Mnats Little PSU = Smoke

Newbie needing some help here. I built the Mnats little PSU (http://mnats.net/psu.html) to use as a power supply for a pair of W492's. As soon as I plugged it in C3 and C4 smoked. I checked and triple checked the orientation of the diodes and the large caps and all is good. Of course I have a healthy fear of power supplies and up unitl this point all the power supplies I built (1176, API 312 and Green Pre) worked perfectly from the first start up. Solder joints are also all clean. What could have casued this? Any help would be greatly appreciated.

SJ Waldner

C3 and C4?  The little monolithic ones?  Weird.  Did you check that you got the 337 and 317 in the correct positions and facing the right way?

Yep, the little guys, I put some poly's in now (its all i have right now in this pic). I checked against the data sheets and the regs look like they are in correctly. Heres a pic

Huh, all looks good from what I can tell.  Do you have 100R in place of 120R?

You definitely have 100R resistors in R4 and R6 when you need to have 120R.

Did you get the PCB from Mnats? The parts? Or is this home-brewed?

What DC voltage do you have at the big filter caps? If you got the power transformer backward, you could have hundreds of volts. (How would I know??) The small caps might pop instantly, the big caps might not EXPLODE for many-many seconds.

100r for 120r will give wrong (un-trimmed) output but does not explain the drama.

I got PCB from Mnats when I bought my 1176 board. The parts came mostly from Digikey, except the transformer.... my Dad bought it off eBay (China supplier)...hm that may be my issue. I didn't have chance to measure the voltage from the caps, also I am certain it was not wired backwards. I will try one of my transformers that I know works and see if the issue is the cheap Chinese transformer.

Without connecting the transformer to the pcb, measure the ac voltage at the secondary with the primaries connectd to mains.
Whats the  secondary voltage?

then connected the transformer to the pcb, what DC voltage do you get after the regulators?

Check under the pcb for shorts in tracks or components solder pads.
check all the component values.