Sharp X68000 Expert CZ603C-bk repaired but defective again, I need help.

[motorola73]

Hi.

I am taking advantage of the quarantine to resurrect my Sharp X68000 Expert CZ603C-bk (+ 2mb memory ram added). Before disassembling it, it gave me a black screen, after disassembling it, and connected to the Trust monitor that switches to 15hkz, the boot screen reappeared with the hand holding the disk. I also used the Master Disk. After reassembling it, I tried it on another LCD, and when I connected the vga cable, I felt like a hit of electric current on my hand, and the TV turned off. Now it gives me again the problem of the black screen also connected to the Trust monitor at 15 khz.
The card where there is the rfu and the rgb, looks good (measured capacitors and they all give the same voltage).
The floppy works and the Master Disk loads it (I hear the floppy turn until loading is complete).
I remember I'm trying the TV_CTRL and LCD_MODE settings when I was tweaking the Master Disk.
Could it be that the black screen is caused by the bios configuration?
Is there a way to reset the bios in factory default?


I have already tried removing the battery.

Thanks.

https://streamable.com/hlat8q

[motorola73]

... this before the black screen defect happened again:
https://ibb.co/BnTYy4f

This Japanese writing indicating an error, I solved it by setting the ram switch, after which the classic regular start screen reappeared.

And these are all the detailed components of my Sharp x68k expert:

https://ibb.co/qJK8wXw
https://ibb.co/W0XrT2b

[Jehuty]

If you remove the battery for 2-3 minutes the sram should be cleared.
Are all voltages ok ?
I had an issue where the 5V had a fail (conector broken on plug), but 5VSB was fine - when i messured on the mainboard all pins had 5V.
But those came backwards from 5VSB and not from PSU 5V.
In this situation i also got a black screen and the x68000 powered on and loads. In my case i tried Alien Syndrome - this Game loads until the startscreen and there i suddenly got a picture.

And also check the connectors between all pcb. And check without scsi and ram in I/O.

[motorola73]

Thanks for the info.  Meanwhile, I noticed a 100mf 16v capacitor in position c410 (in the rfu box) that was not going well, and also the contact was oxidized.  I had to pass the sandpaper on the track.  But I don't have a spare.  Alternatively, which similar capacitors can I resolder?

https://ibb.co/jhBwFKh

[kamiboy]

You can replace with 100uF >=16V. I don't remember what the TV_CTRL and LCD_MODE options do, but I am fairly certain they have nothing to do with anything that can cause you to lose picture. My guess is some component on your machine has failed. Perhaps an IC related to video output which is known to fail when using LCD monitors with the X68000 goes wrong. The jolt you felt might have something to do with it.

Do a search, that IC has been discussed here before, find it and try replacing it, might help.

[motorola73]

I pulled a 100 mf capacitor from a defective motherboard and replaced it.
But I have discovered another particular event.  When I turn on the sharp x68k with the monitor off, it works.  When I turn on the monitor, the sharp x68k turns off!

This is the cable that makes me take the electric shock:

https://ibb.co/80MgRY0
https://ibb.co/5xCxZ2D

Furthermore, it seems that the system is in protection: if I turn the transformer switch off and immediately turn it on, the computer will not turn on. If I leave it off for an hour, then it turns on regularly!

[kamiboy]

Something is wrong with your cable. I suggest you stop using it. Prolly a short. I suggest you throw it away and buy a proper db15 to VGA cable for your machine. You can find them on yahoo auctions. Don't use one meant for MAC PC's as they use a different pinout.

[motorola73]

Hi again. The fact is that the Trust svga monitor is the only one that supports 15khz, I can only test it with that. When I was able to fix the Sharp x68k for the first time, it displayed correctly, and I also used it a lot with that monitor (before I had one of the Sony NTSC, spoiled with time). Meanwhile, after removing the screws that hold the svga cable in place, it no longer goes short, and both the monitor and the computer turn on (even if the ignition is delayed and does not display).
PS: the Sharp x68k psu is connected to the 220v-110v converter because I'm in Italy and the electricity supply is 220v).

[Jehuty]

Wait, you´re writing something About 15khz to test. But the X68000 Outputs 31khz from the bootscreen until booting into the Masterdisk. Only some games switch to 15khz.
Check your VGA Adapter. You Need Red, Green, Blue, H and V Sync and GND, Nothing more.

I had a Problem some times ago that my X68000 won´t power on when a Display was connected. If i unplug the monitor the x68000 powers on and boots games (Hearing the Sound).
Pluged in the montitor cabel and the X68000 powered off immediatley.

It was a the worn PSU so i replaced it with an ATX one and all Problems gone.
I am in Germany and have 230V like Italy - so with my ATX solution i don´t Need a stepdown converter anymore.

Check the VGA cable and the Voltages.

[motorola73]

Thanks for the info. If you saw the photos, my Sharp x68000 displayed start screen on the same Trust monitor and with the same cable. From the photos of the cable, it looks good too (there are no burns and the welds seem ok).
Based on what you told me, my Sharp x68000, once fixed must display the start screen also connected to the 31khz pal monitors (maybe in black and white).
In the rfu rgb box (the one that connects separately on the motherboard through another internal plug) I replaced a 100 uf 16v capacitor. The rest looks good. The motherboard chips do not particularly burn and remain at a regular temperature.
Maybe I have to take a better look at the base board placed below (that it is also full of capacitors) where the keyboard and mouse stick, the one where the red power button is located?

[motorola73]

I did another experiment: I replaced the 470uf 180v large capacitor with a 470uf 200v capacitor (as in the photo):

https://ibb.co/HYhT04s

But no positive results. At the beginning it turned on, but as usual it did not display the boot screen to the monitor. After a few minutes it went out (little by little). Tomorrow I will try to replace the phase and earth electrical wires with the Italian plug, to attach the power supply to the 220 volr wall socket directly. I think nothing serious should happen since the transformer now has a 470 uf  at "220v" capacitor.

[kamiboy]

You cannot just replace a capacitor and expect the PSU to be able to handle a 220V source. If I remember right those high voltage capacitors need a multiple of the input voltage in order to not blow up, so at least 400V I reckon, instead of 200. In addition all other high voltage side components of that PSU are designed with 100V in mind and cannot handle 220V. If you plug that thing in directly at best you will blow the fuse, at worst you will fry your PSU, along with the PC perhaps, so don't. 

[motorola73]

You cannot just replace a capacitor and expect the PSU to be able to handle a 220V source. ... 

Thank goodness I read the answer before I started replacing the wires, and yesterday I didn't set the alarm to start the job. Okay, I won't. At most I will procure the same new capacitors and components to repair the power supply, without any modify. I think that the second problem, the one that the x68k no longer displays, is also a question of power, that is, that the rfu and rgb box is not powered well. With your answer you have prevented me from doing irreversible damage. Many thanks for info.

[Jehuty]

You should use a ATX PSU unstead of the original one. I did the modification on my X68000 and X68000 XVI so i can plug both direct into 230V. Softpower works still fine. You only need a litte 74er IC.
Tut is on Gamesx.

Did you measure the 5V on the psu with unpluged Mainboard ?

[motorola73]

I have measured all the outputs of the primary motherboard carefully and these seem ok. The measurements of the transformer must be made carefully again (however the capacitor I replaced is very similar to the original, it is only 20 volts more (470 uf 200v, original = 470 uf 180 volts), therefore it should not be necessary to replace it again because of the risk to wear down the tracks on the transformer plate).

[Jehuty]

Did you measure on the Motherboard ?
You must measure without the board pluged to the psu because the 5VSB are backwards to 5V rail.
So when you measure 5V it coulb be that this is the 5VSB

[kamiboy]

You can replace caps with ones of higher voltage just fine. In fact it is better as they will last longer.

Cap replacement rule of thumb is same capacitance (uF) same or higher voltage.

And yeah, with your problems I would look to getting a replacement PSU. The power supply on these machines are very flaky.