Toshiba HX-10 Black Screen: What To Check?

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By Rink

Rookie (25)

Rink's picture

12-07-2012, 21:08

Did a little more checking - didn't really discover any revelations but hey...

BASIC is definitely working. I blindly entered
10 BEEP
20 GOTO 10
RUN
and got a continuous beep until I pressed stop. Cool. My first MSX program and I didn't even see it.

The mainboard -> PAL encoder connector (PAL1) has the following voltages on its pins:
0v (maybe a millvolt or two over), 4.7v, 2.46v, 3.01v, 2.58v, 0v

The output from the PAL encoder circuit (PAL2) displays only a millivolt or two over 0 on both of its two pins. So basically, there's no PAL encoded composite signal from that circuit. Struggling to check what's happening on LVA510 itself because I don't know what the pins are supposed to be.

I can't say for certain which TMS99xxx is in there. There seems to be something glued over the top. Sad But I'm reasonably convinced that the European HX-10s used a TMS9929A with YPbPr (ish?) output.

Here's a photo - not that it helps with anything mind.

By Rink

Rookie (25)

Rink's picture

12-07-2012, 21:07

Hi mate,

RetroTechie wrote:

As a quick shortcut you could just grab the Y signal, but the VDP's outputs are not designed to drive 75 Ohm inputs directly. So you'd have to put a transistor buffer (something like this) in between. A lot of effort to find out what we -more or less- already know: that the VDP still works.

That's something I've been wondering about today (to be fair, a guy on AtariAge gave me the idea) and is probably worth a try this weekend.

Routing the Y from TMS9929A back to the composite video out would be easier enough - but might not solve the driving problem since I'm not sure at what point it runs through a buffer. Might be easiest just to grab my own transistor or IC and see if I can take the signal from the PAL1 (not sure which of the pins it might be exactly) connector, and feed that to a phono plug for the TV.

If I could eventually get a full component output then that would do for a while - my TV accepts component and I could also get a VGA box I guess. I'll try replacing LVA510 but if that doesn't work, investigating all the capacitors etc. without a datasheet for LVA510 might be harder than replacing the circuit with a new design of component output and then composite.

Thanks for all the comments and advice so far guys.

By tvalenca

Paladin (747)

tvalenca's picture

13-07-2012, 00:17

Hi folks!

Actually, I have downloaded the TMS99x8A from somewhere on the internet... And it says that on TMS9918A chip you have CVBS on pin 36, but if your HX-10 uses an TMS9928A/9929A you will get Luma (Y) on pin 36. So, regardless the version of the VDP your MSX uses, you will get video (BW or NTSC colored) on pin 36. You have to be extra-careful here, because pin 37 is an clock pin, and it's not rare at all to have the VDP output clock damaged, and the MSX won't boot anymore after this (until you change the VDP or install any "Z80 turbo kit")

Anyway, is more likely your MSX have an bad encoder IC than an bad VDP. Or any faulty capacitor in between those two.

If you're planning to mod your MSX to get component video directly from VDP, I recommend you studying both HX-10 schematics and TMS99x8A datasheet, because you need some small circuitry after the VDP pins that could be already on the HX-10 mainboard (like my Sharp/Epcom HB-8000). But a friend of mine told me a sec. ago that some modern TV sets cannot handle component video with less than 480i lines... Still, it's worth a try. (just try don't shorting anything on your VDP! Big smile)

By RetroTechie

Paragon (1563)

RetroTechie's picture

13-07-2012, 03:30

What HX-10 schematics? Question LOL!

By checking where pins 35 and 38 of the VDP go, it should be easy to tell TMS9918A and TMS9928/9 apart. But I bet it's one of the latter, since with a TMS9918A that PAL encoder IC would have no function.

If a simple component replacement doesn't do it, designing / adding some other circuit to pull video from this MSX1, is not worth the trouble IMHO.

By PingPong

Prophet (3759)

PingPong's picture

13-07-2012, 08:34

Rink wrote:

Here's a photo - not that it helps with anything mind.

Hi, from the photo, in the right-bottom part i see clearly a chip labeled "GIxxxxxx" this is the PSG.Above this there is a toshiba labeled IC that i suspect is the infamous "VDP clone" by toshiba. If this is true, you must find the correct datasheet for this VDP, that is different from the Texas Versions. (I'm not sure however)

By Rink

Rookie (25)

Rink's picture

13-07-2012, 09:50

Morning,

Quote:

But a friend of mine told me a sec. ago that some modern TV sets cannot handle component video with less than 480i lines...

Ah of course - I thought things were sounding too easy for a second there. Smile

PingPong wrote:

i suspect is the infamous "VDP clone" by toshiba. If this is true, you must find the correct datasheet for this VDP, that is different from the Texas Versions. (I'm not sure however)

I'm not sure to be honest (but I'll check). The original photo I took isn't big enough to zoom in so will take a peek at the chip when I get home. Everything I've read says that Toshiba used their own VDP on the HX-20/22 but a Texas TMS9929A on the HX-10.

@Retro - you're probably right about it not being worth the effort. But no doubt I'll end up taking this on anyway... :D

By Rink

Rookie (25)

Rink's picture

13-07-2012, 09:56

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

By Jipe

Paragon (1502)

Jipe's picture

13-07-2012, 11:06

just a idea : check if RCA of video signal is ok !!

By RetroTechie

Paragon (1563)

RetroTechie's picture

13-07-2012, 12:17

Google image search to the rescue-
Top side of the board (large!)
Bottom side (we don't see photos of that too often)

-> That 64-pin IC (TCX-1007) is some sort of custom IC / MSX engine. I'm guessing 'S3527 minus the PSG' (note absence of a 8255 on the board). Videochip is very likely a TMS9929 (40-pin, on the left, cooling plate with marking stripe on top). Video RAM in lower left corner (the 4116's), main RAM next to that (the ...64 chips).

I'd have a good look at where the PAL1 (PALI ?) connections come from on the main board, to make sure what's what & that no supply voltage was overlooked. Shouldn't be too hard (ground, Y/R-Y/B-Y from videochip, and +5V you said? Leaves just 1 pin).

That 2-wire connector from video board is probably 1 ground wire (verify!) and outgoing composite video signal. Likely buffered - coming from one of the transistors, perhaps through a series capacitor.

By Rink

Rookie (25)

Rink's picture

13-07-2012, 12:32

Wow - great find with those images. Thank you.

I'll get back in there with the muiltimeter this evening and see if I can verify some of those connections.

Cheers mate.

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