Mains Fuse blowing up

[fabiomayo]

I'm very newbie to this.

The other day the mains fuse on my Dumble HRM (not built by me)
started blowing everytime I turn the thing on. It was working fine
until now.

What should I look for? Any shorts?

[groovtubin]

I'm very newbie to this.

The other day the mains fuse on my Dumble HRM (not built by me)
started blowing everytime I turn the thing on. It was working fine
until now.

What should I look for? Any shorts?

open the AC leads to the power supply first, and insure a GOOD transformer, if thats good, resolder the wires, LIFT the output of the BRIDGE or half wave rectifier, STILL GOOD? resolder, and LIFT the OUTPUT TRANSFORMER CENTER TAP, fire it up, all good?? Bad tube more than likely, OR worst case scenario, bad OT. then MAKE SURE you test the SCREEN GRID resistors AND BIAS resistors jim

[heisthl]

Start off with a good visual/olfactory inspection and no output tubes in the sockets.
The easiest way to work on an amp that is blowing fuses is to use the "Light Bulb" method. Put a new fuse in your amp of the proper size and rating. Take a light bulb fixture and put it in series on the hot side of an extension cord. Use a bulb size appropriate to your problem. Ex. a 100 watt bulb is approx 1amp, 40 watt approx .4 amps etc. There is no reason to use anything larger than a 100 Watt.
With the bulb in series all your amp voltages will be a little low but by using a voltmeter it should be fairly easy to see where all the voltage is being dropped( where all the current is going) - if nothing else the shorting component will get hot and maybe even start smoking - for this to occur the light bulb will be very bright. A normal amp without a problem will give a 100 watt bulb half brightness on first few seconds of turn on but it quickly fades very dim once the caps charge. The old timers used to say "If you can read the schematic by the light of the lightbulb you have a problem".

[fabiomayo]

Thanks for your rplies, people!


I guess being such a newbie I shouldn't attempt to do this repair myself.
I just can't help it.


Thanks for the tips. I'll probably have time tomorrow... Couple of questions
though...

1) groovtubin, how do I test the transformer? All I have handy is a multi-meter...

2) heisthl, I should hookup the light bulb between the fuse and the AC
outlet, is that right? Then just check the voltages to find any faulty
components?

By the way, without any tubes, the fuse still keeps blowing. This amp also
has a B+ 0.5A fuse and that's okay.

[heisthl]

A normal outlet has 3 connections - the earth ground, the common and the hot. If you look at a normal outlet you will notice the rounded hole(earth ground) and 2 slots - the larger of the 2 slots is the common and the smaller of the 2 is the hot.
You make a "Special" extension cord (or outlet box on a plug in cord) that has the light bulb fixture in series with the hot wire(cut the black wire in the extension cord, use your ohm meter to verify it goes to the smaller slot on the recepticle end and if so, screw each of the cut ends onto the fixture terminals) the earth and the common wires are left alone. Plug the modified extension cord into the wall outlet and plug your amp into this special outlet on the end of the extension cord. This way you don't keep blowing fuses as the light bulb limits the current the amp can draw to approx 1% of the bulbs wattage (I = P/E or restated Amperage = Watts divided by Voltage so with 100 watt bulb, 100 divided by 120 = .843 amps) This allows you to test things without blowing fuses. The beauty of using this method is you can turn on your problem amp and see how bright the light bulb gets. Turn your amp off and disconnect a voltage point at the first dropping resistor or choke. Turn your amp on and see if the brightness changed - If its still the same your problem is the Power Xformer, diodes or main filter caps, If its a lot less bright your problem is in the preamp or splitter stages. You can keep reconnecting and disconnecting the next thing(or previous thing) in the voltage chain until you find the point that is acting like a short. Bear in mind when you use this method you will have an initial bright bulb for a few seconds while the filter caps charge this should settle down to pretty dim if all is well. If you have another(working) amp you can try it on the "Bulb" to see the brightness levels I'm talking about.

IMHO this method works better than using a variac on troubleshooting shorts.
On intial amp builds it takes some of the anxiety out of the first power on tests.
Its also the cheap and lazy man's way of forming filter caps if you start with a really low wattage bulb and progress up.

[groovtubin]

Thanks for your rplies, people!


I guess being such a newbie I shouldn't attempt to do this repair myself.
I just can't help it.


Thanks for the tips. I'll probably have time tomorrow... Couple of questions
though...

1) groovtubin, how do I test the transformer? All I have handy is a multi-meter...

2) heisthl, I should hookup the light bulb between the fuse and the AC
outlet, is that right? Then just check the voltages to find any faulty
components?

By the way, without any tubes, the fuse still keeps blowing. This amp also
has a B+ 0.5A fuse and that's okay.

ALL ya gotta do to test the transformer, is OPEN the AC INPUT to the BRIDGE or half wave rectifier in the power supply, just find the (2) red wires ( if fender) that come off the power transformer. I`d open the BIAS winding as well, and PULL the output tubes. Put a good fuse in and turn on the POWER switch ONLY , if that doesn`t blow a fuse, the tranny is GOOD. Pls read prior post for rest of it, using SIMPLE methods you can fig this problemout w/o building your own test gear in the process, My bet is a BAD POWER TUBE, 99.9% of my repairs involving a TUBE amp blowing fuses is a BAD power tube, after 16 yrs, it HASNT CHANGED!!

[fabiomayo]

Thanks for the replies, guys.


I finally understood the light bulb fixture.

I'll try to make the troubleshooting as you both said tomorrow.

Oh, and one thing, groovtubin. I fired the amp today without any tubes
and still blowing fuses. Incidently, I put one of the 6L6 on my recent
Champ build and it worked fine. Tomorrow, I'll test the other one.
(the house is asleep for now )

Thanks so much!

[fabiomayo]

Hi.

Today I poked around the amp some more.

I couldn't make the light bulb fixture since it's Sunday, I don't have
a spare lamp socket and all stores are closed. I did tried to lift bias
and the AC input (everything else was still coneected) and fired the
amp with no tubes. Fuse kept blowing within a second or two. Only
reaction from the amp was the expected flash of the fuse blowing.

I also did a few of the tests recommended here: http://www.geofex.com/ampdbug/fuseblow.htm

Power supply filter caps appear to be fine (through the resistance
test). I even unsoldered a few. I also checked the choke and the
OT according to this simple resistance tests and it turned out ok.


This amp is very hard work on, access to the tube sockets is very
difficult but I don't think there's an obvious wiring short. From what
you guys were saying I can only assume the PT is faulty. Tomorrow,
I'll try and do the light bulb fixture if I have the time.


Any thoughts?

[fabiomayo]

Well... More than a month later, i finally got to work on the amp again

Hopefully, you be able and willing to help, if you please

Today I did the light bulb fixture (100w). Turned the amp on
(standby) and the lamp was VERY bright. Well, as bright as if I plugged into the outlet... The trannies were dead cold. I couldn't find any voltage
on the power supply capacitors or in any point of the circuit for that
matter, except the heaters. I measured 330mV AC on the jewel lamp
which was very dim of course.

All these were done without any tubes.

Should I try this with a 50w light bulb?

I realize only a megger test would ensure the PT is bad, but what
do you think?

Thank you for any help. It's appreciated!

[Allynmey]

Fabio, you can attach an ohmmeter to each prong of the iec connector (hot and neutral) WITH THE AMP UNPLUGGED AND CAPS DRAINED. Turn on the power switch and you should have very little resistance. If you have infinite resistance, you have a short to ground between the plug, PT primaries, switch ot wiring. If it registers a few ohms (PT winding resistance), Lift the heater CT and the PT Sec. CT and test conductance from the hot prong of the plug to chassis with the amp UNPLUGGED. If you have conductivity to ground, touch the one lead of your meter to each piece in the circuit until you lose the conductivity. The piece before the one you tested good is bad. If everything looks good and you have no conductivity to ground, check your standby switch for shorts. If the amp blows fuses with the standby switch in standby mode and it is not shorted itself, the problem lay before that. If it shorts when you turn on the stanby, the problem is usually a bad tube, arcing tube socket ot bad OT. Hope this helps.

Allynmey

[heisthl]

Make sure your lightbulb fixture is working by plugging in almost anything else (radio or another amp etc.) - it should be barely glowing. If it is working properly disconnect the PT secondary wires a pair at a time starting with the red ones until there is low glow(needless to say unplug the amp each time you remove a pair). If the red ones are the problem clip lead your meter to them and see if there is AC present (>600VAC). If there is you will have to use your ohm meter function to look for shorted diodes, caps etc. in the power supply.

[ayan]

I'm very newbie to this.

The other day the mains fuse on my Dumble HRM (not built by me)
started blowing everytime I turn the thing on. It was working fine
until now.

What should I look for? Any shorts?

Oi Fabio,

You were given some good suggestions already. However, my experience has been that if you get a catastrophic thing like that, out of the blue and provided the fuse blows instantly, there is a good chance your rectifier has shorted out. Bridges fail more readily because all it takes is for one of the 4 segments to short and you're toast. If you don't have a bridge, then usually there is a string of 3 or 4 diodes, so you get a few "lives" before your rectifier totally shorts out. Either way, I would check that first and then go from there.

Gil

[tonelab2]

Tech: I see the problem, you have a short in the power supply!
Customer: Well, can't you lengthen it!

[fabiomayo]

Thanks for all the tips, guys. I'll get back in the amp tomorrow.

The fixture works fine. I tried it in my recent Champ build and the
light bulb is barely lit.

Oi, Gil. This amp has a switch between tube rectifier (a 5U4) and
diodes. I don't know the specs but it only has one diode on each side of
the secondary. I'll check then.

You guys have been very kind. Thank you very much!
I'll run these tests as soon as possible and get back.

[fabiomayo]

Tech: I see the problem, you have a short in the power supply!
Customer: Well, can't you lengthen it!

That's funny!