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Old 31st December 2012, 10:31 PM   #1
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Default Hot regulator

My DAC has both SS and tube output on it.
I somehow managed to "mess up" the tube output a while ago and instead of trying to fix it have been running the SS output only with the tube pulled.
I'm not sure what needs to be replaced to get the tube output working again but I did notice that an L7812C regulator near the tube output is running really hot.
Hot enough that I'm unable to touch its heat sink for more than a second.
The two regs nearest the power supply are barely warm.
I'm wondering if that regulator could be the reason the tube output is no longer working and if there's any risk in operating the DAC's SS output.
I did plan on having it looked at by an experienced tech but if there's a good chance the L7812C is the culprit then I'll start by replacing that and properly resoldering the two 3.3uF film caps in the tube circuit.
Thank you
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Old 31st December 2012, 10:36 PM   #2
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The regulator IC is hot because something is drawing too much current.
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Old 31st December 2012, 11:50 PM   #3
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Quote:
Originally Posted by Frank Berry View Post
The regulator IC is hot because something is drawing too much current.
Like what?
I changed a diode right next to that regulator and the "temporary" solder job on the two 3.3uF film caps in the tube circuit is may not be textbook. It's a tight fit and I accidentally touched one of the tube socket pins with a pair of pliers(saw sparks)
At first I was getting static on one channel, last time I checked the tube was barely lighting up and there was almost no sound at all.
Normally the tube lights up even in SS mode. Right now I'm running it in SS with the tube pulled. Many people do this as it supposedly improves SS sound.
I'm not sure if it's my ears but the sound seems to be more anemic than it was.

Sorry if I'm bombarding you with a bunch of "useless info"
Just hoping something might lead to a diagnosis.
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Old 31st December 2012, 11:57 PM   #4
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The regulator may be internally shorted. Check the voltage at the output pin, it should be 12 volts, if not, try replacing the regulator, and if there are any flyback protection diodes around the reglator check those too for short/open.

Mike
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Old 1st January 2013, 01:12 AM   #5
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I'm getting 14.45 in and 12.67 out
How do I check the diode?
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Old 1st January 2013, 02:40 AM   #6
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Quote:
Originally Posted by McCrackers View Post
I'm getting 14.45 in and 12.67 out
How do I check the diode?
Well I replaced the diode and it's still not working...
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Old 1st January 2013, 03:34 AM   #7
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Your measurements are juust barely within specified limits for the LM7812C. If you can disconnect the load somehow and you see the input voltage go up and the output voltage stay pretty much the same, then the regulator is likely still good. It's only a guess, but the problem could easily be downstream. If you don't have a circuit drawing the next thing you could do is search for heat where the regulator output is getting dissipated.
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Old 1st January 2013, 03:50 AM   #8
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How 'bout telling us the manufacturer and model?
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Old 1st January 2013, 02:01 PM   #9
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Quote:
Originally Posted by Andrew Eckhardt View Post
If you don't have a circuit drawing the next thing you could do is search for heat where the regulator output is getting dissipated.
Unfortunately I don't have a schematic. I'm not sure what you mean by "disconnect the load" and I don't know what other parts could be causing the problem. From what I can see there are only caps and resistors between the regulator and the tube socket.

@ counter culture: It's an EE Minimax. Click the image to open in full size.
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Old 1st January 2013, 02:20 PM   #10
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Disconnect the load means disconnect whatever is connected to the output of the regulator. So far we know the reg is getting hot and is barely within spec re the voltage on its output. This could mean two things

1) The regulator is fine, but whatever is connected to it's output is damaged. This could result in the circuit connected to the output of the regulator, (the load) drawing an abnormally high amount of current, hence the heat.

2) The regulator itself could be internally damaged and this is whats responsible for the device outputting a voltage that is surprisingly high and for it getting worryingly hot.

You wont know which of these it is until you test the regulator without the load connected. This would most likely need you to cut a trace on the PCB, or you might be able to remove one component downstream of the regulator that will effectively disconnect it from the rest of the circuit.
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