Conversion from SVI-738 MSX2 to MSX2+ (HELP!)

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By fernando3k

Rookie (24)

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26-02-2022, 12:45

I have a SVI-738 converted to MSX2 using instructions from @nyyrikki and RTC from @lkpalwa.
Currently, I am trying to convert it to MSX2+.
I changed the VDP9938 for VDP9958 (pins 4,21,27 bent and out of the socket, 1-4-27 and 21-58 connected with wires). I put the @nyyrikki MSX2+ ROMs in and it boots up, but I get a boot logo with wrong colors (not SET TITLE seted!!!). I burned using different EPROMs and always get the same result.
Any ideas? Thanks.

Video: https://youtu.be/gAudVPLv7JQ

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By fernando3k

Rookie (24)

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26-02-2022, 13:07

Also does not do the white to blue gradient effect in boot logo

By Stt1

Hero (610)

Stt1's picture

26-02-2022, 15:01

Check out the leg count, next to 21 is green component, maybe you connected 22 instead of 21.

https://www.electronicsrepairmadeasy.com/2017/08/solving-crt...

By fernando3k

Rookie (24)

fernando3k's picture

26-02-2022, 19:34

Pins 21 and 22 are well connected. The color problem is only in the boot logo, then it looks normal. In fact, I tried placing the V9938 (without wiring) and it does exactly the same thing. With MSX2 ROMs it works fine, boot logo color is correct. Seems like a problem with MSX2+ ROMs but I tried them on openMSX they work fine.

By lkpalwa

Expert (115)

lkpalwa's picture

09-03-2022, 16:43

bad VRAM? or bad lead? also have seen some where people connect a 0.1uF to the solution

By fernando3k

Rookie (24)

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11-03-2022, 02:49

VRAM chips are ok. I tested these using testram. The problem is logo screen color only. The 4 VRAM have 0.1uf coupling ceramic capacitor.

By sdsnatcher73

Prophet (3662)

sdsnatcher73's picture

11-03-2022, 06:25

Sounds like some capacitors in the (analog) video circuit behind the VDP are not ready when the boot logo is shown. I doubt there is an issue in the digital domain (so CPU, VDP, RAM, logic chips). Not sure why it happens with 2+ ROMs and not 2 ROMs.

I imagine it happens only on power on, not with hardware reset (if you have a hardware reset), correct?

By fernando3k

Rookie (24)

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11-03-2022, 07:55

When I reset (it has a reset button) it does the same thing. The output is RGB taken directly from the VDP pins.

By sdsnatcher73

Prophet (3662)

sdsnatcher73's picture

11-03-2022, 11:44

I don’t know if you need any resistors (at the minimum). Normally RGB is 75ohm terminated. Best to look at some service manuals of machines with RGB output and replicate the circuit (you can try to find a relatively simple one).

Are the RGB signals from the VDP also still connected to the circuit for the composite signal? If so test if the color shift happens there as well, if not maybe you can reconnect temporarily to check what happens.

By Jipe

Paragon (1571)

Jipe's picture

11-03-2022, 13:33

see the link of Schematic at the bottom of this page : https://www.msx.org/wiki/Sanyo_PHC-77

only a transitor, 2 capacitor and 4 resistor for making a amplifier for R G B
input G_SIG R_SIG B_SIG output G_SIG' R_SIG' B_SIG'
only a transitor, 2 capacitor and 2 resistor for making a amplifier for CSYNC
input CSYNC output CSYNC'
you can omit coils R115 R123 R111 R119 with ?uH

By fernando3k

Rookie (24)

fernando3k's picture

12-03-2022, 00:41

Thanks @sdsnatcher73. Thanks @Jipe.
I plan to make an amplifier using the THS7374 chip. The color problem in the logo also occurs with composite video (RGB disconnected)

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