Atari Mega STE Power Supply PSM-5341 Schematic Discrepancies

[SirPete]

I was working on a restoring a blown PSM-5341 power supply out of an Atari Mega STE.   

 

Most of the caps were blown, so this power supply had a serious incident.  After recapping I could not get it to power up, even though all other components tested fine.

 

I opened up another PSM-5341 power supply to compare against, and found that in my other power supply had some differences.  I then opened a third to double-check, and that one matched the working second one.

 

R5 on bad/blown power supply is 100 ohms (per the schematics).  On working supplies this was 50 ohms (one of the good power supplies had a 50 ohm resistor in this place, the other one had a 100 resistor with a second 100 ohm resistor piggy-backed on to the first, which effectively made this 50 ohms)

 

R4 on bad/blown power supply is 47 ohms (per the schematics).  On both working power supplies this resistor was cut out (legs from the resistor were still there, so it looks like it was there initially).

 

Anyone have any experience on thoughts with this?  I temporarily clipped in a piggy-backed 100 ohm resistor on R5 and the power supply now works.  I'm suspecting that the original schematics were for 220V and when these got to 110V markets Atari realized that the power supply wouldn't work as it was designed, but that's just a guess.

 

 

[snarkdluG]

R4 according to schematics is 47Kohm. I don't know anything about the rest.

Edited July 23, 2022 by snarkdluG

[SirPete]

Yes, sorry, that was a typo on my part.  R4 is 47K on the bad/blown power supply.

 

One of the good power supplies R5 that I through had a 50 ohm resistor actually also has a 100 ohm resistor and a second 100 ohm resistor piggy-backed on the underside (see pic).

 

I'm suspecting the R5 resistor is there to limit surge/startup current.  That's why the bad power supply can't seem to start.  I see the voltage fluctuate slightly as the voltage regulator circuit is trying to get going, but the IC can't get a stable VCC voltage.  Adding a second 100 ohm resistor to R5 solves that in my case.  If this machine was intended for the 220V market I suspect it would have worked fine, but with 110V it doesn't. 

 

R4 seems to be a there to allow the capacitor to discharge to ground if it's not running (my best guess).  Not sure why it's been removed from the two good power supplies, but since it's on both I think it's a factory mod.

 

I'm planning on probably doing both mods to the "bad" power supply to match the others, although I'm hesitant to remove R4. 

 

Thoughts anyone?

[UnderTheRain]

I have the same psu open on the desk in front of me 220v and it has the doubled 100 ohm for R5 making it 50R and R4 cut out. The schematics I have also show R4 as 47K and R5 as 100R

 

[SirPete]

Great, so I guess we've validated that Atari made some modifications from the original schematics.  I'm going to clip R4 out of this one to match what seems to be the "normal" configuration.

 

Thanks for checking yours!

[UnderTheRain]

Don't worry if you see anything else missing on mine I'm waiting on parts to arrive mines got a blown c17 680uF and r24 I want to say 2.74k burned out (5v shorted to 0v on the motherboard).

Edited July 23, 2022 by UnderTheRain