NOS Canon V20 black screen

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By tilatax

Rookie (23)

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14-08-2018, 19:30

I have recently bought a V20. It looks brand new, unused inside and out. The PSU gives good 5/12/-12volts to the motherboard. The caps lock does not lit up and the composite video gives black screen only. All components look intact, no burnt resistors or puffy capacitors. Any idea where to start?
I am planning to to check the capacitors and resistors and if all good, then remove the CPU an put a socket back and replace CPU.
Thank you for your help.

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By Alexey

Guardian (3093)

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14-08-2018, 23:06

CPU is not likely to be the problem. Most likely RAM or VRAM is busted. I had to repair my V20 recently as one VRAM chip started to malfunction. Those 4116 chips are garbage, especially after so many years.

Also check whether any slot pins (in both slots) are shorted.

By RetroTechie

Paragon (1563)

RetroTechie's picture

15-08-2018, 06:24

Welcome here, tilatax! Running Naked in a Field of Flowers

Next steps would be:

  • Check VDP and Z80 clock signals.
  • Check reset signal(s) on the mainboard.
  • Plug in a known working RAM cartridge.
  • If possible @ the same time as external RAM: plug in a game cartridge that generates sound without user input, and see if sound comes from audio connector on the back.
  • Just in case: see if any ROMs inside are in the form of EPROM. These may be losing their contents 2+ decades after data was 'burned' in there.

Useful tools (besides voltmeter): logic tester, oscilloscope. Any of those available? Or know someone who has/can help with those?

Oh and don't replace IC's at random. Evil More chance of damaging the board & make the problem worse, than getting lucky. CPU would be a poor choice to start with that anyway - Z80's tend to be pretty reliable.

By tilatax

Rookie (23)

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15-08-2018, 08:18

Quote:

Thank you both, I will follow your advice and come back to you.

By tilatax

Rookie (23)

tilatax's picture

15-08-2018, 08:25

Tjank you Alexey. I am novice in electronics. How should I test the ram/vram in PCB?
BW
Tilatax

By tilatax

Rookie (23)

tilatax's picture

15-08-2018, 08:33

Thank you. I will let you know abot the results.
BW
Tilatax

By Alexey

Guardian (3093)

Alexey's picture

15-08-2018, 12:50

tilatax wrote:

Tjank you Alexey. I am novice in electronics. How should I test the ram/vram in PCB?

I usually diagnose the DRAM chip by checking the data out pin(s) with an oscilloscope. If the output is missing or not TTL-compliant (lower than 4.1v), I replace the chip. As said above, checking for 3.58MHz on pin 6 of z80 and checking the RESET signal (should be high) must be done among the first things.

A game cartridge probably won't work if the machine is seriously broken, but may be worth a try.

By tilatax

Rookie (23)

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15-08-2018, 23:06

Thank you Alexey. My Hantek oscilloscope has just arrived so I will follow your guidance. Many thanks.

By tilatax

Rookie (23)

tilatax's picture

18-08-2018, 13:51

All seems to be OK on the CPU, however the reset high is on 3.9V on oscilloscope and with multimeter on pin 26-29(gnd) is only 3V. The 5V on the psu to motherboard cinnector is 5V

By RetroTechie

Paragon (1563)

RetroTechie's picture

18-08-2018, 21:41

How does voltage on Z80 pin 26 (/RESET) respond when you push the reset button? Repeat check for other IC's with a reset input (VDP: pin 34).

By Jipe

Paragon (1484)

Jipe's picture

18-08-2018, 22:28

no reset button on canon V20 !!

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