FM Towns II-HR went from working to nothing....

[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

[solidpro]

I've found this mod https://nfggames.com/forum2/index.php?topic=6499.0 to turn an ATX power supply into a compatible one for the FM Towns. I wondered if, like on ATX you can ground the GREEN pin with a BLACK to auto-power-on, there is a similar pin on the FM Towns PSU....?

[Cyothevile]

I've found this mod https://nfggames.com/forum2/index.php?topic=6499.0 to turn an ATX power supply into a compatible one for the FM Towns. I wondered if, like on ATX you can ground the GREEN pin with a BLACK to auto-power-on, there is a similar pin on the FM Towns PSU....?

Here is how all towns power supplies work:

Power on signal switch goes to GND. Kicks on and off PSU with discrete logic on PSU.

Towns OS off pin goes to GND. PSU turns off.

You can wire an ATX directly to the connector on mainboard if you know the pinout which is trivial to solve. Just desolder wiring harness from dead PSU and find what pins are GND, 12v, -12V and 5V. Then wire an ATX to it. When ATX switch is turned on, the pc will turn on. Only downside is the PSU can only be controlled by ATX on and off switch, and not the towns OS OFF or original power switch.

This is all assuming you cant troubleshoot whats wrong on the original.

[solidpro]

Is there no way to ground a pin on the Towns PSU? Note the II-HR has a soft power on.

I have ordered a mini ATX and I will attempt to power on with that, but I think it's actually something other than the PSU....

[Cyothevile]

Is there no way to ground a pin on the Towns PSU? Note the II-HR has a soft power on.

I have ordered a mini ATX and I will attempt to power on with that, but I think it's actually something other than the PSU....

No. The Towns doesnt use ATX logic to turn on and off.

I think youre totally mistaken and that once PSU issue is solved your troubles will go away. Cheer up

[kamiboy]

I've found this mod https://nfggames.com/forum2/index.php?topic=6499.0 to turn an ATX power supply into a compatible one for the FM Towns. I wondered if, like on ATX you can ground the GREEN pin with a BLACK to auto-power-on, there is a similar pin on the FM Towns PSU....?

Here is how all towns power supplies work:

Power on signal switch goes to GND. Kicks on and off PSU with discrete logic on PSU.

Towns OS off pin goes to GND. PSU turns off.

You can wire an ATX directly to the connector on mainboard if you know the pinout which is trivial to solve. Just desolder wiring harness from dead PSU and find what pins are GND, 12v, -12V and 5V. Then wire an ATX to it. When ATX switch is turned on, the pc will turn on. Only downside is the PSU can only be controlled by ATX on and off switch, and not the towns OS OFF or original power switch.

This is all assuming you cant troubleshoot whats wrong on the original.

Trying to understand the ON/OFF logic better, so correct me if I have reached the wrong conclusion from your writeup.

The signal from the power button: a momentary grounding of the signal causes the flip flop of power ON / power OFF status of the PSU.

The signal from the OS power pin: A momentary grounding of the signal causes the power ON to go to power OFF status.

What is the voltage on the OS and power button pin prior to grounding? +12V or +5V?

[hiker]

I think you can assume that the power button pin is floating when it's not grounded.

When I recently rebuilt the PSU of FM towns tower I used Cyo's Arduino script (https://github.com/cyo-the-vile/FM-TOWNS-TOWER-POWER-SUPPLY/blob/main/TownsPSU_script.ino) as a reference. I did use a 74HC74 flip flop and NAND gates.