Texas Instruments TI-99/4A Repair (Wave/Raster Lines)

Defect:

• Wave/Raster Lines (see photo).

Replaced parts:

• 1 x Variable Inductor 2 -> 4.5 μH (L100).

Gallery of the repair:

CBS Coleco Vision Repair (Black/Garbage Screen)

Defect:

• Black Screen and then Garbage Screen.

Components replaced:

• 1 x 2114 Static Ram (U4)
• 2 x 4116 Dynamic Ram (U11/U14)

Gallery of the repair:

APF Electronics Inc MP-1000 (1978) – Joystick Repair

Defect:

• Both Joysticks dead.

Fix:

• The adhesive tape has dried and the conductive pad move inside the joystick. I have disassembled,cleaned and used a new adhesive tape, now they are like new.
• Replaced the fire button on one of the two joystick completely dead.

Gallery of the repair:

I used a product to regenerate the rubber keypad of remote controls to repair the keyboard flat cable of a Home Computer MSX.

I have used this product many times for my work of Retro Computer restoration but never to regenerate the rubber keypad of remote controls :D

Gallery of the repair:

Keyboard #1

Defect:

• When the keyboard is connected, the computer doesn’t work (black screen)

Components replaced:

• 1 x 74LS132P (U4)

Keyboard #2

Defect:

• The Caps lock flashes three times and the keyboard doesn’t work.

Fix:

• Cold solder joint on the capacitor (C11)

Gallery:

Below the description of the repairing of two Processor Technology SOL-20.

Processor Technology SOL-20 #1 Repair

Defect:

• Dead/Split Screen.

First of ALL!

• You should never replace CD4029 with HCF4029 although apparently could work and maybe it works .. on some motherboard could do bizarre defects or video glitches, like a split screen!

Components replaced and fixes:

• Replaced all IC series HCF 4XXX HCF with the right CD 4XXX series.
• Sprayed DUE-CI G-22 on all Texas Instruments IC sockets.
• Replaced wrong ic 74LS157 in place of a 74LS367 (U29)
• Rebuild broken pcb track near the caps (C15)
• Replaced wrong capacitor of 22uf in place of a 1uf (C67)
• Rebuild  bridges under the IC U46 that has seen better days.
• Replaced Capacitor (C31)
• Remake cold welding on the PSU connector and the socket of CPU Intel 8080A.
• Removed a little piece of solder under a socket (see photo)
• Replaced 74LS138 (U34) dead without reason while typing this text on the SOL-20 (FUCKKKK!!!!!!!!!)

… Some Beers… :-D

Gallery of the repair:

Processor Technology SOL-20 #2 Repair

Defect:

• Black Screen/Dead.

First of ALL!

• You should never replace CD4029 with HCF4029 although apparently could work and maybe it works .. on some motherboard could do a bizarre defects or video glitches!

Components replaced and fixes:

• Replaced all IC series HCF 4XXX HCF with the right CD 4XXX series.
• Sprayed DUE-CI G-22 on all Texas Instruments IC sockets.
• Cleaned the motherboard and connectors with Kontakt PCC (Printed Circuits Cleaner)
• Temporarily removed the IC 74LS136 (U22) which put in short-circuit the personality card and burned the 74LS04 (U24). Probably this is a “homebrew” modification for a particular Personality card.
• Replaced wrong ic 74LS04 (U23) in place of a 74LS20 (U23)
• Replaced RAM 2102 (U15)

Note:

• Many sockets not of the Texas Instruments completely rusted are previously been replaced by the owner of this computer (Alessandro Polito).

Gallery of the repair:

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Commodore 64C (ASSY 250469) Repair (1 of 12)

Defect:

• Black Screen.

Replaced parts:

• 1 x MOS 6526 (U1)

Gallery of the repair:

Commodore 64C (ASSY 250469) Repair (2 of 12)

Defect:

• Black Screen.

Replaced parts:

• 2 x TMS 4464 (U10/U11)

Gallery of the repair:

Commodore 64C (ASSY 250469) Repair (3 of 12)

Defect:

• Black Screen.
• Keyboard Dead.

Replaced parts:

• 2 x TMS 4464 (U10/U11)
• 1 x MOS 6526 (U1)

Gallery of the repair:

Read more…

The motherboard conditions are not good, very badly. All capacitors have released acid, some are gone and the same for other components like the transistor Q233 2N3904 SMD amplifying for the video signal BLUE RGB. The Decoder U12 CXA1145 Sony has the pins eaten by the acid of the capacitors.

Poor Amiga …

Work to do:

• Solder resistance from E232R that is disconnected because the acid of the capacitors has leaked and corroded..
• Solder the transistor Q233 that is disconnected because the acid of the capacitors has leaked and corroded.
• Rebuilding a dozen pf pcb track between the IC U12 and the rest of the PCB and rebuilding also the through holes of the PCB.
• Replacing electrolytic capacitors, probably due to the state of the pitches i don’t use SMD capacitors.
• Replacing the male Harddisk connector some legs are broken at the base because the acid of the capacitors has leaked and corroded.

Gallery of the repair & cleaning:

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I start to saying that this type of repair should NOT be made, are not good although the repair can succeed, these damaged motherboards are good for spare parts.

It took me 10 hours to repair and if i ask 15,00 euro per hour like a housekeeper, would be 150,00 euro excluding components and cleaning case … but who pays 150,00+ euro ? no one in the world. :-D

This is one of the reasons because do not exist anymore electronic repairs or there are very few people can do it, nobody is willing to pay.

Works that have been made:

Foreword almost all pcb pads are leaked of the acid, although i have cleaned more times so as not to heat the electrolyte liquid that is worsening the situation, unfortunately the pcb pad could not resist to a new solder and they are broken.

• Soldered the E232R resistor, it’s no a good soldering because the pad are in a badly state but working.
• Soldered a transistors Q233 (2N3904) not SMD, the pitches are gone when i have tried to solder the SMD version.
• Rebuilt 10 x connections doing a horrible jumpers in bizarre places.
• Replaced the electrolytic capacitors.
• Replaced the male connector for the Harddisk.

It’s a shit … but it works!

Commodore Amiga 600 Gallery (Before & After):

Yet another Commodore 64 (USA-NTSC) repaired.

Defect:

• Garbage screen / Black screen.

Repair:

• Replaced 1 x PLA 906114
• Replaced 1 x MOS 7708 (74LS257)

Note:

• The first time the Garbage Screen was in Black & White and then after replacing the PLA 906114 a new Garbage Screen is back with some colors :-D (see photo)

Gallery of the repair:

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

Defect:

• Wave/Raster Lines.

Replaced parts:

• 1 x Variable Inductor 2 -> 4.5 μH (L100).

Gallery of repair:

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

I should point out that someone has already tried to repair, without success.

Defects:

• Black Screen (no raster, no sync) short circuit between 12v and 5v . . . and then . . .
• Wave/Raster Lines.
• Blue Screen with tiny Vertical lines and deafening sound.
• Garbage Screen.

Short Circuit Note:

• The short circuit was due to a malfunction of the IC: TIM9904ANL (U601)

Replaced parts:

• 1 x Variable Inductor 2 -> 4.5 μH (L100).
• 1 x TIM9904ANL (U601)
• 2 x MCM 6810P RAM 128×8 (U608 – U609)
• 1 x 74LS245 (U614)
• 1 x TMS9929 (U100)

Gallery of the repair:

Commodore PET 2001 Chiclet (1978) Fixed.

Defect:

• Garbage screen.

Repair:

The problem of this computer was caused by the oxide on all pins of the RAM and inside of the sockets as you can see from the photos.

To clean the pins of the IC i always use the same way, gently with a rasp nail on the ic pins and then spray a bit of air, see photo.

For cleaning the socket i scraping with a fine tip gently the contact inside the socket.

I have tested all with the ROM/RAM board and simulating the various BASIC and configurations of RAM and loaded some software, including my demo with the PET MicroSD by Dave Curran (Tynemouth Software)

Everything worked perfectly.

As you can see from the photos i have now a complete workstation for repair PET 2001 out of the box, avoiding to have on the table the computer that takes a lot of space.

Gallery of the repair:

Grundy Newbrain repaired

Defect:

• Dead.

Analysis:

• The acid of the capacitors has corroded some pcb tracks and pitches of the main pcb.
• One of the two flat cable (main pcb -> keyboard/vfd pcb) previously repaired cause short circuits and doesn’t work.
• Removing the Filter capacitor “RIFA” in the external PSU (removal made 7 months ago)

Repair # 1:

• Replaced all the electrolytic capacitors.
• Rebuild the PCB tracks corroded by the acid of the capacitors.
• Replaced the flat between the main PCB and the keyboard/vfd pcb.

The computer now turns on but the VFD (Vacuum Fluorescent Display) displays weird characters and the composite video signal is dead.

Repair # 2:

I have used a Z80 NOP generator that i have made some years ago for repairing ZX Spectrum/Sinclair and that causes the Z80 to process only “NOP” instructions
and consequently on the address lines we can see with a oscilloscope a perfect square wave.

With this simple “trick” i have immediately found the faulty component that break one of the address lines of the CPU (A1). The exact same fault was found by Thomas Gutmeier.

• Replaced 1 x 74LS257 (IC 412)

The computer now works perfectly.

Gallery of the repair:

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Download: Grundy NewBrain Schematics (1532)

Laboratory Bench Transformer for repair Commodore CBM/PET series 2001/30xx/40xx(not fat – 8032 motherboard)

Transformer Output Pinout:

• PIN: 4 – 6 = ~16VAC
• PIN: 4 – 5 = ~8VAC
• PIN: 5 – 6 = ~8VAC
• PIN: 7 – 8 = ~16VAC
• PIN: 9 – 10 = ~16VAC

Input:

• 110v

Gallery:

Thanks to Andrea Pierdomenico for this great donation made some years ago.

2 x Commodore Amiga 600 Full Recap and Repair.

Commodore Amiga 600 #1:

• Full SMD Capacitors Recap.
• Replaced the joystick port with a missing two pins. The joystick port has been recovered from a pcb (not amiga) for spare parts.

Gallery:

Commodore Amiga 600 #2:

• Full SMD Capacitors Recap.

Gallery:

TRS-80 Model 1 L2 – Video Snow Shovel Hardware Fix

Video snow, the black streaks against white characters, is most apparent with large, rapidly changing areas of white graphics. The root of the problem is that the TRS-80 display is memory mapped by the CPU.

The solution is to give the video circuitry higher priority than the cpu.

Note:

On my TRS-80 Model 1 where i use the Quinnterface & FreHD i had to use the version that also includes the 74LS125 which seemed optional but in this case it does not appear to be optional, we have to do it, otherwise doesn’t work.

Thanks to Ian Mavric for the support.

Gallery of the hardware mod:

Download: 80 Microcomputing Magazine 1982 (834)