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

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By Chart2msxtr

Resident (63)

Chart2msxtr's picture

02-01-2018, 13:38

Hello good day
I would like to rescue a Sony HB-75P computer, I tell you the symptoms you have and the checks carried out,
the symptoms is that when the on switch is activated the computer stays in black with the Power led and caps on and the relay of the remote is activated, I have checked voltages in the ic 4116 vram and they are all correct, all the fuses are well and also the tension of the z80, ipp, it seems that the feed issue is correct, could be some integrated, is there a way to check them ?, if you can help me I would appreciate it, I could buy another computer, but it is a matter of trying to fix it .

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By Alexey

Guardian (2349)

Alexey's picture

02-01-2018, 17:27

Check the slot pins for possible short circuit. In my experience a click of the relay usually means some problems with the shorting of data or address lines.

By djh1697

Paragon (1409)

djh1697's picture

02-01-2018, 17:44

Not to sound stupid, but does pressing caps lock do anything? If the light goes on/off your video output might be the issue? I am no longer a technician, at one time i might have tried to repair it, but that is 20 years ago.

By Chart2msxtr

Resident (63)

Chart2msxtr's picture

03-01-2018, 10:28

hello thank you very much for answering, regarding the slots, I have checked it and I do not see anything that could be shorting inside the connector, I have also checked the tracks and I have not seen anything, if I press the reset button the relay will be released. remote of the tape and the caps, but nothing else loosen it is activated again, with respect to the button of caps it does not let me do anything stays with the light activated but it does not change if I pulse it, tension problem can be? I checked all the integrated and the indicated voltage, if it were a problem of capacitors the tension would be correct ?, is there any way to check integrated ?, If I follow the relay of the tape this goes to the integrated pherimetal peripherals and also comes Vram memory, but this is not how it is checked, another thing I've seen is, placing a cable rgb to the TV and give tension the TV is black, but I get the symbol that is in RGB, that means that the rgb signal is giving the vdp ?, I could find another computer but I would like to try to repair it by learning a bit, but the truth is that it is quite complicated.

thank you very much

By Alexey

Guardian (2349)

Alexey's picture

03-01-2018, 10:38

I would start the repair by socketing the RAM and trying different RAM chips. In my HB-75P the problem was in those chips. Do you have an oscilloscope? It's much easier to diagnose the faults using this device.

By Chart2msxtr

Resident (63)

Chart2msxtr's picture

03-01-2018, 11:09

Hello

I do not have an oscilloscope, the integrated ones of the ram refer to those of the VDP? in my case they are the 8 that there is just below, in case if I look for them and I will change them, and if there is luck, I have seen them by ebay and they are not very expensive.

regards

By Chart2msxtr

Resident (63)

Chart2msxtr's picture

03-01-2018, 11:11

Hello

One thing, referring to the integrated RAM, as I have seen in another post to replace them, when I turn on the computer I check and heat them, more or less all by the same, these integrated usually take a lot of temperature?

Thank you

By Alexey

Guardian (2349)

Alexey's picture

03-01-2018, 15:43

Heating RAM chips may not be a good idea, especially such old ones. When desoldering RAM, you should heat the other side of the board.

By Chart2msxtr

Resident (63)

Chart2msxtr's picture

01-02-2018, 22:02

Hello

I have been doing some tests, I have desoldado all the memories vram and I have placed sockets, I have connected the tension without the memories and the computer keeps doing the same, the led of cap is left on and the relay of the datacorder is activated, I understand that if it does the same thing, it is not in memory, it can be some capacitor that even if it has the correct voltages in the integrated ones causing a short circuit.

By Tulac

Supporter (1)

Tulac's picture

11-05-2019, 20:41

Hello,

I'm also new to this forum - but with same issue on my Sony HB-75D. I've cleaned the machine and visually checked all components. Powering the machine, the screen stays black - the caps lock led is always on.

I've detached the keyboard from the mainboard to check, if the keyboard has an issue. Does the MSX boot up without connected keyboard anyway?

I've noticed the 8 RAM ICs are getting very hot, so should be a RAM issue?

Kind Regards

By Alexey

Guardian (2349)

Alexey's picture

12-05-2019, 11:03

Yes, MSX should boot without a keyboard. Start by checking the voltages and clocks. Socketing RAM is a good idea too.

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