Problem with VY-0010 repairing

By Louthrax

Prophet (2093)

Louthrax's picture

24-03-2019, 12:17

Hi all,

I'm having some trouble repairing a VY-0010 Philips disk interface. When I received it, it was preventing all my MSX machines to boot, but that was happaning after the MSX logo. I checked the ROM and it was a Victor / JVC variant of the usual MSX disk ROM.

I unsoldered the ROM, and put a socket with a fast disk ROM instead (based on NMS8250).

Time to retrieve a disk drive, fix the RDY signal and drive select stuff, and... everything was working!

Until I put everything back inside the cartridge box and screwed the GND connector of the cable (the original one that was sold with the VY-0010 interface) on the drive: it still worked for a while... and then my MSX machine was not booting again (but this time with no MSX logo at all, just black screen).

I opened the cartrtidge gain, and tried some things: the MSX is booting correctly if I remove the EPROM from the VY-0011 adpater. So it seems that the address / data / slot select mechanism is kind of broken (memory bus beeing garbled because of the VY-0010 ?). I suspect some of the address decoding chips inside of the VY-0010 to be guilty, but I'm not sure...

Let me know if you have hints on this... I'm currently checking signals sent to the ROM and continuity, but I suspect something wrong on the slot selection chips (MSX not booting at all meaning the VY-0010 ROM is selected in some way at 0000-3FFF ?).

Also, now that I'm looking at the MSX cartridge pinout, I'm wondering what the CS1/CS2/CS12 signals are used for ? There's already a "slot select" line, and all addresses from A0 to A15, so why those extra signals ?

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By lintweaker

Master (159)

lintweaker's picture

24-03-2019, 12:26

Hi,
The CS1/CS2/CS12 signals are used to select a ROM/RAM at specific pages (address ranges).

/CS1 -> 4000-7fffh
/CS2 -> 8000-bfffh
/CS12 -> 4000-bfffh

When only using the /SLT select signal a cartridge will occupy the whole 64k (all 4 pages)

By Alexey

Guardian (2449)

Alexey's picture

24-03-2019, 13:02

I think you should use RD signal for the entire 64kb instead of CSx. The selected slot signal is also needed.

By Wlcracks

Champion (326)

Wlcracks's picture

24-03-2019, 17:28

Be sure all voltages +12V are present. I had this problem when i bought the VY-0010 new for 1030 gulden. The Philips service centre called the unit was fine, if could bring my MSX too. The +12V fuse was blown and it all worked again under warranty. Those were the days...

Also had this in my archive. Not sure what it means. I guess a correction on the service manual.

By Jipe

Paragon (1366)

Jipe's picture

27-03-2019, 17:57

By Louthrax

Prophet (2093)

Louthrax's picture

27-03-2019, 19:30

Thanks guys for the advices and links. I'll check the 12V, but I suspect mainly a problem in the ROM address / selection logic: the ROM either prevents MSX to boot (like if memory at 0000 was garbled), or when MSX boots and that I make a memory dump at 0x4000, it's filled with a sequence of 0xB2 0xB2 0xB2... bytes.

By Louthrax

Prophet (2093)

Louthrax's picture

27-03-2019, 22:12

Ok, I checked the voltages on the ROM when MSX is stuck at start (before showing the MSX boot screen):

  • ROM pin 20 (CE) is at 1.9V.
  • ROM pin 22 (OE) is at 1.8V.

Voltages are not moving at all (looks normal as MSX is stuck on black screen), but I'm curious about the 1.8-1.9V. Should those voltages enable or disable the ROM ?

If I believe the specs: min VIH is 2.0, so the state of CE/OE should be VIL (enabled), that explains that the data bus is garbled and that my MSX does not boot at all... Am I right ? (if yes I'm planning to replace the 74HC139 chip that is connected to the ROM's CE).

EDIT: Ah, and according to my 27128 datasheet, the PGM ROM pin should also be set to VIH for read operation to work. I have not checked that...

By Alexey

Guardian (2449)

Alexey's picture

28-03-2019, 11:05

The CE and OE signals don't stay always at zero or one, so your multimeter shows the average when they are fluctuating. You need an oscilloscope to see the real picture.

By Louthrax

Prophet (2093)

Louthrax's picture

28-03-2019, 23:12

Alexey wrote:

The CE and OE signals don't stay always at zero or one, so your multimeter shows the average when they are fluctuating. You need an oscilloscope to see the real picture.

Agree with that. I anyway checked both CE and OE with a working "F16 fighter" ROM in action, the voltages are quickly moving when the ROM is active (that can be observed even with my multi-meter). Both pins seems very stable with the faulty VY-0010 (MSX is booting anyway, so not too much life-activities expected).

Also, I'm getting the same symptoms (MSX not booting - black screen), if I pull-out pin CE and connect it manually to GND.

I've ordered a pack of 74xxx replacement chips for the address decoding area and will see if replacing them fixes something.

By Wlcracks

Champion (326)

Wlcracks's picture

29-03-2019, 17:31

repairing z80 based circuits I can recommend this channel youtube
View the donkey kong arcade boards, punch out and repair 101's.
He has this led probe thingy for digital lines. I bought one too, you basically don't need a oscilloscope.