Blowing fuses

[octavian]

Hi all,

My Stereomour II has started exhibiting a really weird issue after 2 years of functioning flawlessly. It's blowing fuses. But here's the kicker, it's doing it when turning off! While on, it functions perfectly, no weird noises, usual super-low noise floor etc. I'm stumped. What could be wrong with it? Could the switch have gone bad and is shorting somehow while being flipped to off (not sure this would even make sense as the cause)?

Thanks all.

[Paul Birkeland]

What replacement fuses are you using? 

It is indeed strange that you are blowing a fuse at power off.  What voltage is your AC mains, and are your outlets grounded?

-PB

[octavian]

Outlets are grounded, same fuse as original 1.5AMP Fast Blow 250V. Outlets are grounded, it's plugged into a power filter bar with surge protection (as it's been for the last year).

[Paul Birkeland]

Can you test to see if the amp still blows fuses if you pull the power cord to turn it off instead of using the power switch?

[octavian]

Just tried to turn it off by yanking the power cord, still blew the fuse, so I guess it's not the switch.

[Paul Birkeland]

I'd be interested in seeing photos of your build, particularly of the wiring around the IEC power entry module, and the STP that runs up to the front power switch.

Can you share where you're located and what your AC mains voltage is? 

[octavian]

Thanks! I haven’t checked the voltage in a while, but I’m in Chicago, and if I remember right it tends to be around 118. I’ll have to take some photos tonight, but in the meanwhile, here’s a photo from my original build thread: https://bottlehead.com/smf/index.php?action=dlattach;topic=8117.0;attach=9346;image . Could one of the power tubes cause this issue? I guess they’re getting up there in hours.

[Paul Birkeland]

There isn't a Stereomour II in existence that could have been operated long enough to exhaust a pair of 2A3s (unless they were the ultra cheap Chinese ones).

You could certainly power up, then turn off the amp with no tubes installed.

Can you plug in one of those 3 light outlet testers into your outlet, just to be sure there isn't anything weird going on with the power?

-PB

[octavian]

I don’t have one if thise testers, but I can do a quick voltage check with a multimeter and the power bar tests for ground. Could that be enough? The weird part is that I’ve had no issues with any of the ither devices that are plugged into it, only the amp.

[octavian]

Of course, there is the other option. It could be blowing fuses when turning it on. Would the cause be more straightforward in this case? (Though it’s never done this after changing a fuse)

[Paul Birkeland]

If the fuse blows when you turn it on, then the amp would turn off and you'd get no sound out. 

You can actually test your outlets with your DVM.  Set it to AC volts (at least 200V AC if it's not autoranging).  Put the black probe into the round ground hole, then measure the AC voltage at the narrow slot and wide slot.  The bigger slot should show 0V and the smaller slot ~120V.

-PB

[octavian]

Okay, so I looked closely underneath and there seem to be zero issues - no bug causing a short, etc. It looks just like the day I finished it. I also figured out that it is blowing fuses on turning on. I put a fresh fuse in, turned the power on, then the fuse blew pretty much instantly. I haven't been able to test the socket yet (neither of the sets of probes I have for my DVM fit in a socket! Sigh, I'll have to get an extra set). However, the other stuff I have plugged into this same power bar isn't exhibiting any problems or blowing fuses, so I'm thinking it's probably something in the amp.

[Paul Birkeland]

So have you been able to listen to the amp at all when you've had this problem?  When you posted that the fuse blew when you turned the amp off, there was a presumption that you were listening to the amp prior to having this issue.

[Doc B.]

It's difficult to guess what has happened without more consistent description of when the fuse is blowing, but I would be inclined to investigate the switch and the IEC socket to see if something has come loose and able to short because of it.

One also needs to always be cautious of trying fuse after fuse without determining what is causing them to blow, because that can cause expensive damage like a blown power transformer. If a rectifier has blown or a cap has shorted it must be replaced before powering up the amp.

[octavian]

Hi both,

So I originally (wrongly) suspected the fuses blew when turning it off, as it had never blown when replacing the fuse until the last time. So here's where I'm at. The amp sometimes works just fine, but occasionally blows a fuse on start up. When it's working, it exhibits zero issues, no weird noises etc. I checked the underside and all of the wiring seems just fine on both the IEC and the switch. It blows very quickly when turning it on - the tubes don't light up at all etc. Could it be a defective power tube? Is the switch still a likely culprit?

Thanks all. Sorry for the perplexing puzzle.