Repaired for a friend a Commodore CBM 8296 Motherboard.

Defect:

• Black screen of death without startup beep.

Replaced parts:

• Replaced PLA [UE6] with a OTP EPROM 27C512R 70ns + Adapter.
• Replaced PLA [UE5] with a original one MOS 324744-01
• Replaced [UC8] 3-State Driver Bus MOS 65245 (74LS245)

I have received a Lemon II computer (Apple ][+ / Europlus Clone) in a good cosmetic condition but with a serious problem at the keyboard, here below defects and repairs that has been made.

Cleaning comparison (before / after):

Gallery of the repair:

Defects:

• First problem: When a keyboard key is pressed is always displayed the same character, in this case the character “O”
• Second problem (which i have only noticed after solving the first one): The keyboard works correctly, all characters are displayed in the correct way, but sometimes the character displayed does not match with the key pressed. Example: when pressing many times the letter “I”, after a while is displayed the character “Y” and after a while it show the right character “I”.

Components Replaced:

• 2 x 74LS191 (UP/DOWN Counters)
• 1 x SMC KR2376-ST (Keyboard Encoder)

I have to thank my friend Luca that called me to inform me that there was still a piece of the SMC KR2376-ST available in the store.

Autopsy:

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Olivetti S.p.A. is an Italian manufacturer of computers, tablets, smartphones, printers and other business products such as calculators and fax machines. Headquartered in Ivrea, province of Turin, the company has been part of the Telecom Italia Group since 2003.

The primacy of the first PC can be assigned to Olivetti thanks to Programma 101, which was produced in 1964 and had a big success in the U.S. market.

Olivetti’s first modern personal computer, the Olivetti M20, featuring a Zilog Z8000 CPU, was released in 1982. In 1983 Olivetti introduced the M24 and in 1984 the M21 (the first and last of transportable of Olivetti), a clone of the IBM PC using DOS and the Intel 8086 processor (at 8 MHz) instead of the Intel 8088 used by IBM (at 4.77 MHz).

The M24 was sold in North America as the AT&T 6300. Olivetti also manufactured the AT&T 6300 Plus, which could run both DOS and Unix.

The comparison (before / after):

Repairing & things before of the Cleaning:

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Defects:

• Mirror image.
• Sometimes garbled image with using the ISA Bus.

Replaced parts:

• Replaced a 74LS245N on the motherboard side.
• Replaced a 75LS244N on ISA expansion Module.
• Removed the battery for the Realtime Clock.

Upgrade:

• Bios v1.36 to v1.43 (latest).

source: wikipedia

Autopsy:

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Kaypro Corporation, commonly called Kaypro, was an American home/personal computer manufacturer of the 1980s. The company was founded by Non-Linear Systems to develop computers to compete with the then-popular Osborne 1 portable microcomputer. Kaypro produced a line of rugged, portable CP/M-based computers sold with an extensive software bundle which supplanted its competitors and quickly became one of the top selling personal computer lines of the early 1980s.

While exceptionally loyal to its original consumer base, Kaypro was slow to adapt to the changing computer market and the advent of IBM PC compatible technology. It faded from the mainstream before the end of the decade and was eventually forced into filing for bankruptcy in 1992.

Kaypro began as Non-Linear Systems, a maker of electronic test equipment, founded in 1952 by Andrew Kay, the inventor of the digital voltmeter.

In 1981, Non-Linear Systems began designing a personal computer, called KayComp, that would compete with the popular Osborne 1 transportable microcomputer. In 1982, Non-Linear Systems organized a daughter company named the Kaypro Corporation and rechristened the computer with the same name.

The first product, the Kaypro II, carried the Roman-numeral designation because one of the most popular microcomputers at the time was the Apple II. The Kaypro II was designed to be portable like the Osborne. (When battery-powered laptop computers became available, the larger machines came to be called transportable or luggable, rather than portable.) Set in an aluminum case, it weighed 29 pounds (13 kilograms) and was equipped with a Zilog Z80 microprocessor, 64 kilobytes of RAM, and two 5¼-inch double-density floppy-disk drives. It ran on Digital Research, Inc.’s CP/M operating system, and sold for about US$1,795.00.

Kaypro’s first computer, the Kaypro II, had a 2.5 MHz Zilog Z80 microprocessor; 64 Kb of RAM; dual, single-sided, 191 kB 5¼ inch floppy disk drives; and an 80-column, green monochrome, 9″ CRT.

Early in the Kaypro’s life, there was a legal dispute with the owner of the Bigboard computer who charged that the Kaypro II main circuit board was an unlicensed copy or clone.

he outer case was constructed of painted aluminum. The computer featured a large detachable keyboard that covered the screen and disk drives when stowed. This and other Kaypro computers (except for the Kaypro 2000) ran off regular AC mains power and were not equipped with a battery.

The comparison (before / after):

Defects:

• Black screen of death.

Replaced parts:

• Removed the filter capacitors.
• Replaced some Electrolytic Capacitors exhausted (PSU).

source: wikipedia

Gallery of the repair:

Defect:

• Black screen of death.
• Improper adjustment of Contrast and Brightness.
• Power Supply “tired”.

Replaced parts:

• Removed the filter capacitors.
• Replaced all Electrolytic Capacitors exhausted .
• Contrast and Brightness adjustments.

Like we know well the keyboard of the Commodore 116 may NOT be fixed because you can’t open it without break it.

The only way is break the plastic pins that hold the keyboard PCB and find the right way to close the whole thing, i leave you the right way for do this.

Once you have open it, we realize that this keyboard is a colossal shit, the conductive material of the keys is ridiculous a thin layer that is consumed very briefly.

The only way before you throw it is use the “Kontakt Chemie Graphit 33” but without spraying directly on the tiny conductive part of the key but first try on a plastic surface and then with a very fine brush take the graphite to paint the conductive part of the key.

The “Kontakt Chemie Graphit 33” with the days/months or years tends to dry, this solution make a new life to the keyboard, but not for always.

Another classic failure and is the FPC Connector (18 pins) for the keyboard flat that can be easily replaced if you find an FPC connector with 18 pins ;-D

The conditions of this Commodore 8296 (TAN Case) are really bad. The CBM 8296 is left for over 25 years in a dovecote.

The cleaning, repairing and restoration have taken a very long time, but the result is more than acceptable.

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Defects:

• The Commodore 8296 was completely dead (Black screen of death).
• Little problems of CRT geometry.

Replaced parts:

• Replaced PLA U6 with a OTP EPROM 27C512R 70ns + Adapter
• Replaced the ROM 2 x 2764 Eprom with 1 x 27C128 Eprom (spare parts from a original cbm pcb)

Note:

I have replaced the original ROM (2 x EPROM) pcb because the PLA adapter is fat. I could insert a small castle (photo) between the ROM pcb and the Main Motherboard but in this time i have prefered to replace with a single Eprom.

Gallery of repair and cleaning:

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This is a Commodore CBM 8296-D Motherboard repaired for a Friend.

Gallery of the repair:

Defect:

• Black screen of death without startup beep and after replacing U5 and U6 black screen of death with startup beep.

Replaced parts:

• Replaced PLA U5 with a original PLA MOS 324745-01.
• Replaced PLA U6 with a OTP EPROM 27C512R 70ns + Adapter
• Replaced a dual FlipFlop 74LS74 UD4.

* The replacement of the two PLA has already been made earlier by my friend.

The conditions of this Commodore 8032-SK (Gold Label) are really bad. The CBM 8032-SK is left for over 25 years in a dovecote.

The cleaning, repairing and restoration have taken a very long time, but the result is more than acceptable.

Defects:

• The Commodore 8032-SK was completely dead (Black screen of death).
• Little problems of CRT geometry.

Repair:

• Replaced the CPU 6502.
• Yoke calibration of the CRT

Gallery of repair and cleaning:

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The conditions of this Commodore Dual Disk 8250 LP are really bad. The CBM 8250 LP is left for over 25 years in a dovecote.

The cleaning, repairing and restoration have taken a long time, but the result is more than acceptable.

Defects:

• Floppy Disk Drive completely dead.
• Main motherboard failure: Status LED green but with the Floppy Disk Drive LEDs (red) that blink 3 times and then repeat in loop.
• Directory displayed with extra chars.

Repair:

• Replaced all the electrolytic capacitors of the two Floppy Disk Drive.
• Replaced the IC RIOT MOS 901869-01 (Memory, I / O, Timer array)
• Replaced the IC RIOT MOS 6532 (RAM-I/O-Timer)

Gallery of repair and cleaning:

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Gallery of repair and cleaning:

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This computer is arrived for a repair from a friend. The defect is the classic Black Screen of Death and also the PCB has certainly seen better days.

Defects found:

• Short circuits.
• Tracks interrupted or broken.
• Tracks partially raised.
• Jumpers with flying leads.
• Some components are partially unsoldered and soldered to other components.

Repair:

• Rebuild all the tracks interrupted or partially raised.
• Arrangement of all components removed or partially unsoldered.
• Removing flying jumpers
• Replaced CPU Zilog Z80
• Replaced a RAM 2114
• Replaced a Multiplexer 74LS157
• Replaced the ZX-81 Rom with a Eprom 2532 with programmed the ZX-80 Rom.

Mod:

• Composite video output (simple version without Transistor). It was made only to make easiest the repair.

This gallery shows some stages of the repair of the Sinclair Spectrum 48k.

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Defects found on the Sinclair Spectrum #1:

• Black Screen of Death/Garbage Screen.

Components replaced:

• 8 x 4116 RAM
• 1 x Z80 CPU

• Extra:
  • Composite Video

Defects found on the Sinclair Spectrum #2:

• There are no colors.

Components replaced:

• 5 x Electrolytic Capacitors 22uf / 16v
• 1 x SN94459 (LM 1889) Tv video Modulator IC with Chroma Reference Signal (Color)

Defects found on the Sinclair Spectrum #3:

• Jaundice problems.

Calibration:

• Calibration of Trimmer (VR2)

Defects found on the Sinclair Spectrum #4:

• Black Screen of Death.

Components replaced:

• 1 x RAM TMS4532
• 1 x Transistor ZTX650

• Extra:
  • Composite Video
  • Conversion from Spectrum (Issue 2) To Spectrum +

Defects found on the Sinclair Spectrum #5:

• Garbage Screen and high absorption on the 12v line and 0v (ground) with explosion of the transistors ZTX-650 (651) and ZTX-213

Components replaced:

• 1 x Zener 5v1
• 12 x Electrolytic capacitors
• 3 x 4116 RAM (two of these in total short circuit between pin 8 and 16)
• 8 x OKI 3732 RAM
• 1 x Sinclair ROM
• 1 x 74HCT(LS)32
• 1 x Transistor ZTX-213
• 1 x Transistor ZTX-650

• Extra:
  • Composite Video

• Info:
  • About the explosion of the transistors i have isolated the problem, are the ram (LOW).

    The bad way used some years ago was cut the pin 8 of the all 4116 ram to isolate the bad one, in this case, i didn’t cut the pins of the ram, but i have cut the track of 12v which is part of all the 4116 ram, doing so, the transistor ZTX650(651) doesn’t explode.

    At this point there is a short in one or more RAM between +12 and 0v. I have measured the absorption where i have cut the track and it absorbs too much.

    I just have to unsolder the pin 8 of the 4116 ram and trying to isolate it for find which ram is short-circuited.

• …it was not over:
  • When the repair is finished and i have replaced all OKI 3732 RAM with a new one, another IC has decided to die, a 74HC(LS)32. I have no words.

Defects found on the Sinclair Spectrum #6:

• Faulty / AutoLoad.

Components replaced:

• 1 x TMS 4532 RAM

• Extra:
  • Composite Video

Sinclair Spectrum Faulty / AutoLoad Video:

Small and Convenient retro-keychain.

This gallery shows some stages of repairing the Commodore PET 2001-8C

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Two months have passed since i have received my Commodore PET 2001-8 Chicklet exchanged for a Sharp MZ-80K, was the day 28/08/2013.

The motherboard was in disastrous conditions, it took two months and a lot of patience to repair it.

Defect:

• Garbled characters at boot.
• Tape Recorder (Datassette) Motor Tired.

Replaced parts and various repairs:

A large number of short circuits visible and not visible (under the ic sockets)

• 2 x 6540 ROM Basic 1.0
• 2 x 74LS00
• 3 x 74LS157
• 1 x 7805
• 2 x Electrolytic Capacitor / Tantalum.
• 1 x Tape Motor
• 16 x 2114 RAM

I want to dwell on the ram, i noticed that the PET 2001-8 Chicklet does not digest very well all ram.

These ram don’t work properly for my PET 2001-8 Chicklet:

• TESLA 2114 (2/8)
• MM2214N
• SY2114-2
• HKE2114L-2

I had to buy several stocks of ram in 2114 to find the right ram that work correctly. The ram that work very well are the ELCAP 2114L-3.

I have to thank Andrea Pierdomenico for the ROM test on his PET 2001-8C and Alessandro Polito. for the exchange.

This gallery shows some stages of repairing the Commodore CBM 610.

Defects found on the Commodore CBM 610:

• Random raster lines on the screen.

Components replaced:

• 2 x 74LS245
• 1 x 74S32
• 1 x 74S05
• 1 X 74LS14
• 2 x Capacitor 22uf 10v
• 2 x Capacitor 1uf 50v

I have passed three weeks of suffering for repairing this CBM 610. The finding of the fault is not has been easy, the key component of the failure was the IC 74S05 (U96) followed by the other components.