VG-8235 does not boot into splash or basic

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By FrankyBoy12

Supporter (5)

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29-08-2019, 14:07

Hi all,

I'm new here, so first of all apologies if this is not accoording to procedure. Hope anyone has an idea if my project can be saved or dies early.

I have a challenge with a Philips VG-8235 that I acquired recently. It's not booting anything, simply showing a blue screen. I checked around a bit and performed some checks, below the results:

Power light is on, no other led.
Only a blue empty screen over scart and uhf
Z80 CPU gets warm, ram modules get warm
Typing Color 15,4,4 does not result in anything.
No keyboard sounds, no beep, no diskdrive spin (belt broken though)
Battery is removed
Reset button resets the machine briefly and then goes back to empty blue screen
MSX Music module inserted at boot gives nothing either

Anyone with a similar experience or an idea what's going on, or for next steps?

Best regards,
F.

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By Wolverine_nl

Paragon (1046)

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29-08-2019, 15:18

Could it be the clock chip? I read somewhere it needs RTC to boot. Did you check the fuse? etc
Does it even show the MSX logo at boot?

By FrankyBoy12

Supporter (5)

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29-08-2019, 15:20

No, it's not showing the MSX logo at boot. The screen is blue however.

I read similar things, anyone know how can I check if RTC is functioning properly?

By Pentarou

Master (226)

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29-08-2019, 22:23

Quote:

anyone know how can I check if RTC is functioning properly?

I think you'll need the Philips test cartridge or an oscilloscope. Maybe it's easier to burn a ROM with the RTC disabled?

Does the caps led turn on when you press the caps key?
Has the machine been repaired/modded besides the removal of the battery?
Are you sure the blue screen you are seeing is from the VG-8235 and not what the TV shows when it can't display a signal?

By FrankyBoy12

Supporter (5)

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29-08-2019, 23:10

No caps lock light turns on when pressed.

No modding / repairing the last 30 years before I openened the machine that I know of / can see.

the blue screen is definitely only there when the machine is turned on and disappears when resetting and turning off, in both SCART and UHF signal. (In scart there is some reddish glow on the left side)

Unfortunately my knowledge of burning roms or testing with the Philips test cartridge and osciloscope are limited / to non-existent...

By Pentarou

Master (226)

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30-08-2019, 00:05

OK, so:
It's not the RTC clock, because it would freeze the machine on a black screen, before the blue.
The CPU seems to be working, at least for a while, because it sets the VDP registers... But then it doesn't complete the boot sequence, so no basic prompt, no beep and no keyboard.

Maybe it's a problem related to bank switching or a damaged ROM?

By Alexey

Guardian (2457)

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30-08-2019, 03:26

I would bet on the bad RAM. Do you have an external RAM expansion?

By FrankyBoy12

Supporter (5)

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30-08-2019, 12:08

I also have no RAM expansion avaiable, unfortunately...

Could indeed be damaged ROM, should that get warm as well because this stays cold...

By Alexey

Guardian (2457)

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30-08-2019, 14:57

I think what happens is BIOS starts, initializes VDP, sets the correct color and tries to boot to Basic. But something is wrong with the RAM and the system area gets corrupted. So everything goes to hell. RAM would be my first suspect, ROM - second.

By Wolverine_nl

Paragon (1046)

Wolverine_nl의 아바타

30-08-2019, 15:40

Alexey wrote:

I think what happens is BIOS starts, initializes VDP, sets the correct color and tries to boot to Basic. But something is wrong with the RAM and the system area gets corrupted. So everything goes to hell. RAM would be my first suspect, ROM - second.

I was thinking the same thing, it happens before the MSX logo, which probably wants to use RAM? (someone probably knows this for sure) Wink

By mohai

Paladin (841)

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30-08-2019, 16:29

I would say it is a SUB-ROM issue or a RAM issue.
An easy way to check is to change BIOS+BASIC ROM chip with one from an MSX1. This can be done in 8235 with the big board.
I am not sure if this would work in the one with small board, but I would try anyway.
I am sure it is not an issue with RTC. I have a 8280 that sometimes gets discharged battery, but still boots up.

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