Help requested: fuse blows when powered on

[flatmap]

I've read through the other threads here, but seem to have created a different problem.

My finished Crack blows the fuse when I switch it on.  I've checked the direction of the capacitors and the diodes and these are all correctly oriented.  The fuse blows both with and without the tubes in the circuit.  I've inspected the joints and re-soldered anything that looked questionable.

The resistance check currently gives correct values except for the measurement from terminal 12 to 13.  That currently shows, "blinking Meg Ohm" value -- which is what I get for infinite resistance on this meter.  There's no slow change in value at all.  It just registers as a constant very high resistance.

So here's what I know I did wrong.  During my first check out, I made it through the resistance check and then part way through the voltage check and the fuse did not blow -- initially.  Both tubes glowed The voltages were measuring correctly down to the check of A4.  However when I was measuring A4 I (sigh) accidentally shorted A4 to the adjacent pin A3 with the probe.  That blew out the LED to A3... or at least it stopped lighting up.  I replaced this LED and verified that it was oriented correctly. 

In the interest of full disclosure, I initially had transformer terminal 4 wired to 15 rather than 14.  I fixed that after the first fuse blew and I checked back here for updates. 

 

[Grainger49]

flatmap,

After you look at the diodes and capacitors about twice you can't see if one is backward.  Have someone else look at them.

You are right that is the most often reason for a blown fuse.  The next most often reason is because something is shorted. 

Get a magnifying glass and look at every terminal.  Look at the leads on the diodes.  No terminal should have extra wire coming off of it to touch the top plate or another terminal.  No diode lead should touch another lead unless they are going to the same terminal.

Both tubes glowing says that the heater circuits are good.  The high voltage, the B+, would not allow the heaters to glow.  Any problem there would blow the fuse immediately.  So you are left with the hard problems.

Are you replacing the fuse with a slow blow fuse not a fast acting one?

 

[flatmap]

Thanks, Grainger.

I checked the package and indeed I am using fast acting fuses.  They are .5A/250V... but clearly marked fast acting.  So, 'slow blow' is what I need?

I'll try to get someone else to check the diodes and capacitors.  It may take a while to line up a, ahem, volunteer.

Today I'll go ahead and get the slow blow fuses and a magnifier --- and then carefully inspect each terminal.

 

[Grainger49]

Beer helps to get a volunteer, but make him look before you give him the beer.

 

[flatmap]

Hah, no beer needed.  

I went to the store and got slow blow fuses.  I got my 16 year old to double check my capacitors and diodes.  I bribed her.  I checked each terminal for touching wires, solder splashes and what-not.   These all checked out ok.

Then, other than my earlier change connecting transformer terminal 4 to terminal 14 ( rather than to 15 ), I haven't changed anything.  

But I put in the slow blow fuses, and now the fuse does not blow, the tubes light up again, and music sounds!  So does this mean... I just bought the wrong type of fuse after my earlier klutzy mishap?  

 

[Grainger49]

Sounds like that to me.  Check the parts list and see what is specified.  I don't have the kit or manual so I can't.

 

[flatmap]

Hey Grainger, thanks again for helping me out.  Really appreciate it!

 

[Grainger49]

Not a problem.  I would usually respond faster.  I have been out of town since Friday and had limited access.  Being retired gives me a lot of spare time.

 

[STURMJ]

Go with 1A fuses per Doc B, A bunch of us had the same problem. See these threads
http://www.bottlehead.com/smf/index.php/topic,677.0.html
this one contains direct quote from Doc
http://www.bottlehead.com/smf/index.php/topic,930.15.html

 

[flatmap]

Thanks Sturmj.  I'm not sure how I missed this before.

it seems like the slow blow .5A fuses are working for now, but i'll look for some 1A versions next time the fuse zaps. 

Really nice to have this thing working!

 

[Bones13]

I managed to short out the original fuse while doing some post construction metering.  I read the manual and got a set of 0.5 amp fuses, which never survived turn on.  I was worried I had really messed up the construction.  After reading here, I got some 1 amp fuses and it turns on and gets the tubes going.  Only the left channel is active though, so I need to do yet another run through, but not looking for shorts this time round.

All sorts of things to learn here on the forums, thanks to all that go out of their way to help.

 

[flatmap]

Bones13, how did the resistance and voltage checks go?  Are all of those values on target?

 

[Bones13]

Checks went fine.  But I kept having only one channel active.  Swapped the tubes, and inputs, no go.  I got frustrated and let it sit a while.  This weekend I finally found an intermittent contact on the 8 pin.  The 8 pin teflon socket rocks a bit, causing the break.  I need to take it down and bend the clamp somewhat to put enough pressure on the socket.  Working though, so I will attend to that another time.

Crack/Speedball is now in service, in break in period as I write this.

Very pleasant sound.  It needs to warm up before the frequencies extend, but its already very nice.

Nice kit, fairly pleasant first build.  Thanks Bottlehead, and this forum!

 

[flatmap]

Wow, Bones13, good work locating that flakey socket.  I think I would have still been crying in my beer over that one.