Panasonic FS-A1ST Troubleshooting Guide
Fault-finding on the Panasonic FS-A1ST, built on the pass/fail checks in Panasonic's Technical Guide (described on the Panasonic FS-A1ST Technical Guide page) and on repairs reported by owners. Page numbers are the printed numbers in the Technical Guide. Dismantling and routine work are in the Panasonic FS-A1ST General Maintenance guide.
Quick reference
[edit | edit source]| Symptom | First checks | Section |
|---|---|---|
| Dead, no power lamp | +5 V at Q1 collector; C50 voltage; CN1 AC | Power supply |
| Power lamp on, black screen | Supply rails, then clocks; RAM upgrade wiring; try a game cartridge | Black screen |
| C50 hot or smoking | C50, rectifiers, transformer output | Power supply |
| No picture on one output only | VDP and encoder signals (page 13) | Video |
| No sound, or PSG/FM missing | Test programs on page 14 | Sound |
| Drive does not turn, or "disc offline" | Belt, pulleys, heads | Drive |
| Clock or settings lost at power-off | Backup cells and CN17 | Clock and backup |
Power supply
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The supply is on the main board. The transformer's centre-tapped secondary comes in at CN1 (pin 1 is the centre tap); D2 and D3 charge C50 (25 V, 12000 µF), which feeds the +5 V switching converter (IC14 MC34063 and Q1 2SA1598) and the +12 V regulator IC11. D4 and D5 charge C59 for the −12 V regulator IC13. A crowbar of zener D9 and thyristor D1 sits across the +5 V output after L2.[1][2]
Expected readings
[edit | edit source]| Point | Reading | Source |
|---|---|---|
| CN1, pin 1 to pin 2 and pin 1 to pin 3 | About 15 V AC each | Jipe[3] |
| Across C50 | About 20 V DC | Jipe[3] |
| Q1 emitter; IC14 pins 6 and 7 | 20.4 V | Circuit diagram, page 22[1] |
| IC14 pin 5 (feedback) | 1.2 V | Circuit diagram, page 22[1] |
| Q1 collector (+5 V) | 5 V ±0.25 V | Pass/fail checks, page 12[4] |
| IC11 input / output | About +21 V / +12 V ±1.2 V | Page 12[4] |
| IC13 input / output | About −22 V / −12 V ±1.2 V | Page 12[4] |
Measure everything from the negative terminal of C50.[3] The transformer figures above are Jipe's; one owner measured 18.7 V from pin 1 to each end and 37.7 V across the ends, the same as his FS-A1GT, and another measured 21 V a side, off load, on a replacement 220 V transformer.[3][5]
No +5 V
[edit | edit source]Alexandre Souza's procedure for these Panasonic supplies:[6]
- Remove L2, the small bare-copper inductor about 1 cm across. This separates the converter and the crowbar from the rest of the board. Do not remove L6, the large green inductor: that cuts the feedback and the output rises to about 22 V.
- Measure the converter's output with L2 out. Up to 5.2 to 5.3 V is acceptable.
- If the output is wrong, test Q1 and the MC34063 first; they are the usual failures. A 2SA1643 suits Q1 in all the related Panasonic machines.
- Check for about 22 V at MC34063 pin 6 and at Q1's emitter. If it is missing, look for a burnt track between D2/D3 and the converter.
- An open resistor under the board, or a shorted crowbar thyristor, can also stop the supply. If R96 or R182 in the feedback goes open, the output rises and the crowbar fires.
In the machine Souza repaired, an FS-A1GT, the crowbar thyristor (a 5P6J) had burnt and was replaced with a TIC112; the surface-mount 5.6 V zener under the board can be replaced with a leaded one.[6]
Wierzbowsky suggests checking the voltages in the Technical Guide first, then D2 and D3, the zener D9, D1 and D7, Q1, and finally the DC/DC converter.[3] Jipe adds that diodes should be tested with one leg unsoldered, and that the choke L5 can go open.[3]
A 2025 repair shows that a short elsewhere can look like a supply fault. The owner traced a short to a failed 6.3 V 1000 µF capacitor on the YM2413's supply. After replacing it and the other capacitors there was still no +5 V, and the machine started only after several more parts, a faulty voltage regulator among them, were replaced.[3] The only 1000 µF 6.3 V electrolytic in the parts list is C67.[7]
C50 hot or smoking
[edit | edit source]In a 2007 report C50 (25 V, 12000 µF) ran very hot and then smoked; the machine had a replacement 220 V transformer giving 21 V a side. Jipe said a 35 V 15000 µF part is a suitable replacement.[5] If you replace it, also check D2 and D3 and the transformer output, since C50 sits straight after them.[1]
Black screen with the power lamp on
[edit | edit source]- Check the three rails against page 12 first.[4]
- Check the clocks with an oscilloscope (page 12): CPU clock at IC16 pin 8, VDP clock at IC16 pin 64, RTC clock at IC17 pin 20, FDC clock at the upper terminal of X1, FM clock at IC10 pin 8 and R800 clock at X3 pin 1.[4]
- On a black-screen FS-A1ST in 2026, Omega said the more likely causes on a turbo R are bad RAM or a failed custom chip. Other suggestions in the same thread: see whether a game cartridge starts, check the connector with yellow wires and the drive belt, and, on machines with a 512 KB RAM upgrade, check the upgrade's wiring.[8]
- On that machine, the brown deposit next to C10 (4700 µF) that the owner took for leaked electrolyte was glue.[8]
Video
[edit | edit source]Page 13 traces the picture through the board. Enter COLOR 15,15,15 in BASIC to give a white screen, then check:[9]
| Signal | Where |
|---|---|
| R, G, B from the VDP | IC16 pins 22, 23, 24 |
| Composite sync from the VDP | IC16 pin 6 |
| R, G, B into the encoder | IC6 pins 21, 22, 23 |
| Sync out of the encoder | IC6 pin 11 |
| Composite out of the encoder | IC6 pin 20 |
[9] The RGB connector takes the VDP's output through a clamp, so a picture on RGB with nothing on composite or RF points at the encoder (IC6). S-video luminance passes through Q6 to Q9 and chrominance through Q2 to Q4.[9]
Sound
[edit | edit source]Page 14 gives two test programs.[10]
PSG: check the three outputs at IC17 pins 97, 98 and 99 and the mixed signal at IC8 pin 14.
10 PLAY"V15C","V15C","V15C" 20 GOTO 10
FM (MSX-MUSIC): check the output at IC10 pin 14.
10 CALL MUSIC:PLAY #2,"V15C" 20 GOTO 10
The PSG and the cartridge slot sound are mixed by IC8 before the audio output and RF modulator.[10] FRS describes the balance between slot, PSG and FM sound as badly calibrated at the factory; the fix is in the maintenance guide.[11]
Drive
[edit | edit source]- A drive that does not turn may have a perished belt. Original belts have melted onto the pulleys and can block the mechanism.[12][8]
- After a belt change, a drive that spins and seeks track 0 but gives "disc offline" may need its pulleys cleaned of old rubber, a different belt, or a head-cleaning disk. Owners have fixed it each of these ways.[12]
- The belt specification and cleaning notes are in the maintenance guide.
Clock and backup
[edit | edit source]If the time or the word processor's dictionary is lost at power-off, check the two AA cells, which Panasonic did not fit at the factory, and the lead to CN17.[13][2] The RTC clock is at IC17 pin 20.[4] Page 14 gives a test program that prints the time continuously; set the time, switch off and on, and check that it has carried on.[10]
References
[edit | edit source]- ↑ 1.0 1.1 1.2 1.3 Technical Guide: FS-A1ST, printed page 22: circuit diagram of the main board power section, LED board and switch board. AC 100 V through the double-pole power switch to the transformer. The secondary reaches the board at CN1: pin 1 is the centre tap (0 V), pins 2 and 3 the two ends, all through choke block L5. D2 and D3 charge C50 (25 V, 12000 µF), which feeds both the DC/DC converter and the +12 V regulator. DC/DC converter IC14 (MC34063) with Q1 (2SA1598), R36 (0.2 Ω, 1/2 W), R37 (100 Ω, 2 W), D8, L6 (220 µH), C10 (10 V, 4700 µF) and feedback resistors R96 (3.09 kΩ, 1 %) and R182 (1 kΩ, 1 %); L2 (20 µH) to the +5 V line, with zener D9, R217 (1 kΩ) and a thyristor from +5 V to ground. IC11 (C24M12HF) for +12 V with C51 and C42 (16 V, 100 µF) and L9 (220 µH). D4 and D5 charge C59 (35 V, 470 µF) for IC13 (AN7912T), −12 V, with C60 and C41 (16 V, 100 µF) and L10 (220 µH). D7 and D6 from the +12 V and −12 V lines to ground. DC voltages printed on the diagram include Q1 emitter 20.4 V, base 20.3 V, collector 5.2 V; IC14 pins 6 and 7 20.4 V, pin 5 1.2 V; IC11 input 20.5 V. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf. The power section is reproduced as File:Panasonic FS-A1ST power supply circuit (Technical Guide).png.
- ↑ 2.0 2.1 Technical Guide: FS-A1ST, printed pages 37 to 40, electrical parts list. IC6 encoder CXA1145; IC7 floppy disk controller TC8566AF; IC8 LM324; IC10 FM sound YM2413; IC11 regulator C24M12HF; IC12 mask ROM (4 Mbit); IC13 regulator AN7912T; IC14 DC/DC converter MC34063; IC15 reset IC MN1280R; IC16 VDP V9958; IC17 MSX-Engine T9769C; IC18 mask ROM (8 Mbit); IC21 to IC24 video RAM 1464-12; IC25 and IC26 SRAM 5165; IC27 system IC S1990; IC28 16-bit CPU R800; IC30 and IC32 main RAM MN41C4256-08; Q1 2SA1598; D1 thyristor 5P4M; D2 and D3 30D1F; D4 to D7 RA1501A; D8 SB350; D9 zener DTZ5R6; crystals X1 16 MHz, X2 32.7 kHz, X3 28.6 MHz, X4 21.47 MHz; connectors CN1 power (3-pin), CN13 drive (24-pin), CN17 backup battery (2-pin), CN19 external microphone (3-pin). Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Turbo R ST not powering up (no 5+V either), October and November 2025—link(accessed 2026-10-21) alali.ahmed.t traced a short circuit to a failed 6.3 V 1000 µF capacitor connected to the YM2413, replaced it and recapped the board, but still had no +5 V. Jipe: check for about 15 V AC between CN1 pins 1 and 2 and between pins 1 and 3, and about 20 V DC across C50, measuring everything from the negative of C50; test diodes with one leg unsoldered (about 0.5 V one way); check that L5 is not open. The owner measured 18.7 V from pin 1 to pins 2 and 3 and 37.7 V between pins 2 and 3. kamiboy and sdsnatcher73 on replacement capacitor ratings (sdsnatcher73: stay close to the original voltage, never lower). Wierzbowsky: check the voltages given in the Technical Guide, then D2 and D3, the zener D9, D1 and D7, Q1 and then the DC/DC converter. The owner later reported the machine working after replacing several parts, including a faulty voltage regulator.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Technical Guide: FS-A1ST, printed page 12, pass/fail checks. Power: +5 V from the DC/DC converter, ±12 V from regulators. +5 V at the collector of Q1, 5 V ±0.25 V. +12 V: IC11 input about +21 V, output +12 V ±1.2 V. −12 V: IC13 input about −22 V, output −12 V ±1.2 V. Clock waveforms: CPU clock at IC16 pin 8; VDP clock at IC16 pin 64; RTC clock at IC17 pin 20; FDC clock at the upper terminal of X1; OPLL clock at IC10 pin 8; 16-bit CPU clock at X3 pin 1. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.
- ↑ 5.0 5.1 New Problem Turbo R A1ST Smoked :o, August 2007—link(accessed 2026-10-21) CrazyBoss: on an FS-A1ST whose transformer had long before been replaced with a 220 V one, the large 25 V 12000 µF capacitor next to the supply became very hot and then smoked; he measured 21 V from the centre pin to each outer pin of the transformer, 42 V across the outer pins. Jipe: a 35 V 15000 µF part is fine as a replacement; the capacitor filters the supply before the regulators; he had fitted 220 V transformers with two 15 V windings to five turbo R machines without trouble.
- ↑ 6.0 6.1 A fonte do MSX Turbo-R (e dos FX, WX e WSX tambem!)– Alexandre Souza, 9 July 2018—link(accessed 2026-10-21) The supplies of the Panasonic machines in the turbo R style case (FS-A1FX, WX, WSX, ST, GT) are all the same apart from the switching transistor Q1, and a 2SA1643 seems to suit all of them; ±12 V from 7812 and 7912 regulators, which rarely fail; when the 5 V supply fails, the MC34063 and the transistor are the likely causes, though a resistor under the board can go open or the crowbar can short; the crowbar is a thyristor triggered by D9, a 5.6 V zener, and fires if, for example, R96 or R182 in the feedback to MC34063 pin 5 goes open; remove L2 (the small bare-copper inductor about 1 cm across) to isolate the supply and the crowbar, and do not remove L6 (the large green inductor) or the output rises to 22 V because the feedback is cut; with L2 out, up to 5.2 to 5.3 V is acceptable; the FS-A1GT under repair had a burnt 5P6J thyristor, replaceable with a TIC112, and the SMD 5.6 V zener under the board can be replaced with a leaded one; if the voltage is still wrong, check for about 22 V at MC34063 pin 6 and at the transistor's emitter, and if it is missing look for a burnt track from D2 and D3.
- ↑ Technical Guide: FS-A1ST, printed pages 39 and 40, electrical parts list, aluminium electrolytic capacitors: C1, C2, C7, C8, C9, C38, C52, C61, C66, C68, C70, C71 ECEA0JU471; C10 ECEA1AU472; C19, C40 ECEA1HN010; C20, C34, C47, C62 ECEA1HU010; C21 ECEA0JU220; C22, C26, C28, C37 ECEA1CU100; C23, C44 ECEA0JU101; C27, C35, C41, C42, C51, C60, C162, C163, C164, C165 ECEA1CU101; C36 ECEA1CU470; C43, C54, C55, C58, C63, C64 ECEA1HU2R2; C50 ECOS1EP123DA (25 V, 12000 µF); C59 ECEA1VU471; C67 ECEA0JU102. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.
- ↑ 8.0 8.1 8.2 Looking for someone who can (help) repair my Panasonic A1ST TurboR, January 2026—link(accessed 2026-10-21) An FS-A1ST that had worked when stored gave, after several years on a shelf, a black screen, no drive activity and no keyboard response, with one LED lit. Pat: C10 is the buffer capacitor for the +5 V line; measure the supply lines first. meits and Omega: the brown deposit by the 4700 µF capacitor is glue, not leaked electrolyte. Omega: the supply is unlikely to be the cause since the power LED lights; a black screen on a turbo R more likely means bad RAM or a failed custom chip. gdx: check the connector with yellow wires and the big black capacitor, and check the drive belt, which has probably melted and can stick and block the mechanism. meits: see what happens with a game cartridge fitted. Jipe: the machine has a 512 KB RAM modification; check the solder and wiring of the four RAM chips. sdsnatcher73: such an upgrade can cause this; one of his own failed over time and took out an address line on the S1990.
- ↑ 9.0 9.1 9.2 Technical Guide: FS-A1ST, printed page 13, video checks: enter COLOR 15,15,15 in BASIC; R, G and B at IC16 (VDP) pins 22, 23 and 24, composite sync at IC16 pin 6; R, G and B into encoder IC6 at pins 21, 22 and 23; IC6 sync output at pin 11 and composite output at pin 20. The VDP's RGB goes to the encoder and through a clamp to the RGB connector; the encoder makes the composite signal for the video output and the RF modulator; S-video luminance is amplified by Q6 to Q9 and chrominance by Q2 to Q4. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.
- ↑ 10.0 10.1 10.2 Technical Guide: FS-A1ST, printed page 14, audio and backup checks. PSG test program: 10 PLAY"V15C","V15C","V15C" / 20 GOTO 10, with the PSG outputs at IC17 pins 97, 98 and 99 and the mixed signal at op-amp IC8 pin 14; the MSX-Engine's sound and the cartridge slot sound are mixed by IC8 and go to the audio output and the RF modulator. FM test program: 10 CALL MUSIC:PLAY #2,"V15C" / 20 GOTO 10, with the output at IC10 pin 14. Backup test program: 10 CLS:KEY OFF / 20 GET TIME T$ / 30 LOCATE 10,10:PRINT T$ / 40 GOTO 20; with the cells fitted, set the time, run the program, switch off and on again, run it again and check the time is still right. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.
- ↑ MSX hardware fixes and enhancements—link(accessed 2026-10-21) Section 2.2, Panasonic MSX turbo R audio fix: slot sound too loud, OPLL too quiet, OPLL and output low-pass filters too strong, output level too weak. For the FS-A1ST: C48 and C49 1.8 nF, R24 22 kΩ, R23 15 kΩ, C57 180 pF, C159 150 pF; all 0805 SMD; capacitors polyphenylene sulphide film or C0G ceramic.
- ↑ 12.0 12.1 Turbo-R Panasonic floppy drives, December 2017 to March 2018—link(accessed 2026-10-21) Edevaldo: on an FS-A1ST and an FS-A1GT the original belts had melted in place and stuck to everything; after new belts the drives spun and sought track 0 but gave "disc offline"; cleaning the pulleys with isopropyl alcohol and cotton buds removed a lot of black residue and quietened the drives. gdx: had the same error after a belt change and after long disuse, and the drive worked again straight after a head-cleaning disk. Grauw: tried a couple of belts before one fitted; others were too narrow and would not stay on the pulley, or not tight enough. Manuel: belts last about ten years; Colemu: his lasted five. Moniz: old rubber left on the pulley made a new belt come off.
- ↑ Technical Guide: FS-A1ST, printed page 5, in-service notes: the built-in software switch on the front panel (on, the main menu starts at power-on unless a ROM cartridge is fitted; off, BASIC starts); the backup RAM holds the word processor's learning dictionary and the settings made with BASIC's SET commands, and it and the clock run from two AA dry cells, which are not fitted at the factory; the date and time are set with SET DATE and SET TIME. Hosted as File:Panasonic FS-A1ST Technical Guide.pdf.