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Commodore PET 2001 RAM Replacement

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Commodore PET 2001 with top raised, main logic board and RAM rows
A PET 2001 with the top raised on its stay. The two rows of chips along the front edge of the main logic board are the RAM.

The original Commodore PET 2001 static board has 8 KB of main RAM in sixteen chips at UI1-UI8 and UJ1-UJ8 and 1 KB of video RAM in two chips at UC3 and UC4. Boards 320008 and 320081 use the MOS 6550, a 1K × 4 static RAM; boards 320132 and 320137 use the 2114 in the same positions.[1][2] The 6550 was used almost nowhere else and is now hard to obtain.[3][4] Tynemouth Software describes its usual failure as chip-select logic that leaves a chip enabled when it should not be, so that it fights the other chips on the data bus.[3][5] The procedure below finds the failed chips and replaces them, either with good chips or with modern replacement boards.

Symptoms

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  • A screen full of random characters at power-on that never clears. The video circuitry on these machines displays the video RAM whether or not the processor is running, and a working PET clears that screen within a second or two. If it stays, the processor is not running its start-up code, often because of a ROM or RAM fault.[6][7][8]
  • Characters that are wrong in a consistent way. Tynemouth compares the characters on screen with the ones that should be there: if every wrong character has the same bit set, the video RAM chip holding that bit is faulty, or occasionally the buffer that writes to it or the latch that reads it. One of the two video RAM chips holds bits 0-3 and the other bits 4-7; if both fail the screen fills with a chequerboard pattern or with @ characters.[5]
  • A start-up message that shows less free memory than it should. One of Tynemouth's boards reported 1102 bytes free, meaning only 2 KB of RAM was working, and the fault was in the third pair of RAM chips.[6]
  • A 6550 or 6540 that is very hot. Tynemouth's advice is that such a chip needs removing, not a heatsink, because it slowly destroys the working chips around it.[3]

Other faults give a similar garbage screen: the ROMs, the address decoding and the bus buffers.[6] See Commodore PET 2001 ROM Replacement and the Commodore PET 2001 Troubleshooting guide.

Safety

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Warning: the mains switch, fuse and transformer primary are live whenever the PET is plugged in, and the CRT anode runs at about 10 kV and can hold its charge after the machine is switched off.[9][10] Unplug the machine before working on it, keep clear of the display board and the CRT, and see the CRT Discharge Procedure.

Tools and parts

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  • A spare set of RAM chips of the board's type: 6550, Commodore part 901438-01, or 2114, part 901453-01.[1] Tynemouth notes that TMS4045 chips are compatible with the 2114.[3]
  • A chip tester is the quickest way to sort good chips from bad. Ken Shirriff and Marc used a Retro Chip Tester, which found three bad 6550s on their board.[4]
  • Replacement boards that remove the 6550s altogether: a PET ROM/RAM board, which plugs into the processor socket and replaces the main RAM and ROMs, and a 6550 video RAM replacement board, which plugs into the two video RAM sockets. The video RAM board cannot be used in place of the main RAM.[3][7]
  • A logic probe or oscilloscope.[8]
  • An IC extractor. On 6550 boards Commodore's parts list gives 22-pin sockets for all eighteen RAM positions.[1]

Which chip holds what

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Commodore PET 2001 schematic 320026 sheet 2, 6550 RAM I1-I8 and J1-J8
Commodore's schematic 320026, sheet 2. The upper row, J1-J8, holds data bits D4-D7 through the buffer at H9; the lower row, I1-I8, holds D0-D3 through H8. Each I and J pair covers 1 KB, from 0000 at the right to 1C00 at the left. Chips marked with an asterisk are for 8K machines only.

Each 1 KB of RAM is a pair of 4-bit chips: the I chip holds data bits D0-D3 and the J chip holds D4-D7.[2][3]

Main RAM on board 320008 (Commodore schematic 320026 sheet 2)[2]
Address range D0-D3 D4-D7 Notes
0000-03FF UI1 UJ1
0400-07FF UI2 UJ2
0800-0BFF UI3 UJ3
0C00-0FFF UI4 UJ4
1000-13FF UI5 UJ5 8K machines only
1400-17FF UI6 UJ6 8K machines only
1800-1BFF UI7 UJ7 8K machines only
1C00-1FFF UI8 UJ8 8K machines only

Commodore notes on the schematic that all component locations are the same on all four static boards; only the fitted parts differ.[2] On the 6550 boards the four enable lines of each chip do the address decoding: CS1 and CS2 must be high and /CS3 and /CS4 low, and the chip acts only while the Φ2 clock is high. The 2114 has a single chip select, and the 2114 boards add a 74LS139 decoder instead.[3]

Procedure

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Open the PET and reseat the chips

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  1. Unplug the PET and raise the top; it is hinged at the rear and held up by a stay.[11]
  2. Reseat every socketed chip, pressing each one down and checking for pins out of their sockets. André Fachat notes that PETs are known for socketed chips working loose.[8]
  3. Feel for hot chips with the power on, and remove any 6550 or 6540 that is running very hot.[3]

Test the video RAM without the processor

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  1. Switch off and remove the 6502 from its socket at UF3.[1]
  2. Power up. A PET 2001 shows the contents of the video RAM even with no processor fitted: expect a screen of random characters.[7][8]
  3. A screen filled with one repeated character points to faulty 6550 video RAM. Tynemouth saw this, and a screen of random characters returned when a video RAM replacement board was fitted.[7]
  4. With the video RAM chips removed the data lines float high and the screen shows the chequerboard character, $FF. A chequerboard with the video RAM fitted points to that RAM or its data path.[6]

Find a faulty video RAM chip

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  1. Refit the processor and power up. Compare the characters on the screen with the ones expected; Tynemouth's PET diagnostics module draws a known screen for this, and a table of the PETSCII codes is needed.[5]
  2. Look up the codes of the wrong characters and find the bit they have in common. A bit stuck on in the lower four bits points to the chip holding bits 0-3, in the upper four to the chip holding bits 4-7.[5]
  3. Swap UC3 and UC4. If the fault moves to the other half of the character code, the chip is faulty.[5]
  4. Fit a good chip, or a 6550 video RAM replacement board. Tynemouth's diagnostics module does not work with 6550 video RAM fitted; it needs the replacement board.[7]

A 6550 whose enable lines have failed can still work as video RAM, because those positions do not use the enables. Tynemouth has restored a board by moving such chips into UC3 and UC4 and fitting good ones in the main RAM.[5][6]

Find a faulty main RAM chip

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  1. Tynemouth notes that a static PET will start with all but two of its RAM chips removed, since each 1 KB is one pair.[12] Keep the pair at 0000-03FF, UI1 and UJ1, remove the others and power up.[2] If it starts, refit the other pairs one at a time in address order.
  2. A pair that stops the machine starting, or shortens the free memory, contains the faulty chip. Swap the I and J chips of that pair with a known-good pair to find which one.[2]
  3. Test the removed chips in a chip tester if one is available.[4]
  4. If there are not enough good chips, the machine can run with less memory: Shirriff removed three bad 6550s, moved the good ones about and ran his 8K PET as a 6K machine.[4]

Replace the RAM

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  1. Fit good chips of the board's type with the notch the same way as the others in the row. On a 6550 board they plug into the 22-pin sockets.[1]
  2. Or fit a PET ROM/RAM board in the processor socket to replace the main RAM and ROMs. Tynemouth uses one to get a board running; with it fitted, only the two video RAM chips of the eighteen are needed.[7][3]
  3. Fit a 6550 video RAM replacement board in UC3 and UC4 if no good 6550s are left for the video RAM.[3]

Testing

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  1. Power up: the screen of random characters should clear within a second or two and show the start-up message.[6]
  2. Check the free memory on the start-up screen against the RAM fitted, and run a RAM test program for a while. Tynemouth ran his test several times and left it running before handing a board back.[6]
  3. Type on every part of the screen and check that each character appears correctly.[5]
  4. Leave the machine running and check that no RAM chip gets hot.[3]

Board revisions

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Static PET 2001 boards (Commodore)[2][1]
Assembly RAM ROM
320008 6550 6540
320081 6550 2316B
320132 2114 6540
320137 2114 2316B

The board number is printed in the upper left corner.[2] Tynemouth has only seen 6550/6540 and 2114/6540 boards in practice.[3] The later dynamic boards of the 2001-N, 3000 and 4000 series use 4116 DRAM and are covered by Commodore PET 4000 Series RAM Replacement.

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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Commodore Business Machines, 8K Main Logic Assembly Parts Cross Reference for the PET 2001, hosted as File:Commodore PET 2001 Main Logic Assembly Parts List.pdf, p. 2: UC3, UC4, UI1-UI8 and UJ1-UJ8 6550 RAM 901438-01 or 2114 RAM 901453-01; p. 3: UF3 6502, 22-pin sockets for the eighteen RAM positions and 40-pin sockets for UA5, UB8, UF3 and UG8.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Commodore Business Machines, PET Computer Main Logic, Memory Section, drawing 320026, sheet 2 of 3, for board assembly 320008, hosted as File:Commodore PET 2001 Main Logic Schematic (320008).pdf: I1-I8 and J1-J8, their addresses and data bits, the 8K-only positions, and the note that all component locations are the same on boards 320008, 320081, 320132 and 320137.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 Tynemouth Software (Dave Curran), "MOS 6550 RAM chips", June 2024: the 6550 package and enable lines, the PET 2001 RAM layout, the enable-line failure, hot chips, the 2114 boards and the replacement boards.
  4. ↑ 4.0 4.1 4.2 4.3 Ken Shirriff, "A tricky Commodore PET repair: tracking down 6 1/2 bad chips", April 2025.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Tynemouth Software (Dave Curran), "Commodore PET video RAM faults", August 2017.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Tynemouth Software (Dave Curran), "Commodore PET 2001 Repair - Garbage screen with noise", July 2017.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Tynemouth Software (Dave Curran), "Commodore PET 2001 Repair Part 1", February 2024: the no-processor test, the 6550 video RAM replacement board and the diagnostics module.
  8. ↑ 8.0 8.1 8.2 8.3 André Fachat, "PET repairs", 6502.org: reseating socketed chips, the screen shown without a processor, and tools.
  9. ↑ PET-Shack (P.S. Software House), PET 2001 schematic and data-sheet bundle, c. 1978, hosted as File:PET-Shack PET 2001 Bundle.pdf: console wiring drawing with the mains switch, fuse and transformer.
  10. ↑ Commodore Business Machines, Schematic, Service, Display Circuit for the PET 2001, hosted as File:Commodore PET 2001 Display Circuit Schematic.pdf: the anode supply of about +10 kV.
  11. ↑ Marcin Wichary, Commodore PET 2001 with lifted top, Wikimedia Commons, CC BY 2.0.
  12. ↑ Tynemouth Software (Dave Curran), "Commodore PET Repair Part 7 - DRAM repairs", September 2014: static PETs need only one pair of RAM chips to start.
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