Sanyo PHC-28L repair

By Nprod

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14-02-2018, 21:15

Hello! I recently obtained a Sanyo MSX1 computer with a black screen problem and need some help getting it restored and working again. Initially i noticed that the Composite jack had cracked solder joints, likely from continued use. Reflowing it was an easy fix, but there is still no video. Or at least not quite, there are barely noticeable vertical bands when the computer is on which i'd guess are the 40 text columns. All the PSU voltages check out fine. Probing the composite output with an oscilloscope shows only a sync pulse and then a pretty flat signal. I also checked Pin6 on the Z80 and i do get a clock signal there. The TMS9928A chip is also getting something on the GROMCLK pin.

At the moment my guess would be that the problem is either in the RAM, VRAM or the BASIC ROM, i also have no cartridges for this machine.

I appreciate any help with this

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By Manuel

Ascended (13883)

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14-02-2018, 22:36

Sometimes it helps just to keep it on for half an hour. At least that helped with VG-8000/8010's.

By Nprod

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15-02-2018, 09:54

Really? What does leaving it on do exactly?

By Alexey

Paragon (1350)

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15-02-2018, 11:48

Nprod wrote:

Really? What does leaving it on do exactly?

It makes the machine feel alive again. Smile

Anyway, I would bet on the broken 4116 or 4464 chips.

By Philip

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15-02-2018, 13:21

Does the caps lock light work ?

By Nprod

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15-02-2018, 14:02

Philip wrote:

Does the caps lock light work ?

It doesn't. That doesn't rule out the possibility of a bad keyboard connection, but so far it's looking a lot like a classic RAM fault.

Alexey wrote:

I would bet on the broken 4116 or 4464 chips.

With 4116 being infamous for running hot and failing, is it worth buying new ones or would it be better to modify 4164's to fit in their place? They're easier to find too.

A little off topic - how easy is it to upgrade this machine to MSX2? I've seen people do this for Yamaha CX5Ms but not the Sanyo PHC series. Has anyone tried to do this?

By Alexey

Paragon (1350)

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15-02-2018, 15:28

If I remember correctly the 4116 chips require 3 voltages while 4164 chips require only one. So I am not sure whether it's easy to use 4164 instead of 4116.

By Nprod

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15-02-2018, 15:54

Alexey wrote:

If I remember correctly the 4116 chips require 3 voltages while 4164 chips require only one. So I am not sure whether it's easy to use 4164 instead of 4116.

Yeah i read into it and it seems to be a common thing people do. 4116s are apparently a common failure point with Apple IIs and ZX Spectrums, not just MSX. You just need to bend back Pin8 and connect it to Pin9, then trim off Pin1. It's supposed to work with 41256 chips too. I was mainly asking if there are any known good brands of 4116's or if they're all unreliable. This Sanyo has Hitachi branded ones.

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