[BavariaBarbarian]

Hi guys,

I've searched the forums and it seems I'm the first one to run into serious trouble while building a TREAD.

Makes me feel a bit foolish but here it is in detail:
I stuffed the board according to the schematic and Tangent's excellent guide.
Part values are:

R1 120R
R2 1K
R3 3.3K
VSET 1K
C1-C4 100pf
C5 2200uf
C6 1uf
C7 10uf
C8 22uf
...
The rest also being standard.

Then I connected it to a 20V AC-Adapter, the LED lights up and nothing explodes, the LM317 gets about hand-warm.
TP1-TP3 (rectified input voltage) measures 29.8V DC.

So far so good, now my problem:

TP3-TP2 (output voltage) measures 6V-8V DC depending on VSET and diminishes rapidly.

I've got absolutely no clue what's going on here, can anyone help me please?

Here are two small pictures. In case you wonder: R2 isn't missing, it's just mounted on the underside of the board because I mounted the trimpot a bit awkward.

Many thanks in advance!

 

[MisterX]

I can't tell from the pic so you might want to check that you C7 in the right way
Those tantalum caps are polarized and it would explain the problem.

 

[mono]

Your board has no R2 (resistor) on it, the spot next to the POT. With "some" pots that resistor might be jumpered out of the circuit but "something" must bridge that resistor spot, IIRC, because the POT is an open circuit loop without the resistor.

EDIT: Thanks MisterX, I spaced out... MUST get more sleep. Must read posts. Must Zzzz zzzzz...

 

[MisterX]

Quote:

In case you wonder: R2 isn't missing, it's just mounted on the underside of the board because I mounted the trimpot a bit awkward.

Edit:
mono:
It's understandable.... we all do that from time to time.

 

[BavariaBarbarian]

Quote:

Originally Posted by MisterX I can't tell from the pic so you might want to check that you C7 in the right way Those tantalum caps are polarized and it would explain the problem.

Yep! Pulled C7 and now it works. Hooray!
Thank you very much MisterX!

there's a fat bar on the cap to mark the positive lead.
I figured it marks the negative lead, like on electrolytic caps... I too need more sleep.

But I can't see why the reversed mounting of the cap caused the output voltage to fall off.

Did the cap get damaged, connecting it's two pins?