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
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
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.
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.
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.
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.
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.
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($$$)
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
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
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.
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.
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.
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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