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Old 1st January 2013, 01:38 PM   #11
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Quote:
Originally Posted by 5th element View Post
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.
Cutting a trace in the PCB is a bit scary...I don't know where to start.

I might throw in the towel and bring it to somebody($$$)
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Old 1st January 2013, 11:01 PM   #12
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My money's still riding on an internally shorted regulator based on your comment "I accidentally touched one of the tube socket pins with a pair of pliers(saw sparks)". My advice is to just replace the regulator chip if you can get at it easily enough, they are cheap and easy to aquire. And as I mentioned before, if the are any protection diodes connected to the regulator, check those for short/open.

Mike
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Old 1st January 2013, 11:18 PM   #13
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Originally Posted by Michael Bean View Post
My money's still riding on an internally shorted regulator based on your comment "I accidentally touched one of the tube socket pins with a pair of pliers(saw sparks)". My advice is to just replace the regulator chip if you can get at it easily enough, they are cheap and easy to aquire. And as I mentioned before, if the are any protection diodes connected to the regulator, check those for short/open.

Mike
I had some Schottkys on hand so instead of testing it I simply replaced the diode. I'll follow your advice and order a couple of regulators before giving in.
Thanks
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Old 1st January 2013, 11:33 PM   #14
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The output being a half volt high is a little suspicious of the regulator, but if nothing else were broken the unit would still work.
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Old 1st January 2013, 11:35 PM   #15
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Well cutting a trace/removing a component, it isn't that hard and a cut trace can easily be repaired. Component removal would certainly be preferable. 3 pin regs are usually protected vs over current, short circuit and over heating. I've never blown a 3 pin reg before, but they aren't bullet proof.

Replacing the reg is fine, but if you're going to do that you can probably isolate the output of the reg in question quite easily anyway and test it like that.

I don't know what capacitance is connected on the output of the reg, but it is possible for a short to damage it due to the almost instantaneous currrent demand. If the reg needs a certain amount of capacitance to run stably then it is possible that it could be oscillating. This would also explain the voltage and heat.
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Old 2nd January 2013, 12:05 AM   #16
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Quote:
Originally Posted by 5th element View Post
Well cutting a trace/removing a component, it isn't that hard and a cut trace can easily be repaired. Component removal would certainly be preferable. 3 pin regs are usually protected vs over current, short circuit and over heating. I've never blown a 3 pin reg before, but they aren't bullet proof.

Replacing the reg is fine, but if you're going to do that you can probably isolate the output of the reg in question quite easily anyway and test it like that.

I don't know what capacitance is connected on the output of the reg, but it is possible for a short to damage it due to the almost instantaneous currrent demand. If the reg needs a certain amount of capacitance to run stably then it is possible that it could be oscillating. This would also explain the voltage and heat.
I'm not sure I understand...you mean that one/some of the caps in the tube circuit could have been damaged by the short causing the reg to overheat?
If that's the case is there an easy way of finding out which one(s)?

At one point I suspected one of the 3.3uF film caps in the tube circuit was the culprit so I replaced them but that didn't help - so I guess it's not them. I was having trouble getting the fat leads into the holes on the board - thought maybe it was a bad solder joint so I went back to the stock caps.

BTW - I already ordered the regs but it's no big deal.
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Old 2nd January 2013, 12:49 PM   #17
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It depends where the caps are. The ones, or more specifically the one, most likely, would be positioned right next to the regulator. It would connect between the output and ground.

Of course if there's further supply decoupling local to the tube (this would connect from the power rail to ground) then that too could be causing a problem. It is unlikely that the decoupling caps would be causing the problem, but it's not impossible so I though I'd mention it.

If replacing the regulator doesn't solve the issue then you really do need to isolate it's output and check that the reg on its own is working properly, then if it is, go hunting for where the culprit might be. Also you don't want to go needlessly replacing parts that are fine.
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