 1 x 74LS245 (10 +10 PIN)
• 1 x 74LS373 (10 +10 PIN)
• 1 x 74LS03 (7 +7 PIN)
• 1 x 74LS138 (8 +8 PIN)
• 1 x 74LS74 (7 +6 PIN)
• 1 x 4116 (8 +8 PIN)

After all this work nothing had changed, nothing worked, always BLUE screen.

I redid the measurements, but nothing, everything looked the same.

At this point there are only two static memories MCM6810P but that i don’t have investigated for what are used, because already on the motherboard are 8 x 4116 RAM Memory shared between Memory program and Video memory.

… anyway … i have tried to unsolder and put a socket.

So we add:

• 2 x MCM6810P (12 +12 PIN)

Et Voilà the two MCM6810P are both dead! both!

Obviously i don’t have this component in my laboratory, then i try to recover from the motherboard for spare these two RAM hoping that they are not faulty which however these RAM can be purchased online at a very low price.

The fifth and last motherboard that had the same defect of the previous one did not have the faulty ram MCM6810P.

To repair i had to replace the following components:

• 1 x 74LS04 (7 +7 PIN)
• 1 x TMS9900NL (32 +32 PIN)

A dear friend sent to me a few things to repair or test, i will see to do my best.

Things to repair/testing:

• Commodore 64/128 – 1764 Ram Expansion (dead)
• Commodore 64 Power Supply for the Commodore 1764 Ram Expansion (pcb broken)
• ZX Spectrum Plus (7805 over burned)
• Texas Instruments TI-99/A (black screen)
• Amstrad 464 + (replace case and keyboard)
• Kung-fu Master Jamma pcb Board (no audio)
• Arabian Sinclair Spectrum 2+ (dead + tape problem + keyboard problem)
• Sinclair ZX81 (video problem / testing)
• Sinclair ZX80 (video problem / testing)
• Sega SC-3000 (cpu bad socket)

Things for spare parts or to keep:

• Sinclair ZX80 Original Power Supply.
• Commodore VIC-20 Boxed cartridges.
• Sega SC-3000 Basic cartridge.
• Commodore Dual Floppy Drive 8250 motherboard (spare parts)
• Commodore CBM 8296 motherboard (spare parts)
• Sharp CE-510F Dual Floppy Drive for Sharp PC-5000 (Boxed)
• Chalkboard Powerpad + Expansions (Boxed)
• Tape Recorder for Commodore (spare parts)

TI-99/4A MBX Expansion System Repaired.

The interface MBX Expansion System was dead and the power voltage was in short-circuited. The failure was caused by the filter between the power supply and the 7805.

Video: