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Philips NMS 8250 General Maintenance

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Philips NMS 8250 main board
NMS 8250 main board (Sanyo EB1921). The yellow cell at the bottom left is the NiCd backup battery. This board carries two added EPROMs (MSX Resource Center wiki)

The Philips NMS 8250 is a desktop MSX2 from 1986 with a separate keyboard, made for Philips by Sanyo.[1] Its power supply is the part that most often needs attention: the rectifier diodes run hot and fail.[2][3]

Safety

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The mains comes into the power supply board itself: the line filter L901, the fuse F1001 (250 V, 2 A) and the transformer T901 are all on it, with the mains switch SW901 wired to it.[4] Unplug the machine and wait before working on that board. Philips's manual adds that cartridges must be changed with the computer off and that the ICs are sensitive to static, so work on an earthed wrist strap.[5]

Inside the case

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Inside the main unit are the main board, the analogue unit (the video and audio output board), the power supply board, the cartridge connector unit, the keyboard interface panel and the drive.[6] An NMS 8250 owner on the MSX Resource Center forum calls the analogue unit the "graphic top board", and another member describes the work as "just many screws between you & main board".[7] Philips's exploded views on PDF pages 34 and 35 of the service manual show where each part sits. Handle the front panel with care: it becomes brittle with age.[1]

Power supply

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Dust and the rectifiers

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  1. With the case open, look at the supply board round the bridge D1050 to D1053. Dust on the diodes and a darkened board under them mean they have been running hot.[2]
  2. Blow the dust out of the supply and the rest of the case.
  3. If the board is darkened, replace the four diodes. Forum members report fitting larger diodes than the originals.[2][7]
  4. Check the solder joints of the regulators, IC1050 in particular.[2]

Rail checks

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Measure the rails with everything connected. The forum figures show what the machine draws.

NMS 8250 supply rails
Rail Regulator Fuse Load measured on an NMS 8255
+5 V IC1050, SI-3052V F1050, 6.3 A About 1.50 A; 1.56 to 1.60 A with a drive working
+12 V IC1051, 7812 F1051, 1 A 0.22 A; 0.3 A with a drive working
−12 V IC1052, AN79M12 (R1050)

[4][2] A +5 V rail that is correct with nothing connected but rises when the main board is plugged in points to the regulator; on one NMS 8255 it read about 5.78 V on load.[2] Do not run the machine in that state.

The supply board has wire links marked J220V and J240V that set the mains voltage; the owner of an NMS 8255 found his set to 220 V. Check which is fitted before use.[8][2]

Backup battery

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The real-time clock (RP5C01, IC162) keeps its settings on a NiCd accumulator on the main board, Philips part 4822 138 10213.[9] On the board photographed above it is a yellow cell printed "3.6V 45mAh".[10] Owners of other Philips MSX machines replace or remove these cells because a leaking cell can damage the board; without it the RTC settings (screen width, colours, date) are lost at switch-off.[11]

  1. Inspect the cell and the board round it. Corrosion is dealt with in Battery Explosion, Capacitor or Corrosion Damage.
  2. Desolder the cell, or replace it with one of the same voltage, mounted on wires away from the board.[11]
  3. After fitting a new cell, set the clock. The RTC frequency is set to 32.768 kHz at TP107 with VC101.[5]

Service switch

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SW101 on the main board is Philips's service switch, used with the service test cartridge to put the machine into its test mode.[9][12] If a machine that worked before now gives a black screen after the case has been open, check that SW101 has not been pressed.[3]

Adjustments

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Philips lists these alignments for the main board and the analogue unit. Make them only when a symptom points to one of them.

NMS 8250 alignments (service manual)
Circuit Measure at Adjust Target
RTC clock TP107 (earth TP111) VC101 32.768 kHz
Colour subcarrier (analogue unit) TP303 (earth TP302) VC301 4.433619 MHz
Burst (analogue unit) TP301 (earth TP302) VR301 T2 of 2.5 to 3 µs (figure 1)
FDC read-pulse width TP106, with TP108 linked to TP109 VR102 0.5 µs (figure 2)
FDC VCO TP102, with TP108 linked to TP109 VR104 250 kHz

[5] Remove the TP108 to TP109 link after each FDC adjustment.[5]

Disk drive

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The drive's parts list gives the spindle motor and its circuit board as one part and has no belt.[13] Alignment needs Philips's alignment disk and FDD test cartridge; the procedure is in the manual. For general drive care see Floppy Drive Repair.

Cleaning

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  1. Blow dust out of the case, the supply board and the drive with dry, low-pressure air.
  2. Clean the cartridge slot contacts with isopropyl alcohol on a lint-free swab and let them dry.
  3. Clean the keyboard cable's connectors at both ends while it is unplugged.
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References

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  1. ↑ 1.0 1.1 Philips NMS 8250, MSX Resource Center wiki—link(accessed 2026-10-21) Manufacturer Sanyo; 1986; the NMS 8255 is the same machine with two drives; "the plastic front panel also gets very fragile with age. Special care must be taken when shipping that absolutely no stress is placed on the front panel."
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Philips NMS-8255 power (?) supply question and modding, July and August 2010—link(accessed 2026-10-21) On an NMS 8255, which shares the NMS 8250's service manual and power supply. Wild_Penguin, 17 July 2010: "There was a lot of dust, the rectifier diodes (D1050-1053) had kind of a dust-fur coat that probably acted like a thermal insulator", the board was darkened round them, and replacing them cured a slowly rising and then dropping picture and sound level; a soldered-wire jumper selects 220 V or 230 V. 10 August 2010: the +5 V electrolytic changed (1000 µF, 10 V); +5 V correct off load but about 5.78 V with the main board connected. RetroTechie, 10 and 15 August 2010: check the +5 V regulator's solder joints; the rectifier diodes "are 3A or 5A types & they regularly fail". Wild_Penguin, 15 August 2010: measured on a PC supply, about 1.50 A at +5 V (1.56 to 1.60 A with a drive working) and 0.22 A at +12 V (0.3 A with a drive working).
  3. ↑ 3.0 3.1 NMS 8250 broken, February and March 2023—link(accessed 2026-10-21) Daemos, 20 February 2023: "Power supply on these machines are notorious for long term failures. The diodes are your usual suspects. If you have a scope check for extreme power supply ripple." larsthe18th, 21 February 2023: the white switch SW101 in the centre of the main board, left of the floppy connector, may have been pressed by accident.
  4. ↑ 4.0 4.1 Philips, Service Manual, Home computer NMS8250/00/16/19, NMS8255/00/16/19, PDF page 20 (sheet CS 7 580): power supply U101. 220 V through L901, F1001 (250 V, 2 A) and SW901 to T901. +5 V: F1050 (6.3 A), bridge D1050 to D1053 (DSB15C), C1050 22000 µF (code e), IC1050 SI-3052V, C1051 1000 µF (code d). +12 V: bridge D1054 to D1057, F1051 (1 A), C1052 4700 µF (code f), IC1051 7812, C1053 220 µF (code e). −12 V: R1050 (marked with a safety warning triangle), C1054 470 µF (code f), IC1052 AN79M12. Main board end of the supply connector: C102 47 µF (code d) on +5 V, C103 47 µF (code e) on +12 V, C101 22 µF (code e) on −12 V. Hosted as File:Philips NMS 8250 and NMS 8255 Service Manual.pdf; reproduced as File:Philips NMS 8250 power supply circuit (service manual).png.
  5. ↑ 5.0 5.1 5.2 5.3 Philips, Service Manual NMS8250/NMS8255, PDF page 2 (CS 7 568): caution notes and alignments. RTC clock: frequency meter on TP107 (earth TP111), set 32.768 kHz with VC101. Analogue unit: frequency at TP303 (earth TP302) set to 4.433619 MHz with VC301; burst at TP301, set time T2 to 2.5 to 3 µs with VR301 (figure 1). FDC read-pulse width: link TP108 to TP109, oscilloscope on TP106 (earth TP109), adjust the pulse width for 0.5 µs with VR102 (figure 2), remove the link. FDC VCO: frequency meter on TP102 (earth TP109), computer on, link TP108 to TP109, set 250 kHz with VR104, remove the link. Hosted as File:Philips NMS 8250 and NMS 8255 Service Manual.pdf.
  6. ↑ Philips, Service Manual NMS8250/NMS8255, PDF pages 34 and 35: exploded views; PDF page 38: mechanical parts list (front panel, floppy drives U104 and U103, power-on LED U110, mains switch, keyboard interface panel U105, reset switch U108, transformer, analogue unit U102, power supply U101, main panel U100, cartridge connector unit U107 and others). Hosted as File:Philips NMS 8250 and NMS 8255 Service Manual.pdf.
  7. ↑ 7.0 7.1 Recap recommendations, June 2018—link(accessed 2026-10-21) Thread about recapping an NMS 8250. RetroTechie, 23 June 2018: bypass electrolytics round an MSX board can often be removed without the machine stopping; audio and video capacitors show their condition in the output; the main power supply's capacitors are worth replacing regardless of condition, with the chance to fit a bigger bridge rectifier on the case or new 7812 and 7912 regulators with better heatsinking; use the service manual's parts list against the board; replace one capacitor at a time; on the NMS, "just many screws between you & main board". Wlcracks: Rubycon, Chemi-Con, Nichicon or Panasonic. siddy6581, 24 June 2018: did not unscrew "the graphic top board" to see the main board.
  8. ↑ Philips, Service Manual NMS8250/NMS8255, PDF page 32: power supply board layout 41 674 B12, with the links J220V and J240V, F1001 (250 V T 2 A), F1050 (6.3 A), F1051 (T 1 A) and output connector PC (1 +12 V, 2 +5 V, 3 ground, 4 −12 V). Reproduced as File:Philips NMS 8250 power supply board layout (service manual).png.
  9. ↑ 9.0 9.1 Philips, Service Manual NMS8250/NMS8255, PDF page 36: main printed board parts list (IC162 RP5C01; Ni-Cd accumulator 4822 138 10213; X102 32.768 kHz; SW101 service switch). Hosted as File:Philips NMS 8250 and NMS 8255 Service Manual.pdf.
  10. ↑ File:Philips NMS 8250 main board.jpg, photograph uploaded to the MSX Resource Center wiki by Gdx, 15 March 2016: yellow cell at the bottom left printed "3.6V 45mAh".
  11. ↑ 11.0 11.1 Any danger for Philips VG-8235 / NMS-8245 mainboards?, July 2017—link(accessed 2026-10-21) Louthrax, 26 and 27 July 2017: replaced the battery on all his Philips VG-8235s, because "if they start leaking, that can really damage the board"; removing it loses the RTC settings. hardwaremaker: a replacement on wires, away from the board and insulated, is safer.
  12. ↑ Philips, Repair Method NMS8250/NMS8255, 4822 727 15847, printed page 3: "Switch computer into service mode with test switch on the main unit" before switching on with the service test cartridge 4822 397 30138. Hosted as File:Philips NMS 8250 and NMS 8255 Repair Method.pdf.
  13. ↑ Philips, Service Manual NMS8250/NMS8255, PDF pages 2, 3 and 5: FDD alignment (alignment disk 4822 395 30274; FDD test cartridge 4822 397 30135, started with POKE &HFD9F,&HC9 and CALL FDDTEST; radial alignment with the cat's-eye pattern on track 40, index from IC140 pin 13; track 00 sensor; azimuth ±30′, not adjustable; index burst set with VR2 to 400 ±200 µs) and the FDD exploded view and parts list (write protect switch assembly, spindle motor + PCB, track 00 sensor, complete printed board, carriage assembly, eject hook bracket, stepper motor, eject bracket, disk detect switch assembly). Hosted as File:Philips NMS 8250 and NMS 8255 Service Manual.pdf.



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