Jump to content

JVC HC-7 Troubleshooting Guide

From RetroTechCollection
JVC HC-7E main board
Inside an HC-7E: main board with the processor, video chip, RAM and video RAM; power supply board at the top right

No service manual or circuit diagram for the JVC HC-7 has been published. Fault-finding starts from JVC's check list in the owner's manual, the supply rails and the power supply's inhibit line, then the chips whose pinouts are standard.[1] For mains safety and the inside of the case, read the JVC HC-7 General Maintenance guide first.

JVC's check list

[edit | edit source]
Symptom JVC's checks
Will not power on Mains plug pushed fully home
Nothing on the screen Computer and monitor switched on; leads; cartridge properly inserted; television tuned
Cartridge will not run Cartridge pushed fully home; cartridge contacts clean
Cannot load from tape Leads; recorder volume at 5 or higher; phase switch, if the recorder has one
Computer stops while running Power failure; power switch; mains plug; peripherals connected; interference from other appliances (use a mains filter)

[2]

Supply and inhibit line

[edit | edit source]

Measure at the cartridge slot, against pin 41 or 43: +5 V on pin 45 or 47, +12 V on pin 48 and −12 V on pin 50.[3] The same rails can be measured on the power supply's wires at main board connector CN001: red +5 V, orange +12 V, blue −12 V, black ground.[4]

A working HC-7E measured −11.77 V, +11.60 V and +5.0 V on those wires.[5] The yellow wire is the supply's inhibit input. It sits near 0 V while the computer runs and rises to about 1 V when a cartridge slot cover switch opens, and the supply then shuts down.[5][4]

Symptoms

[edit | edit source]
Symptom Check first Notes
Dead, no power indicator Mains plug and the fuse on the power supply board The power supply board carries a glass cartridge fuse on its mains side.[6] Check every rail and the fuse.[5]
Dead, fuse good The yellow inhibit wire and the slot cover switches If the yellow wire is high with the covers closed, a cover switch or its wiring is open; the supply will stay off.[5][4]
Powers on only sometimes; tapping near the supply helps Solder joints on the power supply board and its connectors Reported on an HC-7E offered for sale.[7] Look for cracked solder joints on the supply board and at its connectors.
Turns off when a cartridge goes in A slot cover switch Normal: opening a cover opens its switch and the supply shuts down.[5]
Blue flash, then black; Caps Lock dead Rails at the RAM chips; then try a RAM cartridge On one HC-7E the machine ran with RAM in a cartridge, but replacing all eight RAM chips did not cure it; the remaining suspects were bent or shorted slot pins and short circuits on the board.[5]
Black screen, no response Clock and reset Look for 3.58 MHz on pin 6 of the Z80; watch the reset inputs (Z80 pin 26, VDP pin 34, 8255 pin 35) change when reset is pressed.[5]
No picture on SCART The SCART lead and the television's input selection Pins 8 and 16 carry the composite and RGB status signals, pins 7, 11 and 15 blue, green and red at 0.7 V p-p, pin 19 sync.[8] Try the monitor socket (composite video on pin 4 of the 5-pin DIN) to tell a video fault from a SCART fault.[9]
Keys dead or bouncing The keyboard's microswitches Each key is a separate microswitch.[4]
Cannot load from tape Volume and phase See JVC's check list above.[2]

Chips with known pinouts

[edit | edit source]
  • Z80 (Sharp LH0080A): clock on pin 6, reset on pin 26.[5]
  • 8255 (Mitsubishi M5L8255AP-5): reset on pin 35.[5]
  • Video chip, under the small heat sink: reset on pin 34.[5]
  • TMM4164AP-15 RAM: +5 V on pin 8, ground on pin 16.[5]
  • The Hitachi HD27L019 is a gate array; a forum reply did not expect a datasheet or pinout for it to turn up.[5][10]
[edit | edit source]

References

[edit | edit source]
  1. ↑ Index of /pub/msx/docs/service_manuals, MSX Archive—link(accessed 2026-10-21) Directory of MSX service manuals; it lists none for JVC or Victor machines.
  2. ↑ 2.0 2.1 Victor Company of Japan, Instructions, Personal Computer HC-7GB, page 33 (PDF page 33), "In case of difficulty". See File:JVC HC-7GB Instructions.pdf.
  3. ↑ JVC, Instructions, Personal Computer HC-7GB, page 37 (PDF page 37), cartridge bus: 41 and 43 ground, 42 clock, 44 SW1, 45 and 47 +5 V, 46 SW2, 48 +12 V, 49 SUNDIN, 50 −12 V. See File:JVC HC-7GB Instructions.pdf.
  4. ↑ 4.0 4.1 4.2 4.3 JVC HC-7E, MSX Resource Center wiki, 8 May 2026—link(accessed 2026-10-21) Community page: 6-pin female Molex connector from the internal power supply, black ground, red +5 V, orange +12 V, blue −12 V; the yellow wire switches the supply off when the cartridge lid is open, at about 0.14 V with the lid closed and 0.93 V with it open; RAM eight TMM4164AP-15, VRAM eight MB8118-12; keyboard on microswitches.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 Got a JVC hc-7e worked at first but now refueso to power on, March to November 2016—link(accessed 2026-10-21) Four-page thread. The HC-7E showed a blue screen for under a second, then black, with Caps Lock dead. Supply readings with the machine on: blue −11.77 V, yellow 0 V, orange +11.60 V, red +5.0 V. Pressing the slot microswitches raised the yellow wire to about 1 V and switched the machine off; a reply called it the supply's inhibit line, normally 0 V. Advice given: check every supply voltage and the supply's fuse, the voltages at the RAM and VRAM chips, the 3.58 MHz clock on pin 6 of the Z80, and the reset inputs on Z80 pin 26, VDP pin 34 and 8255 pin 35 while pressing reset; 4164 RAM has +5 V on pin 8 and ground on pin 16; the owner found +5 V only on pin 8 of the MB8118 video RAM chips, which a reply accepted as correct for that part. The machine ran from a flash cartridge with its own RAM; RAM tests passed; replacing every RAM chip did not cure it. The last advice was to check the cartridge slots for bent or shorted pins and the board for short circuits.
  6. ↑ File:JVC HC-7E power supply board.jpg, photograph of the internal power supply of an HC-7E uploaded to the MSX Resource Center wiki by Mcolom on 26 November 2022. Read from it: glass cartridge fuse, line filter choke, a relay marked SDE8S-E 4A/32A 250V, bridge rectifier and a large electrolytic on the mains side; ferrite transformer F5S-84147 and optocoupler PC1; secondary electrolytics including C13, C14, C16, C17, C19, C20 and C21; regulator IC1; output wires red, yellow, black, blue and orange.
  7. ↑ For sale: JVC HC-7E MSX, January 2017—link(accessed 2026-10-21) The seller reported that the HC-7E did not always power on, but tapping around the power supply made it work.
  8. ↑ JVC, Instructions, Personal Computer HC-7GB, page 38 (PDF page 38), RGB connector: audio out on pins 1 and 3 (500 mV rms, 1 kΩ); blue, green and red out on 7, 11 and 15 (0.7 V p-p, 75 Ω); status CVBS on 8 and status RGB on 16 ("H", pull-up); sync out on 19 (0.5 V p-p, 75 Ω, negative); grounds on 4, 5, 9, 13, 17, 18 and 21. See File:JVC HC-7GB Instructions.pdf.
  9. ↑ JVC, Instructions, Personal Computer HC-7GB, page 35 (PDF page 35), A/V interface, PAL, 5-pin DIN: 2 ground, 3 sound, 4 video. See File:JVC HC-7GB Instructions.pdf.
  10. ↑ Victor HC-7, MSX Resource Center wiki, 30 May 2026—link(accessed 2026-10-21) Chipset: separate ICs including a Hitachi HD27L019 gate array.


Retrieved from "https://retrotechwiki.com/index.php?title=JVC_HC-7_Troubleshooting_Guide&oldid=28038"