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October 23, 2019, 02:13:50 PM

Author Topic: Greetings, and, uh oh...smoke  (Read 105 times)

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Offline VineSerf

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Greetings, and, uh oh...smoke
« on: October 09, 2019, 12:15:08 PM »
Hello All, first time kit builder here. I've just completed assembling a Crack 1.1 kit, and after being very careful throughout I got carried away and neglected to clean up one tag end from a capacitor during the last assembly session.  I did check the appearance against the photo of a completed kit at the end of the manual, and visually it appears to match. All resistance tests were successful.

When I turned the power on to conduct the voltage tests, the tubes glowed, but...  immediately the 5w 207ohm resistor between 15u and 21u gave up a puff of smoke. Damn indeed. I immediately turned the power off and gave the system the careful scrutiny it should have been given, and saw that the tag end from the positive side of the 250v cap at 14u and 15u had gotten tucked underneath (and thus I missed it when it should have been cut off) and was just the right length and angle to touch the contact at 14L. (I subsequently removed it).

No surprise I guess, that now while the tubes still glow, there is no voltage at any of terminals 1 - 10.

I'm guessing that at minimum the 5w 207ohm resistor needs to be replaced, but wonder also about all the other components connected to terminals 14 15 and 21. And is it possible that while the tubes still glow, the power supply has been compromised? Here is where I would very much appreciate any guidance you may wish to offer. Thanks in advance!

Steve R


Offline Doc B.

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Re: Greetings, and, uh oh...smoke
« Reply #1 on: October 09, 2019, 12:45:59 PM »
Use your ohm meter to measure across the 270 ohm resistor, to make sure it is still around 270 ohms. Then you will want to check the UF4007 rectifier diodes. Measure resistance across each one and verify that none have shorted, which would be indicated by a very low resistance like 50 ohms or less.
Dan "Doc B." Schmalle
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Bottlehead Corp.

Offline Paul Birkeland

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Re: Greetings, and, uh oh...smoke
« Reply #2 on: October 09, 2019, 01:00:29 PM »
No DC voltages means that resistor is open.

If you're able to power up the amp and see glowing tubes, your UF4007 diodes are probably OK.  They can provide pretty significant peak current, and for long enough that the 270 ohm resistor will end up being the weakest component with that kind of short. 

If you're in a bit of a hurry to get back up and running, a 200-300 ohm 5-10W resistor can be substituted for that part without any issues.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Doc B.

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Re: Greetings, and, uh oh...smoke
« Reply #3 on: October 09, 2019, 01:18:33 PM »
If you just want to fix it, follow PB's suggestions. If you want to learn something about troubleshooting along the way, make the measurements I suggest.
Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.

Offline VineSerf

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Re: Greetings, and, uh oh...smoke
« Reply #4 on: October 09, 2019, 01:34:42 PM »
Many thanks to both Doc and Paul for getting back to me so quickly. From your posts I'm a bit more hopeful that I haven't done anything catastrophic here. The course of tests and replacement(s) sounds simple and within my admittedly limited electronics skill set.

Much obliged!