Toshiba HX-10 Black Screen: What To Check?

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By Rink

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11-07-2012, 21:25

Hmmm... I've taken a look inside and can't see anything suspicious. In fact, it's probably the cleanest PCB and assembly I've ever seen. No sign of any oxidation or loose chips (the LVA-510 is soldered in, not socketed and it's PCB looks fine too).

The LVA-510 is on a separate board which connects by two cables PAL1 and PAL2. Unplugging those makes no difference (I didn't expect it to make things better but maybe no signal at all). One of the pins on PAL1 is supplying 5v, the others seem a little more variable. Not sure exactly what this does since the RF box is at the other end of the board. Not sure how to test that.

The power supply seems to be pumping out about 18v ish (my multimeter struggled a little here - AC?) on the +-12 connector. Found 5V and 12V/-12V on the two voltage regulator looking things TR4 and TR6.

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

By Rink

Rookie (25)

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11-07-2012, 21:48

Interestingly, I was just fiddling with the tv's tuning to see if the Ctrl+G beep would be picked up through the RF connection (it works from the audio plug) and it doesn't... But when tuned out slightly, you can see a change in noise patterns when that key combo is pressed.

Not sure if this means anything.

By Rink

Rookie (25)

Rink's picture

12-07-2012, 00:44

So... It seems the extra board with the LVA-510 on is a circuit to convert RBY to CVBS and send it back via the two PAL2 wires for the composite video output. I'm guessing that with a faulty card there, or no card in at all, the RF output should still work? The fact that it's a black screen on both outputs that suggests to me that the problem is somewhere *before* the video output.

Not sure what that would be though - some kind of display processor or video RAM maybe?

Does this sound right to you all?

By RetroTechie

Paragon (1563)

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12-07-2012, 04:58

Most MSX1's use a videochip that has YUV output (or something like that). This goes to video encoder IC (like this LVA510) -> composite video signal. That goes to RF modulator -> UHF TV signal.

So with a dead video encoder IC, image would be lost both on CVBS and RF outputs. 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).

By Rink

Rookie (25)

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12-07-2012, 09:51

Ah yes. That makes sense - it was late when I was looking at this stuff, that's my only excuse. Smile

So it definitely looks like something wrong with that PAL encoder circuit. I can replace the IC (when the new one arrives) but there's a shit load of capacitors on that board that I don't fancy replacing. Lol. Suppose I should first check the usual things: like whether anything is coming out of the circuit through the PAL2 wire; that the IC is receiving it's power etc. etc.

By Rink

Rookie (25)

Rink's picture

12-07-2012, 11:30

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?

By Jipe

Paragon (1517)

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12-07-2012, 11:37

have you a multimeter ? you can check voltage on R-Y B-Y and Y signal

By Rink

Rookie (25)

Rink's picture

12-07-2012, 11:43

I do have a multimeter and need to check the R-Y B-Y and Y as soon as I can. I didn't think of it last night unfortunately - though to be honest, I'm not so hot on video circuitry so it wouldn't have occurred to me until I started trying to learn a bit more.

R-Y,B-Y,Y signals are compononent component (LOL) video, right?
I'm wondering if I could use something like this -

as a test to by-pass the need for a PAL encoder altogether???

By Jipe

Paragon (1517)

Jipe's picture

12-07-2012, 11:47

look your model of VDP TMS99xx
after search a schematic with the same model because i don't found HX-10
see on schematic the pinout of vdp and test the signals

look also the VRAM model

i think the encoder no directly plugging on the VDP

By RetroTechie

Paragon (1563)

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12-07-2012, 19:07

Y/B-Y/R-Y and Y/Pb/Pr are probably not exactly (?) the same Wikipedia says that YPbPr and Y/R-Y/B-Y are the same thing: a brightness signal (Y) that includes vert+hor. sync (= monochrome composite video), and 2 color difference signals. Or a complete composite video signal in the case of a TMS9918A. What VDP is in that HX-10?

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.

Likewise one could bypass the video encoder IC, and route this Y signal to where the video encoder 'injects' its composite video signal. Which would be difficult since I've never seen either a schematic of the HX-10, or a datasheet (or even a pinout) of this LVA510.

Given the symptoms so far, and the fact that you have a replacement coming up, I'd just go ahead and replace this IC, then investigate further if that doesn't fix the problem. But in case the problem is elsewhere, it might be wise to not destroy / overheat the old IC.

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