FM Towns PSU

[Monstermug]

Fault:  Doesn't want to power on or off.  When power button is pressed it takes a while for anything to happen.

Cause:  Suspect IC's, primarily at IC51.

Does anybody know what IC51 is or a replacement for it might be?  There doesn't seem to be any markings on it. 

[kamiboy]

I wouldn't hold my breath there, these forums are pretty dead, and FM Towns information in general is hard to come by.

Tell you what, once I go home today I'll try really hard to rummage through the mess in my closet to find those PWM's we were talking about on youtube. That should get you going.

[Monstermug]

Replaced faulty transistor C1815 right next to the nameless ic51 and it boots and shuts down flawlessly now   I did check the other two C1815s too but they checked out fine.

Another FM towns risen from the grave! I'm getting quite a big pile of them now

Was that you I was talking to on youtube?  Haha great video.  Now to fix the CD rom drive.  I'm hoping it's just a faulty lid mechanism.  Going to solder in another wire to link the lid up to see if that works. 

[Monstermug]

Simply soldering a higher guage wire from the cd rom pcb to the lid switches worked.  Cd rom loads first time every time.  If I could reach out and give you a hug I would    Who would have thought it be those silly switches not providing +5 to the CD rom.  Genius.

[kamiboy]

Well, well, I guess this story is a happy end. Cheers.

[Demetris74]

What a small world, had the same issue 2 days ago already posted up new thread,  did replace all 3 C1815 and still the same problem

[Monstermug]

After putting it all back together, the PSU problem seem to have cropped back again.  I guess the faulty transistor wasn't the only thing that's causing the +5v not to engage/disengage properly.  Lucky on a cold start it turns on instantly.  But after that it takes a few tries before it can shut down and restart again.  Could be that IC51 still.  Could be something totally unrelated to the PSU altogether?  Best bet would be to take it all apart and work out what is in between the power button and the PSU.  Before I go do that, I wanna wait and see if Kamiboy can find me what that unknown IC51 chip is first.  That would most likely be our best hope for fixing this problem as I still not worked out how to dissemble the motherboard yet. 

[Demetris74]

I guess Will go trhough this together, wich is good Same simptoms here, After cold start everything was ok, eventually totally died. I have also removed and test the starting relay and works on and off ok, If i find something will update. will work more on it on monday.
I clearly suspect the 5v Standby circuit, all small capacitors area, doesnt give the voltage to start
What also came to my mind the PSU is ok after the repair,  and the problem passed to the MBoard after the PSU Failure, Only way to know is with another good PSU

[kamiboy]

Yeah, mine behaved the same way as described. First it would start up fine, then gradually it required more and more presses of the power button to start, until it just flat out refused.

I am pretty certain the problem is isolated to the PSU, not any component on the rest of the machine.

I guess you'll need that IC after all, I'll continue looking for it then.

[caius]

Can you post, please, a picture of this unknown IC51?Thanks.

P.S.
Which is the model of your FM-TOWNS?I have two of them, maybe PSU is the same.

[kamiboy]

Mystery IC marked below.

[Demetris74]

Is it possible to send also the back of the PSU i need to compare some jumper connections

[kamiboy]

Sorry, no pictures of the backside have survived from the time I did my repairs. I am sure I had them, but running out of space on my iPad meant I deleted everything except the one above.

[Demetris74]

Not a problem,

I am also sure the Non name IC can be the power switch control circuit, Only thing to find it's logic operation

[kamiboy]

this is how it was explained to me: The mystery IC sends a pulse of a certain frequency to a regulator, switching it on and off to produce the correct voltage.

[Monstermug]

My heart skipped a beat for a moment there.  I thought you had found the replacement IC 

Which is the model of your FM-TOWNS?I have two of them, maybe PUS is the same.

I have the FM Towns Model 20F

Looks exactly like this:

[kamiboy]

Bad news I am affraid. I turned over the one box I thought I would have put the IC's in, but they were not there. I have no idea where else I might have put them. For the time being they are lost.

[Demetris74]

I would be able to test the IC via ESR / IC logic gate tester, and identify the Specs, will update soon

[kamiboy]

Good news everyone! I found the IC's. I am guessing the mystery IC replacement is supposed to be the lone one.

[Demetris74]

Great News
Is it the one on the foam? have you done this replacement before on similar trouble?

[kamiboy]

It is indeed the one on the foam, and no, this is completely untested. Use at own risk. I suggest that if you swap the mystery IC with the one in the picture you test the PSU by itself. You need to short two points on the output port of the PSU in order for the PSU to turn on. Then you just need to test voltages on the output port to see whether the IC is indeed a drop in replacement or not.

Do report back.

[Demetris74]

Ok Great

Will do the test and update, will do it out of the case, Can you indicate me the 2 points for startup? Hope you know otherwise i wont test it in the case is risky

[kamiboy]

Sorry, I already forgot. I found the points by doing continuity test from the switch on the front to the PSU.

If I remember right it is a 12V rail that needs to be shorted to ground. It is one of the lines to the far right I think, or maybe to the far left on the PSU. You should be able to identify it by it being sort of isolated. I believe it has a trace running to a leg on an IC.

[Monstermug]

Your a godsend.  Just ordered one.  Had a feeling it was some kind of hex inverter.  I should have some time to take it all apart again next week to test it unless Demetris gets there first.  If this works I am sure it would help a whole bunch of people with the same problem. 

I guess the only difficulties in testing it out of the case would be to find the two pins to short.  I haven't yet worked out how to take off the motherboard without breaking something yet.  Perhaps the easiest way is to solder a socket in, connect wires to the socket that leads out of the psu casing and then test continuity using the wires while the psu is inside the case?

[kamiboy]

I took my FMT completely apart. That way I was free to test everything out as I saw fit. Finding the two points should be easy though. Before the PSU is turned on all it outputs is 12V on one or two lines. First find ground, then test every output from the PSU until you find the 12V lines. Then in turn short them to ground and the one that turns on the PSU and spins the fan is your on/off line. Shorting it will either turn on or off the PSU depending on whether it is already on or not.

[Demetris74]

I have also ordered  the IC as we speak. there is a running 12v line is the first 4 pin pairs at the left side Solder side (Confirmed)
Will do as Kami mention Replace the IC install the fan and short 12v to ground, The fan should start showing the operation. IF that works then  this  solves a big problem many coun't figure out in the past.
Fingers crossed

[Demetris74]

Good news everyone! I found the IC's. I am guessing the mystery IC replacement is supposed to be the lone one.

Have received the ic when after i notice that the specific ic is 14pin not 16 as the original one
So i am guessing thats a big disappointment to all?

[kamiboy]

Wow, what a disappointment. I guess my pappy dropped the ball on that one, he must have been mistaken when counting the legs. Sorry about that. I guess I can't use mine in the future either.

[Demetris74]

Update!

After soldering back the IC with a socket, the computer started normally and power on and off for for 10 minuties with out any problem when suddenly died again, i suspect other source of the problem.

[kamiboy]

I haven't touched mine for months, so I have no idea whether the PSU in mine still works or not. I kind of dread going back to it, but will soon.

If it acts up I am going with my plan B, butcher an old PC PSU and stuff it inside the FM Towns PSU casing. That is my final solution.

[Demetris74]

But how u need the voltage points and the  connector

[kamiboy]

I voltages are easy to map out using the test points on the motherboard, and the connector is simple, I'll just use the one from the original PSU.

[Demetris74]

this guy found the voltage points on the original psu can clearly see them but wonder where is the voltage points can someone indicate the colors that correspond to the proper voltage? if the colors are right ofcourse

https://vimeo.com/166300852

[kamiboy]

Why don't you google it? ATX power supplies use standard colours for voltages.

[Demetris74]

Yeah but i mean dont know if this guy used an atx psu thats what mean but collors seem to be atx

[Demetris74]

Kamiboy

Can you somehow help  highlight the points to solder the voltages? maybe in a new photo? i can't see them clearly on the video, need to pause but still not sure
  i want to test this for all of us, anyway my psu is busted let me nest this one with an atx psu i have the cumming days.

maybe make an official photo of the mounting points for an atx PSU mod. I am thinking of leaving the stock psu in place and use the atx externally obviuslly removing the fuse of the internal psu.
whats your thoughts?

[kamiboy]

Sorry, I cannot. Been too long since I had my unit open, and my PSU might be different from yours. But you can mao it out yourself using a multimeter. You should notice that the majority of the connector solder pads are connected. Most are either gnd or +5v, with a few being +12v or something higher than 5V, one being either -5V or -12, I forget, and one being for switching the PSU on and off when shorted to gnd.

[kamiboy]

Good news everyone! I found the IC's. I am guessing the mystery IC replacement is supposed to be the lone one.

Have received the ic when after i notice that the specific ic is 14pin not 16 as the original one
So i am guessing thats a big disappointment to all?

By the way, about the IC. I asked my pappy about it and it seems I dun goofed. He actually never bought me a replacement for the mystery IC since he tested mine and determined it to be good. The three IC's he bought were replacements for the other IC's on the PSU.

That means the one you bought is a drop in replacement for one of the other IC's on the board, but not the mystery IC.

[yukin]

For the video, it's my fm towns 20F that my friend did repair, he said to me it is a AT power supply. He did add a power switch on the back of the towns so i can power it on. The power switch on the front does not work. You can see on the picture attached the red button he added.  About the mystery IC, my friend was not able to find also a replacement so i decided to simply go with a AT power supply and use the original connector of the power supply to put the right voltage where it need it. He told me the mystery IC chip is the key to make the front power switch working. So thats why we went with this alternative.


SOOOOOOOO, the wires you can see in the video are coming from a AT power supply i guess.

Red= +5V
Yellow= +12V
Blue= -12V
Black= Ground

[Demetris74]

what would be helpull if he can show us where the voltage wires go at the fm towns plug that he chopped, which pins