exxos's Atari, Amiga & retro forum

I use for R35 47 Ohm 1W so the 12V FAN run at 9V max ...

Good idea to do that, which I assume would keep the noise level down.
I suppose it'll still cool down the machine adequately (especially if there's no internal HDD).

I didn't read your posting until after I had fixed the PSU, but the good news is that not only did I find a 10 Ohm resistor in my parts box, but also a BIG one! Not sure about the rating, but it should most certainly handle more than the original resistor ;)
I'll see about replacing it with a 47 Ohm/1W resistor the next time I order components.
So after this the PSU works fine again (I measured all output voltages). The fan works as well, and I've now removed the yellow pin (just in case I plug it in the wrong way again!).
I've also removed the yellow wire/pin from the fan (just in case I connect it the wrong way again).

Is there any chance this mishap could have damaged my Mega STe motherboard?
The computer's dead (black screen) although the "power on" LED (connected to the motherboard) lights up and all the voltages from the PSU give their correct voltages.

Pacman wrote: 19 Mar 2021 15:40 Is there any chance this mishap could have damaged my Mega STe motherboard?

If it had damaged your board, then it wouldn't have worked at all after fixing the PSU.

Well, the screen is black when powering on (both in mono and colour modes), so apparently it doesn't boot.
I've had problems ever since I put in a MonSTer board -more specifically after soldering the additional wires needed for it from U1 (74F244), but it's now removed, a socket put in place, and the wires carefully soldered to the socket pins. I've also tried numerous 74F244 chips without the computer powering up. I've checked all the PCB traces going from the 74F244 and everything looks OK according to the schematic.
Not sure what else to check.
The MonSTer board is of course removed (to make troubleshooting easier), and the TOS EPROMs as well as the 68000 CPU put back into their sockets.

I've also checked for pulsing at the CE pins of the TOS 2.06 ROMs (suggested in the Atari forum) but either my scope (a cheap DIY one) can't handle it and/or my knowledge of handling it isn't good enough. Compared to my (working) STe with the same scope settings it does however appear like the signal isn't present at all.

I've seen different threads concerning dead Mega STe machines, so I'm going to have a closer look and see if there are things I can check and get closer to the source of the problem.

I'd be tempted to check all the voltages at the PSU to see if any are too low for the machine to power up and if other signals on the power plug are OK.

Might also be worth testing using a diag cartridge with serial output to a PC (or other ST) to see if there is any output there.

The voltages appear OK:

+5.054V
-5.038V
+11.970V
-12.424V

And "Power good" is also outputing around 5V as far as I remember.
A package from Exxos including the diagnostic cartridge/cable is right around the corner, so I plan to attach the cable to my (working) STe and see what comes out of it.

I've just removed the U1 IC socket so I can recheck all connections (they all checked out OK, but you never know -and at least one PCB trace had its hole awfully close to a via which may have been shorted when I inserted the IC socket), including all the motherboard vias in the surrounding area.

Instead of putting a socket back in I'll be using "round socket connectors" (item #0169) (also called "SIP sockets") which I just received from Exxos. They'll take up less space (no longer visually blocking the view as a normal IC socket does), and for attaching those wires from U1 which will go to the MonSTer board -I can solder those directly to the sides of the applicable pins (just remove the black plastic with a hobby-knife, revealing the metal pin).


Unlike an IC socket which you just put in place I need to align all these different socket connector pieces so the IC will fit straight on top. As discussed in this thread I think I'm going to plug all the needed round pin sockets into the bottom of a suitable IC socket, push the whole "assembly" into the U1 location, then solder the pins from the underside of the MSTE motherboard. That should keep it all aligned! Then I can remove the IC socket on top and insert the 74F244.

Now all I need it time and patience :lol:

frank.lukas wrote: 01 Feb 2021 17:06 I use for R35 47 Ohm 1W so the 12V FAN run at 9V max ...

The Noctua fan came with a couple of "low noise adapter" cables which are meant to be inserted between the power supply and the fan.


The one with the appropriate connectors (NA-RC12) doesn't work in my MSTE -the fan doesn't start up at all.


It's got a 147 Ohm resistor along the +V wire. If I replace that resistor with a 36 Ohm/1W or 39 Ohm/1W resistor, would that work similarly as replacing the R35 (10 Ohm) resistor in the PSU similarly to your suggestion of replacing the PSU R35 (10 Ohms) like you've done to lower the noise?


Having an easy to insert/remove "low noise adapter cable" would be great as the fan would be useful for when the internal SCSI hard drive is used, but with a MonSTer board and IDE-CF adapter used for storage I think the heat would be much less. Likewise, if I replace the 68000/16MHz CPU (gets very hot!) with Exxos' 68HC000 I don't see why I would need much more than a very slight airflow to keep the machine cool and safe. In normal ST's there's no fan at all, so without the HDD and hot CPU, I imagine the heat level to be about the same, right?
How slow do you think I could run the fan safely and working, and which resistor value would suffice for that?

I used that cable in a TT PSU. Just leave the orange wire unattached. Use black as GND and red as +12V.