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Tandy 3000 General Maintenance

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Tandy 3000 colour and monochrome video jumper (Technical Reference Manual, main logic board p. 14)
Tandy's drawing of the colour/monochrome jumper on the main logic board, between the serial/parallel card in slot 10 and the front corner of the power supply.

Routine care of the Tandy 3000 covers the clock battery, the setup program, the jumpers and adjustments on the main logic board, the option cards, the floppy drive and the cooling fan. Everything here comes from Tandy's Tandy 3000 Technical Reference Manual (catalogue 25-4104) except where a community source is named. The scanned copy of that manual does not include its power supply, hard disk or BIOS sections, and it has no disassembly chapter.[1] It covers the original 8 MHz machine (25-4001 and the 3000 HD); the 12 MHz version of 1988 (25-4002) has a different main board.[2]

[1]

Before opening the case

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  1. Unplug the mains lead and every cable from the rear.
  2. Remove the cover. Tandy's upgrade instructions start with this step and give no further detail.[3]
  3. Note which card is in which slot before taking any out. The serial/parallel card is in slot 10, and the RAM upgrade sockets can only be reached with the cards in slots 1 to 5 removed.[3]
  4. Take care not to strain the cables to the cards when removing them; Tandy's RAM upgrade procedure gives the same warning.[3]

Clock battery and setup

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The configuration is kept in the battery-backed RAM of the MC146818 clock chip (U5), which has 64 bytes of RAM of which 50 are for system configuration. The setup program writes it.[4] The clock runs from the +5 V supply while the machine is on and switches to the battery below about 3.0 V; with the machine off, Tandy gives the total drain as 64.5 µA.[5]

Tandy 3000 main logic board battery, reset and real-time clock circuit (schematic 8000265 sheet 11)
Sheet 11 of schematic 8000265: battery header J21 (right), diodes CR1 and CR2, capacitor C36, the MC146818 clock U5 and the reset network round C24.
Battery connector J21 (schematic 8000265, sheet 11)
Pin Connection
1 Battery positive, through diodes CR1 and CR2 (1N4148) to the clock supply
2 Not connected on the schematic
3 Ground
4 Ground

Sources: schematic sheet 11 and the parts list, which gives J21 as a 4-pin header.[6][7]

Tandy's chassis parts list names a 6 V battery assembly, part ACS-0020, and the theory of operation calls the backup a lithium battery.[8][5] An owner describes the original as a 6 V pack attached to the side of the case with hook-and-loop tape.[9]

  1. Look at the battery and the board round it whenever the cover is off. A repairer has reported a Tandy 3000 whose battery burst and damaged the corner of the main board nearest the drive cage.[10] Treat any residue as described in Battery Explosion, Capacitor or Corrosion Damage.
  2. Measure the pack off load and compare it with its 6 V rating.
  3. Fit a replacement 6 V battery to J21 with the positive lead on pin 1 and the negative on pin 3 or 4. Check the polarity against the board before switching on.[6]
  4. Run Tandy's setup program from a floppy disk to set the date, time, drives and memory. The setup is not built into the ROM; owners on the VCFed forum point to the Tandy 3000 setup disk image on oldskool.org.[11][12]

Main logic board jumpers and adjustments

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Main logic board settings (Technical Reference Manual, section 6)
Setting Position Notes
Video type E2-E3 monochrome (factory); E3-E4 colour Move before fitting a colour card. Remove the serial/parallel card in slot 10 to reach the jumper
ROM type E5-E9 and E7-E11 for 27128 ROMs (factory); E6-E10 and E8-E12 for 27256 ROMs All four ROMs must be of the same type
RAM upgrade Jumper across E13-E14 Fit with the 128 KB upgrade (640 KB total)
Colour burst Trimmer capacitor on the main logic board (C44, 6-70 pF in the parts list) Adjust only with a colour card and a composite monitor or television connected, until colour appears. Tandy says a plastic tuning tool must be used

Sources: section 6 of the theory of operation and the parts list.[3][7]

Adding memory on the main board

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Tandy's procedure for the 128 KB upgrade:[3]

  1. Remove the cover.
  2. Remove the cards in slots 1 to 5. The 18 empty RAM sockets are behind the fan, at the left front of the main logic board.
  3. Fit each chip with its notch or dot towards the slots at the rear of the system unit.
  4. Fit the jumper across E13 and E14.
  5. Refit the cards and check the memory with the diagnostics.

Tandy's design specification gives the parts for this second bank as 64K x 1 DRAMs with an access time of 150 ns or faster.[13]

Option cards

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Memory expansion board

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Tandy's 2 MB memory board is set with DIP switch S1. Switches 5 and 6 tell the board how many of its four banks are filled; switches 1 to 4 set the start address on 1 MB boundaries according to whether it is the first, second or a later board; switch 8, on the first board only, disables its first 512 KB for other options that use that space. Switch 7 is not used.[14]

Memory board DIP switch S1, banks fitted (On = 0, Off = 1)
Banks containing memory Switch 5 Switch 6 Switch 8
Bank 0 only On On Off
Banks 0 and 1 Off On Off
Banks 0, 1 and 2 On Off Off
Banks 0, 1, 2 and 3 Off Off Off
Memory board DIP switch S1, start address (switches 1 to 4)
Board 1 2 3 4 Bank 0 starts at
First Off On On On 100000
Second Off Off On On 300000
Third Off On Off On 500000
Fourth Off Off Off On 700000
Fifth Off On On Off 900000
Sixth Off Off On Off B00000
Seventh Off On Off Off D00000

Addresses are hexadecimal; each further bank on a board starts 80000 higher.[14]

Serial/parallel card

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The baud-rate generator jumper is set at the factory across E1-E3 for the USA standard; for the international standard, move it to E1-E2 and add a second jumper across E3-E4. Tandy's instructions for fitting the board in a Tandy 3000 jumper E5-E6 (parallel port as LPT2) and E8-E9 (serial port as COM2); E6-E7 makes the parallel port LPT1. On the revision B board, interrupts are enabled with staking-pin jumpers: E11-E12 interrupt 7 (parallel 1), E13-E14 interrupt 5 (parallel 2), E15-E16 interrupt 4 (serial 1) and E17-E18 interrupt 3 (serial 2). Only one parallel and one serial interrupt can be jumpered at a time.[15]

Floppy disk controller

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Floppy disk controller jumpers (normal positions marked *)
Jumper Function
E1-E2 * Primary address (3Fx)
E3-E2 Secondary address (37x)
E4-E5 * Board enabled
E6-E5 Board disabled
E7-E8 * Single-speed drive (300 kbit/s data rate)
E9-E8 Dual-speed drive (250 kbit/s data rate)

Tandy notes that the starred functions also apply when no jumpers are fitted.[16]

Floppy drive

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The 1.2 MB drive is a Mitsubishi M4854-3S or MF504A-3. On an M4854, Tandy says the jumpers HC, DC, MM, DS1, RR, UD, SB and TD on the drive's servo/logic board must be fitted, along with one printed as "11H". Tandy's section adds that Mitsubishi's maintenance manual for the drive was not available when it went to print.[17] For cleaning and general drive care, see Floppy Drive Repair; for the 3000 HD's drive, see Hard Drive Maintenance and Repair.

Cooling

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The chassis has a 12 V DC fan assembly (Tandy AM-4026) with a fan filter (AHC-0190).[8] Clean or replace the filter when the cover is off, and check that the fan turns freely.

Capacitors

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The electrolytics on the main logic board and option cards are listed in the Tandy 3000 Capacitor Replacement Guide. See also Capacitor Failure Symptoms.

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References

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  1. ↑ 1.0 1.1 Tandy Corporation, Tandy 3000 Technical Reference Manual, catalogue no. 25-4104, volume 1 part 1, hosted as File:Tandy 3000 Technical Reference Manual (25-4104), volume 1 part 1.pdf. Title page, table of contents and foldout insertion guide, PDF pages 1-6.
  2. ↑ Radio Shack, Tandy Computer Catalog RSC-19, 1988, p. 6, "Enhanced, 12-MHz Tandy 3000", scan at RadioShackCatalogs.com (retrieved 2026-10-21). 640 KB on the main board, serial and parallel ports on the main logic board, nine slots, keylock.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Tandy 3000 Technical Reference Manual (25-4104), part 1, theory of operation of the main logic board, section 6 "Upgrades, Jumper Settings and Alignments", pp. 14-17 (PDF pages 54-57).
  4. ↑ Tandy 3000 Technical Reference Manual (25-4104), part 1, design specification 2.4.6 "Real Time Clock", PDF page 22.
  5. ↑ 5.0 5.1 Tandy 3000 Technical Reference Manual (25-4104), part 1, theory of operation of the main logic board, 4.4 "Real Time Clock", p. 8 (PDF page 48).
  6. ↑ 6.0 6.1 Tandy 3000 Technical Reference Manual (25-4104), part 1, main logic board schematic D-8000265, sheet 11, "Interrupt Control and Real Time Clock" (PDF page 74).
  7. ↑ 7.0 7.1 Tandy 3000 Technical Reference Manual (25-4104), part 1, main logic board parts lists, PDF pages 58-61.
  8. ↑ 8.0 8.1 Tandy 3000 Technical Reference Manual (25-4104), part 1, "Tandy 3000 Chassis Subassembly" parts list (PDF page 62).
  9. ↑ "Tandy 3000 HD", Vintage Computer Federation Forums, March 2026 (retrieved 2026-10-21).
  10. ↑ "Tandy 3000 Rescue - Need Help!!", Vintage Computer Federation Forums, January 2022 (retrieved 2026-10-21).
  11. ↑ "Tandy 3000 HD (no HD) issues", Vintage Computer Federation Forums, December 2019 (retrieved 2026-10-21).
  12. ↑ "Trouble with Tandy 3000", Vintage Computer Federation Forums, December 2022 (retrieved 2026-10-21).
  13. ↑ Tandy 3000 Technical Reference Manual (25-4104), part 1, detail design specification, revision date 7 November 1985, pp. 1-5 (PDF pages 10-14).
  14. ↑ 14.0 14.1 Tandy 3000 Technical Reference Manual (25-4104), volume 1 part 2, hosted as File:Tandy 3000 Technical Reference Manual (25-4104), volume 1 part 2.pdf. Memory expansion board DIP switch settings, PDF page 10.
  15. ↑ Tandy 3000 Technical Reference Manual (25-4104), part 2, "Jumper Settings on the optional Serial/Parallel Board", PDF pages 30-31.
  16. ↑ Tandy 3000 Technical Reference Manual (25-4104), part 2, floppy disk controller section 1, "Floppy Disk Controller Jumpers", PDF page 66.
  17. ↑ Tandy 3000 Technical Reference Manual (25-4104), volume 1 part 3, hosted as File:Tandy 3000 Technical Reference Manual (25-4104), volume 1 part 3.pdf. 1.2 MB floppy disk drive: introduction (PDF page 3), servo/logic board jumper settings (PDF page 5) and section 3 (PDF page 14).


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