Philips VG-8020 Troubleshooting Guide
.png)
Component references below are those of Philips's service manual for the VG 8020/00, the first version of the Philips VG-8020; the /19, /20, /29 and /40 use a different main board, and no service manual for them has been published.[1][2] Maintenance and adjustments are on Philips VG-8020 General Maintenance.
Warning: the mains supply runs inside the case to the switch and the transformer. Unplug the computer before touching anything on the supply side.[3]
Supply rails
[edit | edit source]| Rail | Cartridge slot pins | Supply lead (J7) | Notes |
|---|---|---|---|
| +5 V | 45 and 47 | Pin 1 | Set to 5 V with VR1 |
| +12 V | 48 | Pin 2 | AN7812, fixed |
| -12 V | 50 | Pin 3 | JRC7912, fixed |
| Ground | 41 and 43 | Pin 4 |
| Symptom | Probable cause | Action |
|---|---|---|
| Dead, power LED off | No +5 V: fuse F3, bridge D1 to D4, cracked joints on the supply board, STK770 | See No power below |
| Power LED on, black screen | RAM, data buffer U23, reset or clock fault, VDP connection | See Black screen |
| Picture in black and white only | LVA510 PAL encoder | See No colour or wrong colours |
| Wrong colours | VDP clock (TC1), PAL encoder | Set TC1; check the encoder |
| Some or most keys dead | Loose or dirty keyboard membranes, ribbon or flat connectors | See Keyboard |
| Cassette will not load or save | Data recorder lead, remote relay RY1, comparator U48 | See Cassette |
No power
[edit | edit source]- Check the fuses on the supply board: F3 (1.6 A slow-blow) on the 5 V winding, F1 and F2 (315 mA slow-blow) on the 12 V windings.[3]
- With the computer switched on, measure the DC voltage across C2, the 4700 µF reservoir on the 5 V side. No voltage points at F3, the bridge rectifier D1 to D4 or the transformer winding.[2][3]
- If C2 has voltage but J7 pin 1 has no +5 V, try VR1 (the 500 Ω preset, called VRT in the parts list). If that does not bring +5 V back, suspect the STK770 regulator (U1) and its pass transistor TR1 (2SC2603).[2][9]
- Before replacing parts, reflow the joints on the supply board. The /20 in the forum thread had good fuses, +12 V and -12 V present and no +5 V; resoldering the supply board cured it.[2]
The power LED is fed from +5 V on the main board, so it stays dark whenever +5 V is missing, even if the 12 V rails are present.[3]
If +5 V is present but low or high, set it with VR1 as described on Philips VG-8020 General Maintenance. A supply that will not reach 5 V within the range of VR1 has a fault; do not leave VR1 at the end of its travel. Check the reservoir capacitors as described on Philips VG-8020 Capacitor Replacement Guide.
Black screen
[edit | edit source]With the power LED on and no picture, work through the checks the forum members gave for JimGR's /00:[6]
- Measure +5 V, +12 V and -12 V (table above).
- Try both the RF and the composite output, to rule out a fault in the modulator or one lead.
- Press CAPS repeatedly. The caps LED is switched by the 8255 under program control, so a LED that responds shows the CPU is running and points the search at the video side.[10]
- Check the reset capacitor C36 (22 µF) and the clock capacitors C82, C83 (33 pF), C84 and C85 (220 pF) round the 14.31818 MHz crystal X2.[11]
- With an oscilloscope, a logic probe, or a red LED and 1 kΩ resistor to ground, check that the Z80 address lines A0 to A15 and data lines D0 to D7 are toggling. Pin 26 (reset) should change state when the reset button is pressed; pin 29 is ground.
- A data line that stays still points at the data buffer U23 (74LS245) or at a chip loading the bus. Compare the two sides of each buffer channel, for example pins 9 and 11 of U23 for D1.
- Suspect the main RAM (U8 to U15, 4864) next. JimGR found three failed RAM chips and fitted sockets as he replaced them.
- Once the machine starts, a test program such as TESTRAM checks the RAM and video RAM.
JimGR's last fault was a poor contact at the socketed video processor U30: the picture was perfect while the chip was pressed down. If you fit sockets, use good ones and check every joint.[6]
His problems began after a recap, when a loose keyboard ground lead touched U29 with the machine running.[6] Keep loose leads away from the board when the machine is open and switched on.
No colour or wrong colours
[edit | edit source]A /00 picture that has lost its colour entirely is usually the LVA510 PAL encoder; an MSX Resource Center member describes this as a common failure needing a new LVA510.[7] On the /00 the LVA510 is IC2 on the PAL encoder board U100, with its own 4.433619 MHz crystal and a 12 to 27 pF trimmer, C15.[12] The /20 also has a separate encoder board; the /40 does not.[13]
Colours that are present but wrong can come from the VDP clock. Set TC1 for 445.32 kHz at pin 37 of U30 with a frequency counter.[4] In JimGR's repair the colours changed as a replacement TC1 was turned, and a forum member pointed out that the capacitors round the PAL encoder are critical.[6]
Keyboard
[edit | edit source]When only a few keys work, or the keyboard behaves erratically, take the keyboard out and check the three membranes; the tape that holds them to the base plate is commonly found disintegrated. The procedure is on Philips VG-8020 General Maintenance.[8]
When one group of keys fails together, look at the connectors. On the /00 the keyboard plugs into JN1 (12 ways, the rows, selected through the 74LS145 decoder U19) and JN2 (8 ways, the columns, read by the 8255).[10][14] On an NMS 8220, a related machine, both SHIFT keys failing was traced to a broken ribbon cable.[15]
Cassette
[edit | edit source]The data recorder socket is an 8-pin DIN. The motor is switched by the relay RY1, driven by Q2 from the 8255; the input signal is squared up by the comparator U48 (µPC311).[10]
| Pin | Signal |
|---|---|
| 1, 2, 3, 8 | Ground |
| 4 | CMTOUT (cassette out) |
| 5 | CMTIN (cassette in) |
| 6 | REM+ (motor) |
| 7 | REM- (motor) |
If the motor does not start, listen for RY1 clicking when the cassette is accessed; if it does not, check Q2 and the relay.[10]
Sound
[edit | edit source]The PSG output, the cartridge SOUND line and the BEEP line from the 8255 are mixed by U50 (NJM4558D), which runs from +12 V and -12 V. Each input and the output pass through 10 µF coupling capacitors (C76 to C79).[10] No sound with a good picture points at that mixer, its supplies or the coupling capacitors.
Monitor connector
[edit | edit source]| Pin | Signal |
|---|---|
| 1 | +5 V |
| 2 | Ground |
| 3 | Audio |
| 4 | Luminance |
| 5 | Composite video |
| 6 | +12 V |
| 7, 8 | Not connected |
The installation manual gives +12 V on pin 6. The service manual's circuit diagram draws +12 V on the pin it numbers 8, and +5 V on pin 1.[16][12] Measure which pin carries +12 V on your machine before wiring a lead to it, because the connector carries both supply rails.
Related pages
[edit | edit source]- Philips VG-8020
- Philips VG-8020 General Maintenance
- Philips VG-8020 Capacitor Replacement Guide
- Philips VG-8020 Documentation
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ Philips VG-8020, MSX Resource Center wiki, 7 March 2026—link(accessed 2026-10-21) Versions /00 (separate ICs) and /19, /20, /29, /40 (S3527 MSX-Engine); RAM in slot 3 on the /00 and slot 3-2 on the others.
- ↑ 2.0 2.1 2.2 2.3 2.4 Philips VG8020/20 documentation needed, August 2024—link(accessed 2026-10-21) BartH: a /20 bought dead, power light off, transformer humming where his /00's is silent; all fuses good; with the NMS 8220 service manual he found +12 V and -12 V present and +5 V missing; solved by resoldering the joints on the power board, which he believed had cracked in transport. Jipe: no service manual for the /20; for a missing +5 V, first look for DC on the 4700 µF capacitor; if there is none, check the fuse and the bridge rectifier; if there is, try the VRT preset; failing that the STK770 is dead. Wierzbowsky: RAM or the LVA510 encoder were the likely suspects; he had seen dead STK regulators and blown fuses.
- ↑ 3.0 3.1 3.2 3.3 3.4 Philips, Service Manual, Home computer VG 8020/00, 4822 727 15228, PDF page 22, power supply U101: mains 220 V through line filter L100, a fuse and SW1 to transformer T1; secondary on CN1 pins 4 and 5 through F3 (1.6 A slow-blow) to bridge D1 to D4 (S2V10), reservoir C2 (4700 µF) and the STK770 (U1) with TR1 (2SC2603) and preset VR1, output through L1 to J7 pin 1 (+5 V); CN1 pins 1 and 3 (centre tap pin 2 to ground) through F1 and F2 (315 mA slow-blow) to bridge D5 (1B4B1), the AN7812 (U2) for +12 V at J7 pin 2 and the JRC7912 (U3) for -12 V at J7 pin 3; J7 pin 4 ground; the power LED is fed from +5 V on the main board through R31 (220 Ω). Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf; see also File:Philips VG-8020-00 power supply circuit diagram (service manual).png.
- ↑ 4.0 4.1 Philips, Service Manual, Home computer VG 8020/00, PDF page 2, adjustments: VDP clock, a frequency meter through a 10:1 probe on pin 37 of U30, TC1 for 445.32 kHz; power supply voltage, VR1 on the supply board for 5 V across the output (C3). Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ Philips, Service Manual, Home computer VG 8020/00, PDF page 15, cartridge slots: pins 45 and 47 +5 V, 48 +12 V, 50 -12 V, 41 and 43 ground, 49 SOUND. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ 6.0 6.1 6.2 6.3 6.4 Philips VG8020 black screen problem...., October to December 2011—link(accessed 2026-10-21) JimGR's VG8020/00: after a recap it worked, then a keyboard ground lead touched U29 and the screen went black with the caps LED on; replacing U29 and U23 and the Z80 did not help; Jipe: on the /00 U29 (74LS273) is the printer port and U23 (74LS245) the D0 to D7 buffer, a broken data line stops the machine starting; check +5 V, +12 V and -12 V, the reset capacitor C36 (22 µF), the clock capacitors C82 and C83 (33 pF) and C84 and C85 (220 pF); RetroTechie: check whether the caps LED responds to the CAPS key, try both the RF and composite outputs, check the supply fuses and look for a clock signal; Jipe: a red LED in series with a 1 kΩ resistor to ground as a probe on the Z80 address lines A0 to A15 and data lines D0 to D7, which should flicker, and on pin 26 (reset), which changes with the reset button; pin 29 is ground; for a stuck D1 compare pins 9 and 11 of U23; JimGR found and replaced three faulty RAM chips, fitting sockets, after which the machine started with unstable colours; all RAM and video RAM passed TESTRAM; blue appeared as cyan and green as orange, and the colours changed as the replacement TC1 trimmer (original 12 to 27 pF or 25 pF, replaced with 2 to 22 pF) was turned; Daemos: the capacitors round the PAL encoder are extremely critical; the final fault was a bad contact at the socketed VDP U30, cured when the chip was pressed down.
- ↑ 7.0 7.1 Philips vg 8020 black and White screen, November 2022—link(accessed 2026-10-21) A VG8020 showing only black and white on RF; sdsnatcher73: the LVA510 needs to be replaced if the output loses colour, and it is a common problem.
- ↑ 8.0 8.1 Cleaning Keyboard Philips VG8020/NMS8220, 4 November 2021—link(accessed 2026-10-21) sdsnatcher73: VG8020 and NMS 8220 keyboards very often have faults; the tape holding the three membranes to the base plate commonly disintegrates, leaving them loose and the keyboard erratic.
- ↑ Philips, Service Manual, Home computer VG 8020/00, PDF page 7, electrical parts list: power supply U101 4822 216 92255, U1 STK-770, U2 7812, U3 7912A, Q1 2SC2603, D1 to D4 S2V-10, ZD1 HZ2CL, R4 0.22 Ω 2 W, R8 56 Ω 3 W, VRT 500 Ω potentiometer, L1 150 µH 3 A, T1 transformer; main board U23 74LS245, U29 74LS273, U30 TMS-9929A, U48 UPC311C, U50 NJM4558D, Q2 2SA720, TC1 25 pF trimmer. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ 10.0 10.1 10.2 10.3 10.4 Philips, Service Manual, Home computer VG 8020/00, PDF pages 19 and 20: keyboard matrix with JN1 (Y0 to Y11, driven by U19 74LS145 from PPI port C) and JN2 (X0 to X7, read on PPI port B); data recorder J8 with remote relay RY1 driven by Q2 (2SA720) from PPI PC4 and protection diode D6; cassette output from PC5 through C1 (1 µF), R30, R25 and C71; cassette input through C6 (22 nF) and R11 to comparator U48 (µPC311); sound mixer U50 (NJM4558D) on +12 V and -12 V, with the PSG output through C77, cartridge SOUND through C78 and BEEP through C79, and its output to AUDIO through C76; caps LED driven from PPI PC6 through U39 and R33 to JN3. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ Philips, Service Manual, Home computer VG 8020/00, PDF page 15: reset circuit with switch, R73, D5, R21 and C36 (22 µF) driving Q3 (2SC2603); clock oscillator with X2 (14.31818 MHz), C82 and C83 (33 pF), C84 and C85 (220 pF) and gates of U53, divided by U49 for the CPU clock. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ 12.0 12.1 Philips, Service Manual, Home computer VG 8020/00, PDF page 18: PAL encoder U100 with IC1 (74LS04), IC2 LVA510, crystal X1 (4.433619 MHz) with trimmer C15 (12 to 27 pF), transistors Q1 to Q3, inputs Y, R-Y and B-Y from the VDP through JN10, CVBS output through C13 (470 µF); RF modulator U102 fed with CVBS through C87 (100 µF) and +5 V through L1 and C86 (33 µF); monitor connector J9. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ File:Philips VG-8020-20 main board.jpg and File:Philips VG-8020-40 main board.jpg, photographs uploaded to the MSX Resource Center wiki by Gdx on 7 March 2016: the /20 board carries the PAL encoder daughter board EMC-PX3607, the /40 board has none.
- ↑ Philips, Service Manual, Home computer VG 8020/00, PDF page 8, mechanical parts list: keyboard 4822 219 80639, keyboard connectors 8-pin and 12-pin. Hosted as File:Philips VG-8020-00 Service Manual 4822 727 15228.pdf.
- ↑ shift key doesn't work., 2003 to 2016—link(accessed 2026-10-21) minus_signs: both SHIFT keys dead on a Philips NMS 8220, fixed by replacing a broken ribbon cable; a VG 8020 owner reported the same symptom with no follow-up.
- ↑ 16.0 16.1 16.2 Philips, VG 8020 MSX Computer installation manual (English pages), appendix A, printed pages 51 and 52: data recorder, joystick, monitor and printer connector pinouts. Hosted as File:Philips VG-8020 Installation Manual.pdf.
