amplifier Keeps blowing Fuses troubleshooting 101

[walters]

If a Amplifier keeps blowing fuses where to start and how to start troubleshooting ?

How to use the Variac in this situation have to bring up the voltage but when you bring it up to the operating voltage it blows the fuse so what does a tech. do in this situation?

If i use the Variac at HALF the voltage then the output of the power supply and regulator is going to be HALF B+ supply voltage so how do i know whats drawing to much current? thats blowing fuses?

If Changing all the power Transistors and it still blowing fuses and drawing to much current what would be the next this that is drawing to much current and make the fuses blow?

The Regulator being open or shorted would cause this?
A Filter Cap in the power supply would this cause this?

How do i use the Variac to track down the current draw? and flow blowing?

 

[walters]

How can i Test the Power transformer for shorts or opens in the secordary to see if its causing the Fuses to Blow ?


Should i Disconnect Cut/Break the traces to the power supply to the unit to isolate the problem to see if the power supply output is putting out the voltages?

If i put the oscilloscope probe on the rectifier i see half wave. If the power supply is outputing 300DC volts the recitifier half wave should be 150volts DC or 300vdc on the oscilloscope displayed from the probe?

If i put the oscilloscope probe on the (+) positive side of the Filter cap. i see the ripple discharging if the power supply is outputting 300DC should the oscilloscope
display 150vDC on the (+) side of the filter caps

If i put the oscilloscope probe on the output of the Regulator it should be a FLAT line with 300VDC outputing?

 

[Dean Huster]

Isolating the various loads is one way, but that can be somewhat difficult when a printed circuit is involved. Just don't forget that a blowing fuse is a secondary failure that's always caused by something. Don't ever let anyone talk you into the "weak fuse" theory.

The typical fuse blowers are shorted output transistors, shorted filter/bypass capacitors and shorted power supply rectifiers. The lists doesn't end there, of course, but those items probably account for 90% of the short causes.

Look for darkened/burned resistors that might be in series with overloads.

The crazy way to troubleshoot (I've done it after giving up all hope) is to replace the standard 1/4 x 1.25" cartridge fuse with a 1/4-20, 1.25" long bolt. Then the cause of the blowing fuse will often rear its ugly head in the form of smoke, fire, destroyed circuit boards, etc. I do not recommend this method.

This is where a schematic of the equipment is mighty handy. Then you could discover loads that can be disconnected by pulling a low value resistor or a jumper.

The Variac is used to bring the line voltage up slowly while monitoring the current through the problem fuse. Let the line voltage hover just under the fuse's blowing threshold and start checking voltages, looking for low voltages. Alternativly, you can measure cirucit currents, which is a bit more difficult unless you have a DC current probe and access for the probe in the equipment.


Dean

 

[walters]

The Variac is used to bring the line voltage up slowly while monitoring the current through the problem fuse. Let the line voltage hover just under the fuse's blowing threshold and start checking voltages, looking for low voltages.

So im looking for low voltages when im probing around the power supply or circuit?

The voltages are going to be HALF of the voltage because the line voltage is lower because of the Variac so am i looking for voltages around 5 volts and lower near ground because its a short?

 

[spuffock]

Looking for shorts with a variac is NASTY. The variac is low impedance, and you will either blow the fuse again , or find nothing by measuring voltage. Take the fuse out and connect a large bulb of suitable voltage rating across the fuseholder.If it lights brightly, then there is a short, and you find it by disconnecting things. If it's not so bright, then there's a heavy load, and something's getting hot. When you've sorted things and the bulb is very dim, then it's worthwhile putting a fuse in again.

 

[walters]

How does the Light bulb tell you stuff?

Is it the power supply how would the light bulb light if its

how would it light : dim,bright,normal
a.) shorted?
b.) open?
c.) loaded?
d.) good?

 

[Oznog]

Yeah and the variac can supply just enough current that it doesn't blow the fuse, but smokes previously undamaged components in the current path of the short. Be careful.

Also look for insulator failures on the transistors. Basically first you want to see if there's any continuity from any of the the transistors to heatsink (with it off). Then find the + and - rails and see if they have any continuity to a speaker output with it still off.

Sometimes you can disconnect stuff on the supply without desoldering it. If so you might be able to disconnect the power supply and see if that powers up by itself without blowing a fuse.

 

[walters]

Also look for insulator failures on the transistors. Basically first you want to see if there's any continuity from any of the the transistors to heatsink (with it off).

So my Red probe on the transitors chassis? and the black probe on the heatsink?


Then find the + and - rails and see if they have any continuity to a speaker output with it still off.

I put my Red probe on the (+) rail on the output of the power supply?
and put my black probe on the (-) rail on the output of the power supply?

What + and - rails are u talking about ?

 

[audioguru]

Your amp's output or rectifier tubes (valves) has a heater to cathode short, and the transformer's heater winding has a grounded center-tap.
Unless it is a transistor amp with 4.4kW output into 8 ohms with those 300V rails. :lol:

 

[walters]

So test the Output transformer primary side to see if 300volts is there
does the secordary of the output transformer have 300volts?

 

[Nigel Goodwin]

So test the Output transformer primary side to see if 300volts is there
does the secordary of the output transformer have 300volts?

If you have such a complete lack of even the slightest hint of electronics knowledge you SHOULD NOT be anywhere near a valve amplifier!.

 

[walters]

Can you please give me some basic 101 troubleshooting for a value amps?

Where should i put my oscilloscope probes and what should i be looking for ?

 

[zevon8]

walters, your interest and perseverance are commendable, but I fear you are attempting things that are beyond your present grasp. I am afraid you are only going to get hurt ( electrocution, ) or at the very least damage alot of gear and test equipment. When I see you post about testing a live chassis, then later post questions regarding how to make a very simple measurement it does not sound as though you have a good enough understanding of basic electricity to be doing what you are attempting.

I think you need to find some text books, or better yet, a structured electronics course.

P.S. no offence intended, but is it just me or do these threads remind anyone else of an Eliza test?

 

[walters]

If a Amplifier keeps blowing fuses where to start and how to start troubleshooting?

Where do i put my probes at ? where should i start ? what can go bad?

 

[monkeytree]

"what can go bad" in short (no joke) your life

 

[walters]

So can you help me so i don't Die when i touch the chassis how to i
protect myself from a LIVE chassis and what causes a LIVE chassis?

My meters and oscilloscope can get damages from a LIVE chassis how can i protect them also?

 

[wayneds]

Troubleshoot Guide

If a Amplifier keeps blowing fuses where to start and how to start troubleshooting ?

How to use the Variac in this situation have to bring up the voltage but when you bring it up to the operating voltage it blows the fuse so what does a tech. do in this situation?

If i use the Variac at HALF the voltage then the output of the power supply and regulator is going to be HALF B+ supply voltage so how do i know whats drawing to much current? thats blowing fuses?

If Changing all the power Transistors and it still blowing fuses and drawing to much current what would be the next this that is drawing to much current and make the fuses blow?

The Regulator being open or shorted would cause this?
A Filter Cap in the power supply would this cause this?

How do i use the Variac to track down the current draw? and flow blowing?

Hi! You can use the following to start trouble shooting
How To Fix the Fuse on Your Guitar Amplifier | How To Do Things.com
Although the article was intended for a Marshall Valvestate, the Power Supply section is similar...if you face any further challenges, feel free to contact.