Crack Build blown fuses after wiring recheck

[Sh7eleven]

Hello,

Here's the story so far:

I completed the build of the crack.  All the resistance measurements checkout.  Though, I will include three that don't meet the 15% off barrier but are off just in case - RCA center pins are 106 and 108 as opposed to 90-100.  Terminal 13, * slowly rise to 270K, starts out by dropping towards zero and at about 3K ohms turns around and climbs towards 270. 

On my first start up, the transformer got very hot, but nothing else happened - no LED's or Filaments lit up. I turned it off and checked the fuse, which was not blown. Went back and retraced the wiring, checked the orientation of the parts that required it - everything checked out. Started up again and fuse blew. Re-traced wiring again and I found that I missed soldering terminal 3L. 

Now I retraced the wiring, checked orientation of parts, checked all solder joints, soldering 3L - everything checked out. I started it up and the fuse blew immediately. I again retraced and checked orientation, and I re-soldered every joint.  The fuse blew again. 

I have three questions:

1. The replacement fuses I bought were radioshack 1/2 amp 250v fuses slow blow fuses - are these the right fuse to use?

2.  What should I do next - A blown fuse according to the manual indicates a miswiring, but, I have checked it from start to finish 3 times the route and orientation of the parts that needed it and, apart from the small variations i mentioned, my resistance checks seem correct. Would a cold solder joint lead to a blown fuse? I tried to make sure that each mechanical connection was solid, but should I vacuum the solder and check the mechanical connections at each joint?

3. The only variation from the manual I made was the 5/10 revision on page 19 where instead of routing a black wire from power terminal 4 to terminal 22L, I routed it 14 U as suggested by the revision - 14U became pretty crowded - should I remove it and route it to 22 instead?

Any help wold be appreciated,

Thanks,

Sean

[Doc B.]

Most often a blown fuse is a symptom of a blown rectifier diode. You may have blown a rectifier before the connection at 3L was corrected. If your meter has a diode check setting use that, otherwise set your meter to a range around 2K ohms. Check each diode, looking for short, like a reading under 100 ohms.

[HiFi Builder Guy]

Hi Sean,

If the transformer got very hot very quickly it makes me question the orientation of the power transformer.  Please verify that the sticker actually matches up with the numbers molded into the transformer terminals.  Also, the crack takes a 1 amp slo-blo fuse (the first run of cracks we had 1/2 amp but have changed to 1 amp). 

Shawn

[Sh7eleven]

Thanks for the replies,

Nice pick-up Shawn, Looking at the sticker orientation on the transformer and the numbers on the plastic - they appear to be reversed - so glad I included that detail in the description - 1 is where ten is; 5 is where 6 is; 9 is where 2 is etc. So I'll have to take it out and reverse it, then rewire it. And I should also pick up some 1 amp fuses, Can I at least test it with half amp fuses or should i head back to the shack? (Just a heads up, the parts list print out I recieved in the package and in the cd still lists a half amp.) 

Any other damage I Should expect expect from having this backwards? Should the transformer be ok?  I'll check the rectifier diodes after I break it down and reverse it.

sean

[HiFi Builder Guy]

Sean,

Our apologies for the mislabel.  If the transformer is toast we will send out a replacement right away.  If there was no smoke you are probably ok .

I just took a PT-3 off the shelf and measured the HV secondaries at 82 ohms each.  I'd recommend measuring and comparing yours once it's pulled/disconnected before re-installing.  You could try the 1/2 amp fuse, keeping in mind if it blows you may still be fine and just need the higher value.

Shawn 

[Sh7eleven]

No worries,
 
There was no smoke. I got the transformer out, but I not sure what you mean by HV secondaries - If you mean terminals 6, 7 and 9, 10, then they measured about 83 ohms each.  terminals 1 and 2 measured 12 ohms each.  and Terminals 4 and 5 measured 0.  I'm a novice so to let you know, I measured by attaching black to one terminal in a pair and red to the other - ex. Black 9 red 10 for ten's reading, and then swtiched to get 9's reading.   

So I am ready to put the transformer back in or should the readings at 1 and 2, and 4 and 5 also read 83 ohms?

[Paul Joppa]

For a transformer, the direction of measurement does not matter. Looks like your transformer is fine.

[Laudanum]

Just to add, I think my Crack has a 1/2 amp fuse and it has been fine.  So as mentioned, if the 1/2 amp doesnt blow now that you have the transformer squared away, you should be fine.

[Sh7eleven]

Ok, I took out the power transformer, switched her around, put her back in with the correct orientation and wiring, and grabbed some 1 amp fuses.  Powered up and Filaments started to glow and LED's lit up - awesome!

I measured the voltages and found:

T5: measured 123 - supposed to be 90.
T9: 128 - 100
A1: 116 - 90
B4: 114 - 90
B6: 126 - 100

I noticed that A1-T5-B4 are connected together as well as T9-B6.  As a novice - it brought joy and was no small revelation to note that the places I had readings that were off by more than 15% were wired together.   I plan to re-solder these joints and check the mechanical connections of the 160 v capacitor attached at T9 and the 22.1K ohm 1W resistor attached at T5.  The resistance readings by these components still read normal.

As an aside - is it ok to solder the socket tabs while the tube is attached, it seems like a lot of hear or should I remove the tubes.

PS Not really experienced with volt meters and curiously all the values read on my volt meter had the negative sign in front of them - Is that just the oddity of the volt meter or does that indicate something when everything is being read as negative? (all values correct or otherwise were negative)  

[Grainger49]

Sean,

I would remove the tube just to be careful.  If you get solder down into the hole the pins go in you have a problem.  But, at least, if you take the tube out then you don't ruin a tube along with the socket. 

If you don't use too much solder you shouldn't have a problem.

The negative sign on your volt meter means you had the red probe on ground and the black lead doing the testing.  No biggie there.  It is refreshing that those points that are wired together read the same voltage.  Many times with newbies there are substantial differences between points wired together.  There shouldn't be any difference.

[Sh7eleven]

Thanks Grainger,

Maybe i have the red probe plugged in where the black probe should be. 

After re-soldering, the voltage readings start high but over the course of ten minutes drops steadily and flattens out:

A1 - T5 - B4 - all read 105, instead of 90.

T9 - B6 - all read 119 instead of 100. 

Why would the readings at these  be dropping over the course of ten minutes? Is it from the tube warming up? All my other readings stay relatively steady. Are these values good enough to go forward?  The A1-T5-B4 is at 16%, but the T9-B6 is still at 20%, should I go at it with the soldering iron again?

[Paul Joppa]

How do A1 and A6 compare? They are the plates of the driver tube for the 2 channels. If they are far apart, the tube is unbalanced - that's not necessarily a problem unless the mismatch is bad, but something to be aware of.

The voltage may be dropping because the tube's cathode is still forming. In theory this is done at the factory, but we've seen this more than a few times - the tube does not conduct the full current at first, but once formed the cathode is stable. You can check the voltages again after 50 hours or so of operation; if this theory is right they will be closer to the manual values (and probably better matched).

It may also be that your power line voltage is a bit high; that would raise all the other voltages too.

[Sh7eleven]

A6 reads 87 and A1 just read 102 (so did T5, B4). Is this too far apart so as to make the mismatch bad?  Also, Do you mean that the 6080 and 12au7 are a bad match - or one of the tubes is unbalanced - ie. 6080 only (learning, what can I say ). What would be the result of using a poorly balanced tube, volume differences, clarity?

T9-B6 stills read 118. This is close enough to proceed to the last step of checking the voltage at the Headphone jack?  Also If I did the start up mod, would I see the same 9v drop to zero effect or is this what the mod is intended to rectify?

So until about 50 hours or so when the cathode forms - should i give the amp about ten minutes then to warm-up before plugging the headphones in to avoid damage? 

[Paul Joppa]

It looks like A6 is still coming down, so don't sweat it. I think what you have is fine, unless you have really bad OCD  :^)

I was referring to the balance between sections of the 12AU7. The 6080, configured as a cathode follower, is less sensitive to mismatched sections. That's good, because really well-matched ones are hard to find!

Finishing the cathode-forming is just a matter of operating time, there's no reason not to listen right away. The startup switch wiring should eliminate the voltage rise.

[Sh7eleven]

Thanks Paul,

I went ahead and hooked her up and she sounds beautiful.  Absolutely dead quiet - no noise when the volume adjusted up or down.  Paired with HD-600's and fed by a marantz sa 8001. Its hard to describe the sound being a relative novice - delicately lush - does those even go together? I had forgotten what I had in the cd player, First Track turned out to be Andrew Bird's Sovay from Mysterious production of eggs. It has *Acoustic guitar*, violins, vibraphone, whistling: wow. Didn't turn it up the volume more than an eighth of a turn and got great separation of instruments too.  

Thanks to all for your help and guidance, gotta listen for a while!

Edit: *electric* guitar was in the next track oops - uhh yeah no OCD here