Need some help, fuse blows

[jflorek]

I put my amp together, checked by BSEE, found some mistakes and corrected them but I am blowing fuses EVERYTIME i power up.  Cant find the short!  Seems I did some damage but just not sure where or how to find it.

Suggestions appreciated!!

[Natural Sound]

John, you did not specify which amp you have. Since this is posted in the General category more information is needed.

Have you done the resistance checks? What were the results?

[jflorek]

Apologies.  i have a stock crack.  all resistaces check out perfectly.

thanks

[Grainger49]

Then you should start by posting pictures of your power supply.  We will want to check the orientation of the diodes and capacitors.  Also the transformer wires should be visible.

[Chris65]

But before that, make sure you are using the correct fuse. What is the rating of the fuse & is it fast-blow or slow-blow? (Should be 1A slow-blow.) And what is your mains voltage?

To quote from PB:
Blowing fuses generally means that something is in backwards, or two things are touching which shouldn't be.  Your diodes and capacitors are correctly oriented, but you should carefully inspect the power supply built on the two 6 lug terminal strips.  Look for adjacent terminals that have untrimmed component leads that might be touching.

And yes, photos help!

[Adrian]

More fuse info:
http://bottlehead.com/smf/index.php?topic=7082.msg68589#msg68589

[jflorek]

I'm using 500mA fuses.  Blew the one that came with the kit and replaced at the local electronics store.  Pics as requested by Grainger.

Thanks all.

[Paul Birkeland]

If these two bare diode leads touch, you'll blow fuses all day.

Then there's the blue elephant in the room...

[jflorek]

Apologies for the long interval.  I work out of state so the only time I have to complete this is weekends.  Thanks for the advice!

I still have the short.  The diode leads do not touch so that's not it.  I changed the cap (yes same voltage and micro farad designation).   This was one of my mistakes but has been corrected AFTER I overheated and (possibly) fused the cap.

So I believe things are installed correctly at this point.  The question is, what could I
have blown when I fused the cap?  Is there some other damage that I might have done?

I'm stumped. 

[Paul Birkeland]

Try removing the single red wire at power transformer terminal 9 on one end, then leave it poking up in the air.

Next, try turning the amp on with no tubes in the amp.  Does the fuse blow?

Next, try turning the amp on with the tubes in the amp.  Do they glow? Does the fuse blow?

[jflorek]

Thanks PB.  I took the red wires off spade 9.  The 1A fuse doesnt blow but the voltages are less than half the expected values.  When I put the tubes in, they do not glow.  Voltages are WAY OFF.

My outputs as follows:

1=53v
2=.3v
4=4v
5=5v
7=0v
9=2v
etc.

Any thoughts?  The problem seems to be on the power side.  Thanks again for the help.

[Strikkflypilot]

You have the correct transformer.? Live in US? Europe?

[jflorek]

I live i the US.  Using the correct transformer, as described in the literature with the kit.  I don't  know but I'm looking at DC voltages from the outputs and they are below expected values.

[adydula]

Hey,

If the fuse blows this means that something after the fuse is drawing too much current and the fuse blows etc...simple.

So this means that something after the fuse is most likely shorting something to "ground" when it should not.

Things that cause shorts to ground that draw excessive current are many.

They can be miswiring, parts in wrong, shorted resistors etc..

I would check all my wiring from the switches to the transformer, and beyond over again. Look for anything that might be grounding to the metal plate.

You can also leave power off, and look at the schematic. You can start unhooking things to see where the short might be. I would remove the diode bridge from the transformer and see if the fuse blows most likely not unless the transformer has a short to ground on the secondary side. If the fuse doesn't blow with the bridge rectifier removed then check all the diodes with the wires from the transformer secondary windings removed one at a time for resistance forward and backwards...if one of them is very low resistance in both directions you have found you culprit here. If all this checks out ok and the diodes are ok, you can put the bridge rectifiers back in and start looking at the sext step in the power circuit..the 220Uf caps....in correctly, remove and check for shorts. Also you can check at the B+ output for both channels to ground with an ohmmeter to see if there is a short here.

I would remove the tubes and check from the Plate of the tubes to ground...if there is a short here this could be an issue.

I don't know if u stated this but if you remove tubes and power on does the fuse blow?

Good Luck.
Alex

[Paul Birkeland]

I live i the US.  Using the correct transformer, as described in the literature with the kit.  I don't  know but I'm looking at DC voltages from the outputs and they are below expected values.

Transformer measurements are AC voltages, and they have to be measured between pairs of lugs.

Try just having the 12AU7 in to see if that glows.

-PB