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10k resistance on output of σ25

sigma25, sigma26, sigma24

10k resistance on output of σ25

Postby therobbstory » November 4th, 2016, 11:16 am

Long story short, my Alpha 10 preamp wouldn't come to life after work today. The transformer enclosure was burning hot to the touch, and I'm not sure how long it'd been that way. I've just cracked it open, and verified output voltage of the 9vac transformer.

I get about .01vdc on the output. The voltage regulator is very hot to the touch, and the LED isn't illuminated. Is there a way to test the regulator for fault before I order replacement? What would cause that? What else should I be looking for?
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Re: 10k resistance on output of σ25

Postby amb » November 4th, 2016, 11:39 am

Check to make sure that your C4 isn't installed backwards. That would cause it to become short-circuited and make the voltage regulator go into shutdown.
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Re: 10k resistance on output of σ25

Postby therobbstory » November 4th, 2016, 12:20 pm

Don't think that's it. It's been in use daily for over a year.

image: http://imgur.com/U5JltFk
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Re: 10k resistance on output of σ25

Postby amb » November 4th, 2016, 12:35 pm

OK, what happens if you disconnect the load? You can do this by flipping the σ25 around and plugging it into the backplane "the other way" -- i.e., the σ25 input is still connected to the input pins on the backplane, but now the σ25 will be hanging free on its output side with no load connected. Does the voltage regulator and transformer still get hot?

This will help to isolate the problem, whether it's on the σ25 itself or somewhere downstream.
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