sony problem hb-75p does not start and remains black screen

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By Nprod

Expert (100)

Nprod's picture

14-05-2019, 22:03

I had a black screen issue with an HB-75 and i started by socketing of the BIOS and Personal Databank ROMs and cleaning the traces which had a lot of crusty rosin residue around the pins. After this it booted just fine, so it was probably a bad joint somewhere. I've also seen reports on this forum about traces "rotting" in that area - they would turn black from (possibly) moisture getting under the soldermask layer.

Do all of the RAM ICs get hot or is it just specific ones? Have you tried re-seating the two ROMs (if they're socketed)?

By

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Anonymous's picture

17-06-2020, 10:13

Hi,
I'm new in this forum :-). I'm trying to fix my Sony HB-75F which was my first computer. I really love it and if I can fix the black screen, I would be very happy.
I already had perform an internal cleaning.
When I press the PowerOn button, my monitor stay black and tell me there is a monochrome signal. When I press the Caps, I can switch On/Off the red led of this button. When I press Reset, I can see it rebooting and it give me a new black screen. Some keys give me sound on the screen. By reading the begining of this post, I have also verified any shortcut in the rom slot.
If I understand well, the first thing to do is verify the tension inside the computer ? and if the tension are good, the main suspect is the 8 RAM ICs ?
I recently watched a video on youtube (Noel 's retro lab) and the guy have the complete schematics of the computer. I tried to find the same information for my msx model without success, where can I find this kind of ressource ?
thanks for help
(sorry for my poor english).

By Grauw

Ascended (10684)

Grauw's picture

17-06-2020, 11:15

The Sony HB-75 wiki page has a service manual link at the bottom.

If the system boots and caps / key click works, it sounds to me like the CPU and main RAM are okay. The key click and caps indicate that the system has booted and is running, and if the CPU or RAM or ROM is bad this won’t be the case.

A problem with VRAM would also typically not result in a black screen. VRAM issues are normally quite recognisable from specific patterns on the screen. Even if all the VRAM ICs would have failed, you should still see a white-on-blue screen. So I do not think the VRAM is bad either.

The issue sounds like more of a problem with either the video encoding circuitry or the video chip. Is your HB-75F connected via RGB SCART? I’m not sure if the French version has a composite video output, but if it does then try that, see if you get a different result.

An additional test:

  1. Turn it on and wait a little for the internal firmware to start
  2. Press the DOWN arrow key four times and press ENTER to go to BASIC. Each key press should cause a key click.
  3. Type BEEP and press ENTER.
  4. If you hear a beep sound, then CPU, RAM, ROM and VRAM are definitely okay.

p.s. Noel’s retro lab is a nice channel, I’ve watched all his videos! :)

By Grauw

Ascended (10684)

Grauw's picture

17-06-2020, 11:21

cfm59 wrote:

When I press Reset, I can see it rebooting and it give me a new black screen.

What do you mean exactly by “you see it rebooting”? Does the screen turn blue for a while and then black? Are any characters visible at any time?

When you start the system it first shows the boot message on a blue background, and then goes into the firmware menu, which is on a black background. So if the screen colour visibly changes, this is important information.

By cfm59

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17-06-2020, 11:55

Thank you for the advises, I downloaded the service manual.

By the way, I tryed your procedure and I heard the BEEP (I'm so happy, he is alive) !

By "I can see it booting", I just wanted to say the monitor return in scan mode to find a source and finally give me the black screen. There is not any boot message or something like that.

I tryed the msx both on my TV (with SCART to SCART wire and) and on a PC monitor with a SCART to RCA wire (with this kind of adapter https://www.media-rdc.com/medias/5ee4e8014f08387d89825a8db7f...).

I'm not sure the SCART wire I used on my TV was the original one.

To try the composite output, I need a SCART to composite wire / converter, right ?

By Grauw

Ascended (10684)

Grauw's picture

17-06-2020, 14:13

No, the SCART to RCA adapter is equivalent to composite. SCART includes composite on its pins.

About your SCART to SCART cable, could you double check with a multi meter whether the RGB and composite / sync pins are wired up properly? Just to eliminate that possibility. The pinout can be found here.

By cfm59

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17-06-2020, 18:00

It Works !

Thank you so so much Grauw ! I have tears in my eyes. This computer make me what I'm now (a sysadmin). My parents bought it for christmas when I was 9 years old (37 years ago).

Finally, I used a SCART to HDMI converter which is composite enable. I tried the same yesterday but it was the first time I used it so I didn't insist enought with buttons.

By Grauw

Ascended (10684)

Grauw's picture

17-06-2020, 18:29

Great, congratulations!

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