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

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Par 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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Par 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

Par Stt1

Hero (607)

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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...

Par fernando3k

Rookie (24)

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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.

Par lkpalwa

Expert (115)

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09-03-2022, 16:43

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

Par 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.

Par sdsnatcher73

Prophet (3550)

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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?

Par 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.

Par sdsnatcher73

Prophet (3550)

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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.

Par Jipe

Paragon (1561)

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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

Par fernando3k

Rookie (24)

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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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