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

I ran the repair of the membrane that had a micro interruption, i have used a micro-drop of vinilic glue mixed with the graphite that i have scraped from the pencil ikea, i made a thin film on the microfracture and now it works.

There were other problems, including: graphite consumed in some keys and the keyboard connector (pcb side) don’t work very well.

I have received this motherboard of a CBM 8032 to repair from Rossano T.

The motherboard was devastated by rust, many components are short-circuited due to rust that was abundant on the pins side of the chip. Unfortunately the computer was turned on to test it, causing the total breakdown of more components.

I have installed 40 socket because some components are rusty other are dead, however some traces under the chips are eaten by rust and and i had to rebuild.

The repair and cleaning took me a very long time and a lot of patience, it took me about 10+ hours.

Components Replaced:

• 1 x 4650 (6845P) CRT Controller
• 3 x 4116 RAM
• 2 x 2114 Video RAM
• 5 x 74LS244
• 3 x 74LS74
• 1 x 74LS02
• 1 x 1uf 100v Capacitor

Phases of the repair:

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This gallery shows some stages of cleaning the computer Apple IIe and the Apple Monitor ///.

I have also made minor repairs which i describe below:

• Fixing the keyboard connector on keyboard side (some keys not working)
• Adjusting the yoke of the CRT tube (the screen is lightly rotated)
• Replaced the LED of power-on (CR1) of the Apple IIe (LED was off)
• Removing the RIFA filter capacitor (the capacitor was exploded)

Merging between two  Amstrad 464+ partially functionals to make it one fully functional and in a decent aesthetic state.

This computer was in very poor condition, almost all the traces and pitches on the Z80 CPU side was interrupted.

I had to remove the old socket and install a new one and rebuild all traces/pitches interrupted.

The failure was due to a cold solder on the ram socket and a bent pin on the ic MOS 8276.

This power supply for the REU 1764 is arrived for repair destroyed, the internal transformer was completely extirpated from the pcb.

I have disabled the mod to make the screen in reverse (white chars on black background), i have also made the composite mod with the right resistor value for the best quality video output.

Replacement:

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Simple steps to replace the keyboard Membrane of the Sinclair ZX Spectrum 128k (heatsink).

• Buy a new Sinclair Spectrum 128k Keyboard membrane.
• Unscrew the eight screws on the bottom.
• Kindly disconnect the keyboard from the connectors on the main pcb.
• Remove the back of the keyboard.
• Unscrew the block that holds the cable.
• Remove the old membrane.
• Clean the keyboard with air-spray and a soft brush.
• Install the new membrane.
• Close the back of the keyboard.
• Very kindly put the block of the flat cable and screw in.
• The keyboard is ready to be connected.
• Very kindly insert the flat cable into the connector.
• Your Sinclair ZX Spectrum 128k is like new.

source: rwapsoftware.co.uk

About 3 months ago i have repaired my Tatung Einstein with stability problems, after a few minutes the screen was filled with random characters. Below the replacement parts:

• 2 x 74LS157 (IOO6/IOO7) Multiplexer.

On the same day has broken the Tatung TC-01 Monitor, the vertical sync is died without any way of adjustment. Yesterday, instead of celebrating Easter, i have tried to repair the monitor, the faulty component was as follows:

• 1 x TDA 2578A Sync Mixer.

Very happy i have connected the Tatung Einstein to the monitor, i have tried a few games and after 15 minutes the Tatung (computer) decided to die. I spent more hours to understand what could have happened, oh! found it, some ram burned. Below the replacement parts:

• 3 x RAM 4264 (IO11/IO10/IO8)

What could happen again?

I also did the dump of the eprom (rom), the download below.

• Tatung Einstein Rom v1.21 (1206)

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I had several problems with the repair of the Hewlett-Packard HP-85.

The first problem was the way to reconstruct the pinch roller using a latex tube, this is a very bad choice, then i have used a 10mm heat shrink, this choice instead is very good.

The second problem was the rubber parts of the pinch roller are literally dissolved and the rubber is went to cover the optical diode and part of the pcb and of course nothing has worked and the tape drive was always in stalled state.

After cleaning, the drive started to work perfectly. The last problem are the magnetic tapes 3M DC-1000 instead of HP DC-100, for the correct operation you must perform a modification by adding a resistor of 2kohm in parallel to the already present on the pcb. After this mod you need to format the tape with the “erasetape” command.

I must also thank John of the site vintagecomputers.freeserve.co.uk for the support.

Download:

• 3M Data Cartridge Compatibility List (1701)
• Athana DC100A / 3M DC-1000 Fix (1424)

source: voidware.com/calcs/hp85rep hp9845.net vintagecomputers.freeserve.co.uk

Replacement:

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Simple steps to replace the keyboard Membrane of the Sinclair ZX81. The removal of the old keyboard is not illustrated but it is a very easy job.

• Open the Sinclair ZX81.
• Place the membrane.
• Remove the adhesive protection.
• Place the membrane and press firmly.
• Check the Membrane Connector.
• Gently insert the connectors on the pcb side.
• Close the Sinclair ZX81.
• Enjoy.

source: rwapsoftware.co.uk