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Yesterday I assembled an OTG board purchased from http://www.opentherm-gateway.com.

Now there seems to be a problem with the PSU part. At the 7805 VCC pin I do measure slightly less then 3V DC.
At secondary pins of transformer I do measure roughly 3V AC. Also the transformer turns warm within seconds.
With the transformer taken from the board I measure about 20V DC open ended at the secondary side.

My first guess was I overheated one or more of the diodes D1-D4 during soldering. I've just replaced these, but the situation remains as before.

I'm wondering whether the 3V AC is a result of the transformer's short circuit protection. Can anyone either confirm or deny?

If it is not the short circuit protection I guess the most likely fault would be the transformer itself?
If it is the short circuit protection what would be the next step in troubleshooting? The zenerdiodes?

BTW:
- No caps blew.
- There are no signs of magic smoke.

Are you sure all components are mounted correctly and on the right position? Most of these problems are caused by this.
If you expect there is a malfunction of some part, please contact me on the contact form of the website. I can send you replacement parts for free. I can also offer you to take a look at the non-working gateway.

Maybe Schelte can clear something up. If not, you've got my offer.

Indeed, sounds like a wrong transistor or something like that. Note that Q4 is the NPN transistor (BC548) and all others are PNP (BC558).