Multiple Build Problems with my Crack

[dpitch40]

My Crack OTL Amp finally came last Thursday... My attempts to build it have not gone well. I will try to describe what happened.

I meticulously followed all the instructions, double-checking that everything was properly connected, all the diodes and capacitors were oriented correctly, all of my solder joints were solid, and nothing was touching or shorting. In the resistance checks, everything checked out except while the resistance of the left (black) RCA pot was 95.2 Ohm, right in the expected range, the right (red) RCA jack pot was 108.3, and I couldn't figure out why. In the live voltage checks, everything checked out except the six connected points that were supposed to be 90V were about 70V. Then just as I was finishing the checks on the octal socket, I heard a crackling sound and my tubes were going dark. Disconnecting everything, I discovered my fuse was blown, probably because I'd accidentally shorted something with my probe.

So i go to Radio Shack and buy more fuses (0.5A 250V fast-acting). My electrical engineering major friend and I both take a look at it. At one point the system I was attempting to use for plugging it in while looking at the underside fails and it falls a few inches onto my desk; some of the large capacitor leads snap off their connections and have to be redone. When we try it again, the tubes never turn on. I discover that the next fuse has also blown, immediately upon powering it on. we repeat this process again just to be sure that something has gone wrong in the circuit and it now doesn't power on at all. We discover that the resistance between terminals 1 and 2 on the transformer (where the live and neutral wires connect to) is about 12 ohms, which means a current of about 10 amps--no wonder I was blowing fuses!

So anyway, I have three main questions:
1. Is the mismatched resistance on the RCA jacks something to be concerned about? If so, what could be causing it? All the other resistances check out according to the instructions.
2. Again, is having some components at 70 V instead of 90 V a problem, and if so, how should I investigate it?
3, And is my transformer damaged? Or could something else be blowing the fuses?

[berniebanks]

If you have the 240V its posible you might have your transformer in backwards, mine was mislabelled.There's a lot to read in this thread that may help you

http://www.bottlehead.com/smf/index.php/topic,2453.15.html

Bernie

[STURMJ]

The fuse should be 1A slow. See the link here
http://www.bottlehead.com/smf/index.php/topic,930.15.html
(2ond to last post)
Hopefully that will take care of the start up problem.

[Doc B.]

Sounds like it was working right at first. When you shorted something with your probe you may have taken out one or more of the rectifier diodes in the high voltage power supply. You can check them with the diode checker function of your meter. I'm pretty sure there are multiple descriptions of how to do this on the forum. Basically you are measuring the resistance of each diode to see if any of them have shorted.

[dpitch40]

berniebanks: I am in America, mine is the 120V version.

Sturm: The parts list clearly said 0.5A fuse, and the slow-blow fuses had dots of something in the middle, which the original fuse did not.

Doc: I checked all four diodes. With my DMM's black lead on the silder banded side of the diodes, I got a reading of about 0.428 (appears to be volts, in the diode checker mode) for all four of them. Should I check anything else?

[galyons]

Doc: I checked all four diodes. With my DMM's black lead on the silder banded side of the diodes, I got a reading of about 0.428 (appears to be volts, in the diode checker mode) for all four of them. Should I check anything else?

Measure the diode resistance in both directions. One way the meter should indicate an open circuit. The other way you should get a resistance reading ,  then diode is good. If you measure an open circuit both directions, the diode is open, (bad). If you measure low resistance both directions, the diode is shorted (bad).

Cheers,
Geary

[dpitch40]

Gotcha. I measured the rectifier diodes the other way, and they all show open circuits, so they seem to be fine.

Also, possibly worth mentioning is that my kit came missing a few screws and the switch, so I bought another switch that fit. It is rated for 20 A at 125 V AC, so I didn't think it would be a problem.

[Grainger49]

It does sound like for once the fuse opened saving other more precious components.  It us usually the components that protect the fuse.

Do you remember where you were when you shorted?  The capacitors might have seen some damage.  Usually they will bow one end or both if that happens.

[Doc B.]

If the rectifiers look OK it could indeed just be that the 1/2A fuse couldn't handle the startup current. A 1A slo blo is what we ship in the kits, even though the manual may say otherwise. Give a 1A slo blo a try and see if it works. If there is still a short it will take out the 1A fuse same as the 1/2A.

[dpitch40]

Grainger: I was testing the voltage either B7 or B8. Presumably the probe touched one of the nearby contacts. Although B7 and B8 were both supposed to be at 0 V, so could connecting them have caused the short?

Doc: Okay, I will pick up some 1 A slow-blow fuses soon. I'm sure future buyers would appreciate having that reflected on the parts list.

[Laudanum]

To answer the unanswered ... Your measured resistance differences between potentiometer sections is normal with those pots.  Those resistance measurements are fine.

[Grainger49]

Ok, I looked B7 and B8 are the heater leads.  That is AC.  The most likely tube pins to short to are B1 (6080 grid/12AU7 plate) and B6 (6080 cathode).  Also, you cold have shorted B7 to B8, shorting the heater winding.  I see these as mostly drawing current and blowing the fuse.  I wanted to determine if you could have caused more current through the power supply caps than they could take. 

Go with the 1A Slo-Blow fuse and try again.

[Paul Birkeland]

You can also short the heater winding to the chassis, which would be B8 to the plate.

I'm sure you'll get your Crack back up and running, those caps are pretty tough!

If you feel the need to troubleshoot the amp by section (after getting everything back together), let me know.  It only involves temporarily lifting some connections and observing behavior.

-PB

[dpitch40]

I was testing B7 or B8 (can't recall which) with the front of the underside of the chassis towards me and the probe in my right hand in a pencil grip. Based on that, I think there aren't too many ways it could have shorted.

Laudanum: Great, that was what my computer engineering major friend thought as well. Is the 90 vs. 70V voltage difference a cause for worry?

Anyway, I will get those fuses on my way home today and let you know the results.

[dpitch40]

With the proper fuses it works the same as before I blew the first one. Trying it out for real now and it sounds pretty fantastic. Let me know if I should worry about that voltage though.