Sony hb 75p freezes

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By AndreV

Expert (106)

AndreV's picture

14-11-2020, 12:35

Hi,
I've got a sony hb 75p last week.
After a couple of minutes the computer freezes or resets it self. The image is good,sound is good. cartridge works fine, games load perfectly with a data recorder. the pcb looks like new. Keyboard works good actually the whole computer looks like new. no strange noises, no relais klikking random. the computer reacts normal.
I only have a solder iron and a multimeter. I think that the computer reaches a certain temperature and some solder points get lose? I am not a electrician and can't read a scheme. Is there somebody who had the same issue,and has a simple solution?, fore examle: threw it out of the window, pick it up, connect it, and look, it works fine again Smile

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By gdx

Prophet (4043)

gdx's picture

14-11-2020, 14:15

Check if the power supply voltages are properly rectified. The capacitors may be defective.
Otherwise, there may be a false contact and the computer crashes at the slightest shock.

By AndreV

Expert (106)

AndreV's picture

14-11-2020, 15:17

By AndreV

Expert (106)

AndreV's picture

14-11-2020, 15:19

By AndreV

Expert (106)

AndreV's picture

14-11-2020, 15:21

thanks fore helping, which capacitor can i check?

By AndreV

Expert (106)

AndreV's picture

14-11-2020, 15:26

gdx wrote:

Check if the power supply voltages are properly rectified. The capacitors may be defective.
Otherwise, there may be a false contact and the computer crashes at the slightest shock.

I think i can rule out the slightest shock, it happens all the time a the exact same moment,

By Wild_Penguin

Hero (623)

Wild_Penguin's picture

15-11-2020, 18:35

As already requested, measure the +5V, +12V DC lines (at least; also -12V, and -5V if the 75P has it, but IIRC it does not have it - this one does have the SM linked in the wiki).

An MSX can work apparently fine with some overvoltage, but "reset/hang" when an overshoot/momentary short (at an capacitor? some other component?) happens because of the overvoltage. I've had this kind of problem. (The "overshoot" I've described is just a guess; I'm not actually sure how a computer circuit fails if there is overvoltage. Without these "apparent" errors an overvoltage is sure taxing on components and reducing their lifespan!).

Now you've measured the mains voltage, and it is not surprising it is 227V. If it wasn't, you've got bigger electrical problems than your MSX.

By Wild_Penguin

Hero (623)

Wild_Penguin's picture

15-11-2020, 18:37

The capacitors on the small PCBs of the +5 and +12V rectifiers circuits do at least sometimes fail. Mine were practically exploded when I got my HB-75P (curiously, it still worked without any issues, despite exlpoded capacitors - I tried it when I got it since it was sold as "tested").

By AndreV

Expert (106)

AndreV's picture

15-11-2020, 21:23

I've i understand correctly i have to mesure this (picture), and it has to have a value of 12 volt or 5 volt.

By AndreV

Expert (106)

AndreV's picture

16-11-2020, 08:36

mesured this so far.

By gdx

Prophet (4043)

gdx's picture

16-11-2020, 10:41

Look the service manual (chapter 5.23) for the voltages.

https://hansotten.file-hunter.com/uploads/files/sonyhp5575sm...

Also note that a voltmeter doesn't indicate if voltages are properly rectified.

To check capacitors:
https://www.youtube.com/watch?v=qDABYKoVO4Q

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