These two computers had the classic fault: Black screen of Death.

The first one was repaired by replacing a ram chip (4264) partially interrupted, the other one had One of the two chip PLA dead.

I have to thank my friend Andrea for providing me a motherboard of a 8296 for spare parts, where i have recovered the PLA which i needed.

The composite video mod for the computer Microdigital TK-83 unlike the Sinclair ZX81 it’s more easier.

You no need to install the usual transistor with 1 or 2 resistors / capacitor. The transistor is already installed on the motherboard of the Microdigital TK-83, so just pick up the video signal as described in the picture.

I remind you that the output signal will be in NTSC standard for the Microdigital TK-83 produced in Brazil (PAL-M)

Autopsy:

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from Wikipedia:

The Amstrad CPC (short for Colour Personal Computer) is a series of 8-bit home computers produced by Amstrad between 1984 and 1990. It was designed to compete in the mid-1980s home computer market dominated by the Commodore 64 and the Sinclair ZX Spectrum, where it successfully established itself primarily in the United Kingdom, France, Spain, and the German-speaking parts of Europe.

The series spawned a total of six distinct models: The CPC464, CPC664, and CPC6128 were highly successful competitors in the European home computer market. The later plus models, 464plus and 6128plus, efforts to prolong the system’s lifecycle with hardware updates, were considerably less successful, as was the attempt to repackage the plus hardware into a game console as the GX4000.

The CPC models’ hardware is based on the Zilog Z80A CPU, complemented with either 64 or 128 kB of memory. Their computer-in-a-keyboard design prominently features an integrated storage device, either a compact cassette deck or 3″ floppy disk drive. The main units were only sold bundled with a colour or monochrome monitor that doubles as the main unit’s power supply. Additionally, a wide range of first and third party hardware extensions such as external disk drives, printers, and memory extensions, was available.

The CPC series was pitched against other home computers primarily used to play video games and enjoyed a strong supply of game software. The comparatively low price for a complete computer system with dedicated monitor, its high resolution monochrome text and graphic capabilities and the possibility to run CP/M software also rendered the system attractive for business users, which was reflected by a wide selection of application software.

During its lifetime, the CPC series sold approximately three million units.

Replacement/Adaptation of a power switch for Amstrad CPC 464:

source: wikipedia cpcwiki.eu

If you are the lucky owners of these eprom: Motorola MCM 68766C you can do something really cute, hardware permitting. Example: use this EPROM like a ROM for some Devices and Computers Commodore without using an adapter ;-D

I had to use the EPROM programmer Promenade C1 for Commodore 64 to burn these eprom because all my programmers don’t support these type of eprom.

Download:

• Eprom Motorola MCM68766C Datasheet (1863)
• Prom-Shell v2.00 (739)

Autopsy:

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This is the translation of the Arabic boot screen:

© 1987, 1988 Sinclair of Egypt
presented by Matsico Corp, an authorized dealer of Sinclair & Amstrad in Egypt
prepared by Dr. Nabil Nazmi
(Version 3)

The computer is arrived in pitiful condition. The inside of the computer and the keyboard were are full of sand and dirt.

Restoration and repairs that have been made:

• Replaced the top case with a new one.
• Replaced the bottom case with a new one, then replaced again with the old one to keep the original appearance. (the hole for the switch is standard in the Arabic version)
• Replaced the tape recorder.
• Replaced the motherboard with a issue #2 instead of issue #1 (was not convenient the repair)
• Cleaned all keys and membrane.
• Replaced all rusty springs of the keyboard.
• General cleaning.
• Moved the ROM/EPROM mod on the new motherboard.
• Replaced the original switch with a new one, then replaced again with the old one to keep the original appearance.

from Wikipedia:

The ZX Spectrum +2 was Amstrad’s first Spectrum, coming shortly after their purchase of the Spectrum range and “Sinclair” brand in 1986. The machine featured an all-new grey case featuring a spring-loaded keyboard, dual joystick ports, and a built-in cassette recorder dubbed the “Datacorder” (like the Amstrad CPC 464), but was in most respects identical to the ZX Spectrum 128. The main menu screen lacked the Spectrum 128′s “Tape Test” option, and the ROM was altered to account for a new 1986 Amstrad copyright message. These changes resulted in minor incompatibility problems with software that accessed ROM routines at certain addresses. Production costs had been reduced and the retail price dropped to £139–£149.

The new keyboard did not include the BASIC keyword markings that were found on earlier Spectrums, except for the keywords LOAD, CODE and RUN which were useful for loading software. This was not a major issue however, as the +2 boasted a menu system, almost identical to the ZX Spectrum 128, where one could switch between 48k BASIC programming with the keywords, and 128k BASIC programming in which all words (keywords and otherwise) must be typed out in full (although the keywords are still stored internally as one character each). Despite these changes, the layout remained identical to that of the 128. The ZX Spectrum +2 power supply was a grey version of the ZX Spectrum+ and 128 power supply.

The ZX Spectrum +2A was a variant of the Spectrum +3 housed a black version of the Spectrum +2 case mouldings. The Spectrum +2A/+3 motherboard (AMSTRAD part number Z70830) was designed such that it could be assembled without the floppy disk controller or associated logic and a +2 style “datacorder” connected. Originally, Amstrad planned to introduce an additional disk interface for the +2A/+2B called the AMSTRAD SI-1, however this never appeared. If an external disk drive was added, the “+2A” on the system OS menu would change to a +3.

The power supply of the ZX Spectrum +2A used the same pinout as the +3. However, the power supply purchased with the +2A/B had “Sinclair +2″ written on the case.

source: wikipedia

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This gallery shows some stages of cleaning the computer Apple ][ Europlus, Apple Disk ][.

I have also made minor repairs which i describe below:

• Replaced the Lamp of the power-on.
• Removing the RIFA filter capacitor (the capacitor was exploded)
• Fixed the center hub of the Apple Disk ][.

I have decided to repair my Coleco Vision Secam version (RGB) purchased some years ago in a full working state, today is died. Below a description of the defect and of components being replaced.

Defect: dirty sprite

• 3 x 4116 (RAM)

I have received this computer in excellent cosmetic condition but unfortunately with some dead keys of the keyboard.

The trick of the repair is cleaning the two little strips and the small conductive button (see the photos)

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

Details:

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I have found this RGB mod for the Philips Videopac G7000 console fom the gamesx site and i have decided to build the circuit to see if it works.

This mod is not easy to do, especially if one has no experience in electronics, however the result at the end is spectacular.

RGB DAC Schematics (click to zoom):

Download: Philips VideoPac G7000 Schematics (1865)

source: gamesx.com

This gallery shows some stages of cleaning the Commodore CBM 8050 Dual Drive Floppy Disk. Below the defects and replacement parts.

Defect:

• Died – Status LED: RED (5 Flash) Zero page error.

Replacement parts:

• 1 x 6502
• 1 x 6532
• 1 x 6522
• The connectors of write enable sensors are inserted incorrectly.
• Missing +5v on the PCB of the sensor circuit.
• Dirtiness in the write enable sensor drive 0
• Cleaning floppy drive heads.
• Cleaning belt of the floppy drive.
• Removing the filter capacitor.
• The Analog PCB of the Floppy Drive is not fixed properly.
• The Locking screws of the external box are not original.
• Lost the felt which push the Floppy Disk vs the head.
• Disk is present (Switch Motor) dirty.
• General Cleaning.

Test formatting floppy disk:

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)

We must always remember to remove the filter capacitor RIFA before turning on a computer off for over 30 years. Otherwise it might happen that you can see from the photos and the video.

This time is my fault because i had completely forgotten.

The Magic Smoke of a RIFA capacitor:

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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)

Autopsy:

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I have repaired the PSU with the failure of stanby mode of the Sharp X68000 (gray version). The failure was caused by two electrolytic capacitors and the voltage regulator 7805 in short circuit.

I have also tried to repair the external case of the Sharp X68000 (gray version) that during the first transport in Italy many years ago was destroyed.

from Wikipedia:

The Sharp X68000, often referred to as the X68k, is a home computer released only in Japan by the Sharp Corporation. The first model was released in 1987, with a 10 MHz Motorola 68000 CPU (hence the name), 1 MB of RAM and no hard drive; the last model was released in 1993 with a 25 MHz Motorola 68030 CPU, 4 MB of RAM and optional 80 MB SCSI hard drive. RAM in these systems is expandable to 12 MB, though most games and applications did not require more than two.

The X68k ran an operating system developed for Sharp by Hudson Soft, called Human68k, which features commands very similar to those in MS-DOS (typed in English). Pre-2.0 versions of the OS had command line output only for common utilities like “format” and “switch”, while later versions included forms-based versions of these utilities, greatly improving their usability. At least three major versions of the OS were released, with several updates in between. Other operating systems available include NetBSD for X68030 and OS-9.

Early models had a GUI called “VS” (Visual Shell); later ones were packaged with SX-WINDOW. A third GUI called Ko-Windows existed; its interface is similar to Motif. These GUI shells could be booted from floppy disk or the system’s hard drive. Most games also booted and ran from floppy disk; some were hard disk installable and others require hard disk installation.

Since the system’s release, Human68k, console, and SX-Window C compiler suites and BIOS ROMs have been released as public domain and are freely available for download.

Early machines use the rare Shugart Associates System Interface (SASI) for the hard disk interface; later versions adopted the industry-standard small computer system interface (SCSI). Per the hardware’s capability, formatted SASI drives can be 10, 20 or 30 MB in size and can be logically partitioned as well. Floppy disks came in a couple of different formats, none of which are natively readable on other platforms, although software exists that can read and write these disks on a DOS or Windows 98 PC.

source: wikipedia