Help wanted troubleshooting newly assembled OTG.

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Help wanted troubleshooting newly assembled OTG.

Postby The Lord » Tue Feb 19, 2013 11:07 pm

Yesterday I assembled an OTG board purchased from

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?

- No caps blew.
- There are no signs of magic smoke.
The Lord
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Re: Help wanted troubleshooting newly assembled OTG.

Postby flue » Thu Feb 21, 2013 12:06 am

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.
Buy opentherm gateway PCB's, kits or assembled PCB's:
Special discount for this forum using the following coupon code:
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Re: Help wanted troubleshooting newly assembled OTG.

Postby hvxl » Thu Feb 21, 2013 5:45 pm

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