Sanyo PHC-28L repair

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By Philip

Champion (317)

Philip's picture

13-03-2018, 20:18

It can be tested by replacing it Smile It seems for you replacing it is just as easy without a socket though Smile

By Nprod

Supporter (12)

Nprod's picture

13-03-2018, 22:24

Philip wrote:

It can be tested by replacing it Smile It seems for you replacing it is just as easy without a socket though Smile

I got a desoldering gun last month Tongue

Update - i took out some 150ns Toshiba 4164s from a Commodore64 that are known to work 100% and put them in the VRAM sockets.

- Now all their DOUT pins show digital data on the oscilloscope, whereas before some of the chips would just show a regular clock-like signal that doesn't change.
- The VDP, VRAM and CPU now get hot, especially the VDP - they were all pretty cold before.
- The INT pin on both the VDP and CPU is permanently high at around 4V.
- The DOUT pins on the RAM show garbage that looks like this:

- The CD0-CD7 CPU data bus pins on the VDP show the same garbage
- The OE and CE pins on the ROM show normal activity

Just for comparison, this is what the DOUT on the VRAM chips looks like:

I hope this has been exhaustive. My guess currently would be that either the other 4164's im using have something wrong with them, or one of the four 74LS157P multiplexers isn't doing its job.

By Philip

Champion (317)

Philip's picture

14-03-2018, 14:31

The int is generated by the vdp, but the vdp needs to be initialized by the cpu.
So it seems everything that has to do with the z80 doesn't work properly, so perhaps it's the z80 ?

By Nprod

Supporter (12)

Nprod's picture

14-03-2018, 20:50

Well, i ordered a Z80 and a VDP, i guess we'll see what happens in about 2 weeks... For the most part ICs aren't hard to source, but there is this Fujitsu MB64H120 gate array in the middle of the board that worries me as it's impossible to find any info on. Are there any known replacements for it?

By Grauw

Enlighted (7213)

Grauw's picture

14-03-2018, 23:01

No, the MSX-Engine ASIC is the one chip that you generally don’t want to break.

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