"Blows Fuses" Debugging

[jwhousley]

I just looked at this thread as I had that fuse issue happening with flashing from the rectifier too.
I was a bit short on time so I had to find the issue quickly (other work to be done inside the amp).
Found out one of the first filters caps (two in serie) was shorted causing too much current draw from the rectifier.

Very nice How-To btw ! Thanks !!

Thanks for the information. How fast did the fuse blow upon power on? Was it random and inconsistent or ?

[PboBSG]

Please excuse me while I introduce myself as one of the infinite supply of newbies and a brand new member of this forum. My name is Rich and I am 72 years old 2 years retired from corporate life here in the USofA. I needed a hobby because I was failing retirement (again) and about to fail my marriage ifI didn't find something to keep me out of my wife's hair.

Being that I enjoy learning and new challenges and being that I enjoy music and I love guitar music of all kinds I went down this rabbit hole with my head down and me arse in the air.

I am a learning amateur, I have no desire to be in the business of amp repair and therefore no threat to anyone in that business. I am simply a hobbyist who wants to learn the obsolete technology of electronic audio as it plays out in the world of guitar amplification.

That, and I have 11 tube/valve amps that seem to be in a constant stage of needing something. I cannot afford to send them to a "tech" every time they need a new fuse, lamp, tube or biasing.

Thank you R.G. for a very helpful short treatise on blown fuses and also for the http://www.geofex.com/ link where I am headed next.
Cheers!

[pdf64]

I suggest that the best first step you could take would be to build a light bulb limiter and track down a selection of matching filament type bulbs in a range of wattage ratings.

[Matthews Guitars]

I use a variac and an accurate clamp-on ammeter across the AC line, and a hand held thermal camera. I bring the voltage to the unit up to the point where current draw is close to the fuse limit, and start scanning with the thermal camera for something getting hot that shouldn't be. This has been pretty effective.

That can save a lot of time. But fault isolation procedures shouldn't take a lot of time, either.

[R.G.]

@Matthews
Yes, there are a lot of other ways to run down a "blows fuses" fault. . The one I really like is feeding in a sine wave signal current and following the current down wires with a cassette deck read head. You can literally follow the current along wires to the short. There are many others.

I deliberately limited this procedure to things someone could do with a DMM, fuses, and a soldering iron. Kind of like can you do it with one hand tied behind you.

There are better processes. But a lot of newbies are not equipped either by training and experience or equipment to follow them.

[pureoldsound]

Hi all,

Didn’t want to start a new thread about the same issue.

I built an amp, a plexi based amp, 50w 2 EL 34, 3 12AX7 pre amp. I finished the amp and all the debugging after a new built was done. Voltages were great, and sounded great. Upon cleaning my desk, I thought I’ve disconnected my AC power cord, had 2 cords thought I disconnected the right one, and a piece of solder wire fell into the power switch…It was a bullseye…..it blew my power switch. I always add a MOV on the switch, Ala Trainwrecks, my home’s fuse tripped. I cleaned everything up. Replaced all the switches, as well as all the AC wires. Fired the amp up again and was working fine.

A week later went to play the amp and started blowing the main fuse only when switching the standby on. I have only 1 fuse (it is the correct amp slo-blow). I went to the geofex page and checked all the troubleshooting pages. I replaced my OT ( as I thought the 2ry was gone), all my filter caps, diodes.

It is still blowing my fuse on standby? I tried it w/ot tubes. Have about 3.33 volts on my heaters, 475v on primary filers. Could it be the PT secondary winding that is done? Is the only thing that I haven’t change.

[R.G.]

[...] a piece of solder wire fell into the power switch…It was a bullseye…..it blew my power switch. I always add a MOV on the switch, Ala Trainwrecks, my home’s fuse tripped. I cleaned everything up. Replaced all the switches, as well as all the AC wires. Fired the amp up again and was working fine.

A week later went to play the amp and started blowing the main fuse only when switching the standby on. I have only 1 fuse (it is the correct amp slo-blow). I went to the geofex page and checked all the troubleshooting pages. I replaced my OT ( as I thought the 2ry was gone), all my filter caps, diodes.

It is still blowing my fuse on standby? I tried it w/ot tubes. Have about 3.33 volts on my heaters, 475v on primary filers. Could it be the PT secondary winding that is done? Is the only thing that I haven’t change.

Short answer: Yes, it > could be < the PT secondary. But you can actually test the PT and OT electrically to find out if one is dead or not. Replace-on-suspicion is an expensive way to repair things. And if you're getting 475V on your first filter cap, it's highly likely that the PT is just fine.
I would get out my light-bulb limiter and quit paying $0.50 to $2.00 per fuse for each test. It's far cheaper and less confusing.

Failing that, if I didn't have a light bulb limiter, I would disconnect >>>every single wire<<< from the PT secondary; high voltage winding, and all bias and heater wires. I would wrap a bit of tape over every wire end to ensure that it's open. This leaves only the primary winding and the AC mains wiring connected. Now put in a fuse and turn on the AC, standby, etc that previously made the fuse blow.
If the fuse blows, something in the AC mains wiring is causing the overcurrent, because there is no secondary load. If the fuse does not blow, there is a flaw in something past the PT, and you can sigh in relief at not having to buy a new PT.
If the fuse blew, disconnect the PT's primary windings, all of them, with tape over each wire end. Now the PT is electrically removed from the circuit. Again, put in a new fuse and flip on the mains switch, standby, etc. If the fuse blows now, the fault is in the AC mains wiring and/or switches.
If you suspect a transformer is bad, you can test it with your DMM and an NE-2 Neon bulb. First, disconnect all the windings, > all of them <. Make sure they will not accidentally touch. Now with your DMM set to read resistance, check resistance within each winding. All of the primary leads will show a low-ish resistance to each other, for instance. The heater windings will have low resistances within the winding. Check every winding. This verifies that the windings do not have open circuits; for instance, it tells you that half the high voltage winding isn't open, etc. Note that a winding may have more than two leads. A heater winding and the high voltage secondary, for instance, will have three leads each. A multi-voltage primary may have several leads - common, 110, 120, 208, 220, 240, maybe. All the leads inside one winding will show a resistance below 1K, sometimes a lot lower.
Next, check resistance from each winding to the others. Pick any primary lead, and test resistance to each of the other windings. Since you have already verified that each winding is resistive to all its own leads, you only need to test one lead of each winding to all the other windings. All of these should be OPEN. If any winding shows a low resistance to another > winding < then there is an internal short between the windings. If not, you have verified that (a) the windings are internally connected, not open and (b) they are not shorted to one another.
The last test is a test for internal shorts within a single winding. For this, you need a battery capable of maybe an amp. A fresh 9V, a few C or D cells in series, or a 6V lantern battery will all work; and a neon bulb. An NE-2 bulb works GREAT. Solder a 100k resistor in series with it, and tack solder or wire-clip-lead it across the highest voltage winding. Make sure the other windings are all OPEN, not touching anything. Now touch the battery across any winding (the same one as the neon, or any other, they all work) and remove it. If the neon bulb flashes, there is no internal shorted winding. If no flash, repeat the test until you're sure you don't have an external short causing a false result. No flash means an internal shorted turn. Flash means OK. This works because the battery loads up the core like a car coil, and you get an inductive kickback when you remove the battery. The kickback rises until something eats up the kickback energy. If the neon is the easiest flash over point, it flashes. If an internal short exists, it eats the kickback voltage and the neon does not flash.

If a transformer ever once worked, and then passes these tests, it's almost certainly good.

Random other observations:
3.33V? Was that a typo? was it really 6.33V?

[pureoldsound]

Thanks for the reply. Will try disconnecting the PT 2ry wires.

I see some small burn marks on the 2ry wires that are on my star ground.

[pureoldsound]

It always measured 3.33v

[martin manning]

That’s normal if you have a grounded center tap on the heater winding and you are measuring from the heater pins to ground. Measuring across the heaters you will get 6.66 VAC.

[pureoldsound]

That’s normal if you have a grounded center tap on the heater winding and you are measuring from the heater pins to ground. Measuring across the heaters you will get 6.66 VAC.

Thanks Martin….

[Ranalog]

Good!
I am in the problem of step 2!
but my problem is that when I power on, sometimes the fuse blows, sometimes it doesn't.

The amp is a Bogner Helios 100w.
with a 2A slow blow fuse (in Europe, 230V)

I don't know if it's a problem with the power transformer or if the ignition Peak is so high that a 2 A fuse is not enough.

What is your opinion?

[R.G.]

[...]
but my problem is that when I power on, sometimes the fuse blows, sometimes it doesn't.
[...]
I don't know if it's a problem with the power transformer or if the ignition Peak is so high that a 2 A fuse is not enough.

What is your opinion?

I don't know which one it is either. I do think that the right way to debug this is to (1) get or borrow a light bulb limiter (2) perform testing, looking for the few times that the bulb remains brightly lit instead of going bright at turn-on, then fading lower. Note that if it never stays fully bright (i.e. fuse-blowing high current) then the limiter is holding down the inrush current to a value lower than would pop the fuse, indicating that it's an inrush surge problem, not an intermittent component or wire.
I also have an opinion that you need to be very careful messing around inside your amp with the AC mains connected and the power on. No amp is worth dying for.

[pureoldsound]

Well,

Just a follow up. It turns out is not the PT, OT, power caps. Changed the switches, and it still blowing the main fuse?

[pureoldsound]

Fixed it . I use Ken’s trick with the diodes in series at the PW tubes. It turns out one set was burned and could not see it due to the heat shrink disconnected both and it did not blow the fuse.

It turns out it wasn’t the ot, pt or the pw caps.

I’ll save those for another project then.