Building Preamp from kit but a resistor keeps burning up

[Jumpybuns]

I purchased a preamp kit from classic api, the vp26. It should be no problem but everytime I plug the completed circuit into the lunchbox, A 200ohm resistor starts to smoke. Everything seems to measure as normal and im out of ideas.
Thanks for your time!

 

[(*steve*)]

Can you post the schematic, indicate the 200 ohm resistor in question and also post photos of both sides of your board indicating again, the 200 ohm resistor in question?

 

[Jumpybuns]

Here's a link to all the docs I was given.

The problem resistor is R6. I left the 2nd burned one on there so you can see what kind of condition its in!
Thanks a bunch!





[Mod edit - deleted a risky site addy]

 

[brevor]

I cant find the burned resistor in the photo, check for solder shorts around the resistors pads. Also make sure C9 is not in backwards.

 

[davenn]

I cant find the burned resistor in the photo, check for solder shorts around the resistors pads. Also make sure C9 is not in backwards.

agree with brevor .... what is cooking ? Steve did ask you to indicate which component it was

R6 ?? which one is that ?

 

[Jumpybuns]

It's the one at the top
Sorry about that

 

[davenn]

OK

one side goes to that electro cap above it make sure that that cap ( and the other 2 electros below it) are in the circuit the right way around
I cannot make out where the other side goes to

BTW it looks like one solder joint on that T1 transformer isn't soldered correctly
what do those 3 switches in the middle do ?

I didn't want to risk opening that zip file form some unknown site .... what was in the zip file, can you post it directly to the forum ?


Dave

 

[Jumpybuns]

i cant tell where it terminates either, it looks like right there. meaning one side goes to that cap and the other doesnt go anywhere

ill touch up that T1 for sure!
im self taught so thanks for going easy on me!

the 3 middle switches are a mute, phase, and pad

 

[Jumpybuns]

that website is pretty cool if youre into music stuff. good price on the DIY op amps.

 

[(*steve*)]

A short across C9 or C9 connected around the wrong way would be most likely.

Given what I can see of your soldering, if I were you, I'd get a little practice soldering and re-solder pretty much every joint on the board.

It appears that the components go through plated-through holes. If the joints were soldered properly you would be able to see solder filling the holes from the top side of the board (it would be almost level with the top of the board). I don't think I can see even one example of that.

 

[Jumpybuns]

So I should get a new cap for c9?

Will re-soldering everything help solve my issue? I know I need practice but I'm afraid of beating up the pcb

 

[davenn]

OK so SW1 is supplying 48V phantom power to In-Lo and In-Hi inputs via R6 200 Ohms
R7, R8 and SW2


wouldn't hurt to replace C9
But initially leave it out and see if the resistor still smokes, if so the fault lies elsewhere

make sure there is no short cct between either J1-8 or J1-10 to ground (0V) rail

 

[Jumpybuns]

Ok, I resolderd this section of the board. Still smoke.
I replaced C9, still smoke.
I removed C9. No smoke.

About to test for a short but Ive never done so Take my results with a grain of salt

 

[brevor]

So you tried a new C9 and it still smokes, but if you leave C9 out it doesn't smoke. Thats very strange ! Are you sure the 48 Volt source you are using is OK?

 

[davenn]

I replaced C9, still smoke.
I removed C9. No smoke.

OK that is a major hint

Again ... are you absolutely sure you are putting C9 in with the correct polarity ?
I suspect not

 

[brevor]

Again ... are you absolutely sure you are putting C9 in with the correct polarity ?

From the photos and drawings he posted it appears to be correct.

 

[davenn]

From the photos and drawings he posted it appears to be correct.

I cannot see any marking on the PCB to indicate which way around all those electros are supposed to go
There probably is a marking on the board under the cap

 

[Jumpybuns]

Im using this thing, seems like it has good reviews.

http://www.lindellaudio.se/index.php/506-power/

 

[(*steve*)]

Did you use a new capacitor and resistor when you did your replacement above? Can you show us a closeup of the soldering of the capacitor? Have you measured the resistance across the capacitor?

 

[Jumpybuns]

yep, new caps new resistors. ill measure the resistance tonight, i havent done that with new cap

heres a pic of the solder of the cap, its dead center (the 2 vertical points)