FM Towns II-HR repair

[solidpro]

Hey Everyone

I bought an FM Towns II-HR which was powering up fine and then just decided not to do anything. I disassembled the machine and completely recapped the power supply with new RS-Components caps and it still doesn't power on.

There is about a 10% chance that on the PSU, I got capacitors C58 and C52 the wrong way around (1 of them is 2200uf and the other is 1000uf) - does anyone have a point of reference for the II-HR PSU as to what these two caps are?

Having said that, I suspect it would power on even if they were switched, so it must be something else. I can hear a very slight hum when the PSU is plugged in, me thinking it's getting power. The power button is 'soft' so I don't know if there is another way to insipire a power on?

Also there are *a lot* of electrolytic caps on the mainboard, so I didn't replace them - I could not spot any signs of leakage. But could that be worth doing?

Thanks

 

[Maikudou]

If nothing else to suspect I would start with checking if power supply is in fact supplying the expected voltages

 

[GiGaBiTe]

There is about a 10% chance that on the PSU, I got capacitors C58 and C52 the wrong way around (1 of them is 2200uf and the other is 1000uf) - does anyone have a point of reference for the II-HR PSU as to what these two caps are?

The caps are for filtering. Since this is an old PSU, the 2200uF is probably on the +5v rail and the 1000uF is probably on the +12v rail. Almost all power supplies have silk screened polarity markings on the PCB. Even if they don't, it's usually pretty obvious which way they go. Just follow the ground plane on the bottom of the board back to the pads where the cap goes.

If you got those caps backwards, they most likely would have exploded, or bulged out and vented.

When you recapped the PSU, did you replace ALL of the capacitors? You need to replace all of them, even the small ones buried in between other components. Those are especially subject to failure because of their vicinity to smoking hot components and little airflow.

Having said that, I suspect it would power on even if they were switched, so it must be something else. I can hear a very slight hum when the PSU is plugged in, me thinking it's getting power. The power button is 'soft' so I don't know if there is another way to insipire a power on?

Having a backwards capacitor can most definitely prevent a power supply from powering on if it has short circuit and/or over current protection. You REALLY don't want to power it on and find out, if you think the caps are backwards, you need to remove them and make sure they're not.

As for soft power, it sounds like it's one of those uncommon soft power AT supplies. You'll have the normal P8/P9 connectors, and there will be a separate 3 pin connector that has +5vsb, ground and PS_ON. If you short PS/ON to ground, it can force the PSU to turn on, but I DO NOT recommend doing this. There's clearly a fault and forcing it could kill the entire machine stone dead forever.

Also there are *a lot* of electrolytic caps on the mainboard, so I didn't replace them - I could not spot any signs of leakage. But could that be worth doing?

If they're SMD electrolytics, yes, replace them all. FM Towns machines are like Macintoshes, the SMD caps are ticking timebombs.

The FM Towns machine I repaired had hundreds of SMD caps in it, and all of them were bad.

 

[solidpro]

Hi

I definitely got the polarity the right way around on all the caps. I checked before and after and it's clearly marked on the PCB itself. When I wrote backwards, I mean C58 and C52 are 1000uf and 2200uf caps and as I noted them all down I may have mixed them up in my notes.

I did replace everything. There were no SMD caps on the board for the II-HR - all through-hole.

I tested all the ones I removed and about 20% were bad.

I would check the PSU voltages but there is no obvious way to get it to power up. I wondered if, like ATX, there is a pin I could short to do that? Seeing as the power switch on the FM Towns II-HR is 'soft' I expect there is but I don't know which pin!

 

[GiGaBiTe]

I would check the PSU voltages but there is no obvious way to get it to power up. I wondered if, like ATX, there is a pin I could short to do that? Seeing as the power switch on the FM Towns II-HR is 'soft' I expect there is but I don't know which pin!

Need a picture of the pigtails coming out of the power supply. There should be a P8/P9 and a separate small 3 pin connector.

 

[solidpro]

I got a small ATX power supply with it's newer 24-pin ATX adaptor and bought an adaptor to downsize it's power connector to the older 20-pin ATX standard, which has exactly the same keying on the pins as the 16-pin FM-Towns HR (just with 2 pairs chopped off one end). It was £9.45 here.

Using the adaptor meant I could adapt 'any' ATX power supply rather than the one I bought to test with (a cheap one for testing). This one actually fits in place of the old one, along with all the slack on the cables just fine and it was just £24.

With the orange wire on the FM towns connector at the top and closest (along with the plastic lug to hold the connector down)

I re-wired it as follows:

Pin 4 to 8 (black) = ground => BLACK ON ATX PSU
Pin 9 to 12 (brown) = +5V => RED ON ATX
Pin 13 to 14 (brown) = +12V => YELLOW ON ATX
Pin 15 (brown) = -12V ( used for sound) => BLUE ON ATX
Pin 16 (black) = ground => BLACK ON ATX

Then finally wired the ATX GREEN to a spare BLACK so your FMT can power on when using the rear switch of the ATX psu. Alternatively you could wire this all the way to the front of the FM-T HR.

And.... it worked. Machine works fine! So the old PSU has more than cap problems.