Video failures on a HB-55P?

By UFO

Supporter (8)

UFO's picture

18-10-2020, 19:15

Cheers!

After a long time without the possibility of enjoying some retrotinkering, life has given me some spare time and I'm trying to fix some device I have laying around.

I am currently dealing with a HB-55P which shows some video problems. The computer is PAL and it is connected to a TV/monitor Hannspree T191H. When connected to my main TV in the living room, it shows same behaviour.

To sum up:

- The problem is it displays the image on B/W, both on RCA and RF. I have ordered a new cable, just in case.

- When I turn from PAL to SECAM format on my TV, the image starts shaking and alternating between B/W and colour, like shown:

https://imgur.com/GjutKTU

  • The computer boots and works correctly, without any noise or smell.
  • It loads cartridges properly.
  • I have checked the board and it does not show problems with caps, cold solders, rust, etc. It is quite clean.
  • I have checked voltages on the top board, where the video connectors are, and I have seen that the 5V regulator gives correct values but the 12V one gives 16V. If you look at the picture, the connector labeled as CN6, coming from the power supply, has 4V on the 5V points and 0V on the 12V points.

  • RAM, VRAM, TMS9929 and Z80 give correct voltages.
  • I have checked that VRAM heats a lot, as well as the TMS9929.
  • If I turn on the computer it only shows B/W image but, after some time and heating up, image starts shaking and alternating between B/W and colour.

What is happening here? Could you please give me a hand, so I can fix this thingy? :)

Thanks in advance for your answers! :)

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By bsittler

Expert (72)

bsittler's picture

19-10-2020, 01:20

I am not an expert but it looks to me like a problem in the PAL color subcarrier signal, perhaps a crystal or a capacitor has developed a problem, or perhaps the video encoder chip (YUV-to-composite converter) is dying somehow. Is there a crystal whose frequency is 4.43361875 MHz? That should be the colorburst. It could even be a failed solder joint given the change in behavior after sustained application of power, so reflowing joints or resoldering components in the YUV-to-PAL section could solve it

Also you could consider tapping those YUV/YPbPr signals directly and adding a component-to-RGB encoder to get a higher quality (sharper) color video signal than PAL though this would also typically require a new connector and SCART cable and of course a television capable of handling RGB SCART

By UFO

Supporter (8)

UFO's picture

21-10-2020, 10:45

Thank you very much for your answer, bsittler! I will check some caps around the VDP. Smile