Toshiba HX-10 Black Screen: What To Check?

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By PingPong

Prophet (3788)

PingPong's picture

13-07-2012, 12:44

Rink wrote:

Apparently this is a Toshiba HX-20 with the T6950 visible in the top left

here, if useful you can find the pinout of the T6950

http://imageshack.us/photo/my-images/62/t6950pinout.jpg/

By Jipe

Paragon (1511)

Jipe's picture

13-07-2012, 16:10

it's easy to determine the VDP model : T6950 have 42 pin and TMS 99xx only 40 pin

By Rink

Rookie (25)

Rink's picture

14-07-2012, 14:30

I built a small circuit (following the schematic Retrotechie posted) to amplify/buffer the component video Y and feed it to a portable TV's composite input as a test... and now have a black and white screen on the TV from the MSX. Everything fine there - I can finally see that beautiful BASIC prompt.

Still deciding whether to wait and replace the LVA510 and try that... or replace the circuit entirely with a component or vga output.

But i'm bloody happy to have got this far.

By Rink

Rookie (25)

Rink's picture

18-07-2012, 19:45

Bloody Samsung TV just reports "mode not supported" when this thing is plugged into the component input.

Don't suppose anyone can confirm what "resolution" the TMS9929A actually outputs YPbPr?

By Manuel

Ascended (18233)

Manuel's picture

18-07-2012, 21:56

Probably a 15kHz signal?

By Rink

Rookie (25)

Rink's picture

19-07-2012, 00:52

Hmmm... Guessing 240p if it's a 15Hz signal. TV supports 480i allegedly and since it doesn't work...

God damn. Cannot catch a break on this bit.

By Rink

Rookie (25)

Rink's picture

19-07-2012, 16:18

On to plan C... might be D or E by now...

Convert YPbPr to RGBS and output through a SCART connector. If that works, I *may* be able to run the RGBS through a chip for PAL Encoding to restore the Composite+RF output... will see I guess. Smile

By matthew180

Resident (48)

matthew180's picture

19-07-2012, 16:37

Rink wrote:

In fact, it's probably the cleanest PCB and assembly I've ever seen.

I had the exact same thought when I opened my HX-10. Very nice layout.

Rink wrote:

Don't suppose anyone has a schematic for this damn thing?

If you find one, *please* let me know! I want to add a hardware reset to mine, and schematics would be very helpful.

Rink wrote:

I can't say for certain which TMS99xxx is in there. There seems to be something glued over the top.

I don't know if Toshiba made an non-European version of the HX-10? Either way, the HX-10 uses the 9928 or 9929 (PAL), and *not* the 9918A. The difference between the 9918A and 9928/29 are that the 9918A only has composite output (pin 36), the CPUCLK (pin 38) signal, and EXTVID (external video input, pin 35), and the 9928/29 replace those thee pins with the component video output (B-Y pin 35, Y pin 36, R-Y 28).

RetroTechie wrote:

When the videochip itself is dead, chances are more stuff doesn't work (usually the Z80 gets its clock signal from the videochip, and without interrupts from the VDP, keyboard wouldn't be scanned etc).

Sometimes, but it depends on the system. Only systems that use the 9918A can get a main clock signal from the VDP (3.5MHz-ish from pin 38) because the 9928/29 do not have the CPUCLK output. In this case, the HX-10 uses the 9928/29, and the system has its own clock circuit. I have personally only seen (which does not mean a lot) two systems that use the VDP's CPUCLK to run the main CPU: Tomy Tutor and SpectraVideo 328 (but only the NTSC version, the PAL unit has a dedicated system clock).

The interrupts do keep some things from working, however, on the HX-10 specifically most things will still work without the interrupt working properly.

Rink wrote:

As a slight side note: if I have to replace the encoder board (assuming changing the IC doesn't work) - are there any better options out there in the scene rather than trying to replace it like for like? E.g. VGA output etc etc?

Yes. Check out the F18A, which is a direct pin compatible replacement for the 9918A/9928/9929, and 100% tested in the HX-10. You can see the HX-10 being used during prototyping here: http://codehackcreate.com/archives/30

I also used my HX-10 for some extensive feature testing. You can still pre-order and I have started shipping the final version.

Matthew

By Manuel

Ascended (18233)

Manuel's picture

19-07-2012, 21:08

The reset button is, if I remember correctly, a matter of connecting a switch circuit to the proper Z80 pin.

By matthew180

Resident (48)

matthew180's picture

19-07-2012, 22:45

Eh, yes, you can do that I suppose... Some people would, but that's not my style. That kind of reset bypasses the system's power-on reset circuit and could cause problems if you don't pay attention to how the system reset is designed. Most of these systems have an RC-time circuit that is buffered via an inverter, or sometimes a dedicated chip designed for such things. It is best to tie in at that point and short the capacitor to ground via a switch and a weak pull-down resistor.

My need for the reset has not over come my resistance to tracing out the motherboard to find the reset circuit. I keep hoping I'll find a schematic first. :-)

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