Panasonic FS-A1 Mk II - no video signal

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By Alexey

Guardian (3163)

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31-05-2015, 22:42

If you get a stable 12v, no need to change anything. Also check other voltages: -12v, 5v and -5v (if there is). If all voltages are ok, then the problem is in one of the video components. I am not a professional electronics engineer, so I can't help with any piece of advice. I would probably check the RGB signals and Csync generation with an oscilloscope. This may give some idea where to look next.

Alexey

By Alexey

Guardian (3163)

Alexey's picture

01-06-2015, 16:09

If you give up repairing it, don't recycle your computer. I would gladly buy it from you.

Alexey

By sabre470

Resident (35)

sabre470's picture

01-06-2015, 18:59

Thanks for your help, Alexey, if I do, you'll be the first to know Wink

I really want it to work though Smile

By RetroTechie

Paragon (1563)

RetroTechie's picture

01-06-2015, 22:08

If +5V and +12V are both present, computer is running & producing sound, and you still don't get an image even through RGB, there's not too many options left:

  • VDP broken. R/G/B not likely as that would cause a "missing color" problem. Totally dead also not likely, as it generates interrupts, so without that cartridges wouldn't run normal / produce sound etc (I think). So maybe CSYNC output?
  • Maybe some transistor buffer in sync signal between VDP and RGB connector is blown?
  • Or things are actually working but the cable you're using now, isn't doing its job. For example, the TV/monitor on the other end gets 12V on one of its pins, correct?
  • VRAM chips broken so badly that VDP doesn't manage to produce any image whatsoever. Again: unlikely, bad VRAM chips tend to cause image corruption not complete 'blackout'.

Checking the V9938 datasheet, I see it has a composite video signal on pin 21 (that would be a color signal, I think). You could try hooking that up to a TV's "AV" (RCA) input directly. That is: without any buffer parts / transistors or whatever in between. Simply as a quick check of "is VDP producing image?".

If that gives some kind of image, you're looking at a blown transistor in CSYNC output, cable problem or similar. If that produces no detectable image, probably a dead VDP.

By sabre470

Resident (35)

sabre470's picture

01-06-2015, 23:54

Thank you very much RetroTechie, that's extremely valuable information Smile

I will try all that Smile

The bad cable is one of my theories too, the machine didn't come with one. I've built an RGB21 cable but I don't think it's complete, nothing is coming out of it. I'm not all too familiar with RGB21, living in PAL land means I never had to worry about it much. I found diagrams on how to make one, I'm pretty sure I've got the cables in the right spots but I couldn't find what exact components I need to solder on top of the cables on the scart output plug.

Recently, I've built a new RGB cable for my other MSX a Yamaha YIS-503F that works perfect on that machine with fake stereo sound and all. On the FS-A1 on RGB & Composite I get sound but no image as you know, I run everything through an XRGB Framemeister, maybe the video signal is too faint for it but I doubt.

Could I make an RGB cable for the FS-A1 or do I have to stick to RGB21?

I will try the VDP trick, as soon as possible. I have an composite in on my monitor so will bypass the XRGB just to be sure.

Another oddity I found while fixing this machine is that before I changed the caps on the IC board, I could hear the music from the DESKPAC firmware playing albeit very faint. With the new caps I can't hear it anymore so I guess DESKPAC firmware is not loading by default anymore, without an image it's difficult to gauge what's happening. But hey so far I can play F1-Spirit with just sound but I do crash a lot but I still get the SCC goodness.

By l_oliveira

Hero (531)

l_oliveira's picture

02-06-2015, 19:25

The little board is the VIDEO and AUDIO processing (video encoder). The leaked capacitors corroded the traces away and broke the circuit. That's why you have no video.

It has nothing to do with VDP being bad. Please don't ruin the computer. The only part you should be messing with is the video/audio daughterboard.

The encoder chip in it is a SONY CXA1145. It has a RGB clamping circuit which requires the sync signal from the VDP to work correctly. If the encoder chip receives no sync it will short circuit all the output lines to gnd (aka clamp) and it will make appear that the VDP is bad when it is not.

By sabre470

Resident (35)

sabre470's picture

04-06-2015, 14:22

Thank you very much l_oliveira, not too worry I won't ruin it Smile I won't have time until next week to look at it properly. I had a close look with a magnifier and couldn't see anything obvious. I'll share some pictures later.

By sabre470

Resident (35)

sabre470's picture

21-06-2015, 00:53

Hello Folks,

Had another pass today, checked and tested all the traces at risk of acid on the hybrid board. Still no image Sad Check the cart slots they both work fine.

MSX 1 & 2 games are working on either slot, keyboard is responsive but no image Sad

I might try to change the VDP to rule out any problems with it, if only I could find a new hybrid board, my options are running out.

By l_oliveira

Hero (531)

l_oliveira's picture

21-06-2015, 01:30

There's 0% chance your VDP is faulty.

Both composite video and RGB come from that board so if it's faulty you will have no video at all.

RGB is not straight from VDP. It goes through the SONY chip.

By Alexey

Guardian (3163)

Alexey's picture

21-06-2015, 02:35

Here's the CXA1145 document:
http://www.aliot.com.ua/pdf/cxa1145.pdf

And here's the implementation of this chip in FS-A1WSX MSX2+ (page 5):
http://www.msxpro.com/hardware/esquemas/a1wxsm.zip

So what I would check first:
1. Power supply of +5v on CXA pins 12 and 19
2. Proper ground on CXA pins 1 and 24
3. Selector PAL/NTSC (+5 or ground) on CXA pin 7
4. Oscillation of 3.58 mHz on CXA pin 6
5. Csync input signal on CXA pin 10
6. Csync output signal on CXA pin 11

The first 3 may be checked with multimeter, the rest require oscilloscope.

Also you might want to try to get a composite video from CXA pin 20 through 220uF or 470uF capacitor and 75 oHm resistor to your TV or monitor's video input. If you don't get the composite video, the chip may be busted.

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