May have just blown my FX1DJ & A1F - looking to repair. Suggestions?

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By gdx

Prophet (3037)

gdx's picture

20-10-2019, 16:34

MSX_User wrote:

I opened the F1XDJ to see if there was any obvious burning, but all *seemed* clear. I'm stumped. Both computers were confirmed working before the shipment.

When I connected a tuner output to the antenna output of my TV, I heard a snap and smoke came out. I opened it to see if there was an obvious burning, but all seem normal. Even fuses are OK.

By MSX_User

Supporter (10)

MSX_User's picture

20-10-2019, 22:41

sd_snatcher wrote:
Quote:

Would plugging a Japanese unit into a North American mains cause this behavior on two different computers, though?

No. People have been using Japanese MSX computers in Brazil (127V) for more than 30 years without any problem. Only the models from Toshiba are known to be picky about the voltage. The two models you're using are well known to take up to 127V without blinking an eye.

In the case of Toshiba machines, then yes you MUST use a step-down converter. Their power supply is poorly designed and gets hot even when used at perfectly normalised voltages. Check this part of the thread for some recommendations on how to improve their power supply.

I'm not doubting the ability to do so. I'm sure people have done it for years. This is why I find it odd that two working computers were used without a step-down, made weird sounds, smelled like burning electronics, and now no longer work properly. I still have to go through and test rails. Update soon...

By MSX_User

Supporter (10)

MSX_User's picture

22-10-2019, 05:16

Ok, so I tried to test for power on the connector near C91 & D10 on the F1XDJ and it looks like I was only able to get 5v. I'm not positive I had my voltmeter on the correct setting (it's old and worn) or if the connector even had the power rails I was looking for (no schematics atm). I did make something of a discovery - the buzzing was coming from the floppy drive and when the buzzing occurs, IC 903 (SN74LS00N) near the center of the motherboard gets EXTREMELY HOT. So hot in fact, that's where the smoke was coming from.

I bumped the machine when I touched the IC (burned my finger) and the buzzing stopped. After a few minutes, the IC cooled down.

So, it would seem as though bumping the machine both helps and worsens the issue. I'm going to assume IC 903 is toast. I'm not sure why the floppy drive was screaming in pain, but it's all clearly related. Beyond all of this, the caps look good and I'm not seeing any corrosion.

Any suggestions on where to go from here?

By Alexey

Guardian (2387)

Alexey's picture

22-10-2019, 13:49

Disconnect the floppy, disassemble it and find out what the problem it was. Do not connect the drive again until you figure this out.

Does the machine show signs of life after disconnecting the floppy (both power and interface cables)? Replacing the LS00 could be a good idea, taking into account what you said about it.

You can check the voltages on the cartridge slot. The first two leftmost pins closer to you are -12v and +12v. The 2nd and 3rd pins on the opposite (still left) side of the slot are +5v. Take the ground for measuring from the board. Be careful when measuring - do not connect adjacent or the opposite pins to each other.

By MSX_User

Supporter (10)

MSX_User's picture

22-10-2019, 16:45

I forgot to mention I removed the floppy drive and there were still no signs of life when powering the F1XDJ on without it. I'm going to assume the drive isn't the problem here. Instead, I believe it is falling victim to something else, causing it to buzz loudly, but until I dig deeper it's all hypothesis anyway. Wink

A quick search isn't showing anything in regards to schematics for the F1XDJ. Are these available anywhere?

I will test the other power rails tonight.

By MSX_User

Supporter (10)

MSX_User's picture

23-10-2019, 03:26

It looks like I'm getting the following:

5v
12.40v
-11.81v

By MSX_User

Supporter (10)

MSX_User's picture

23-10-2019, 04:08

Ok, so I took the drive apart and it looks fine, except for the belt being completely disintegrated Wink I'll have to buy a replacement. I did make something of a discovery though: the top two wires on connector C902 (on the brown board near the middle of the motherboard) were completely detached. These were literally floating around and likely sending a power surge to anything that touched it! It's not outside the realm of possibility these were a cold solder joint and came loose during shipping. This would explain why IC903 was getting hot to the point of smoking when bumped.

So the next steps are:

- figure out how to remove the motherboard completely (I can't seem to remove it from the case - any pointers?)
- determine where to resolder the wires
- see if IC902 is needed on the brown PCB (mine is missing, but it looks stock and I'm seeing images of it both populated and missing)

Best case scenario is that the F1XDJ turns on without issue once the wires are soldered back on, but I've never been so lucky.

We're getting close, everyone!

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