Simple equipment to test audio amplifier and speaker

[rthorntn]

Hello everyone!

One of the channels in my 3.1 setup stopped working last night, just no sound from there at all, can I ascertain whether it's the AV processor, monoblock amp or speaker with my beginner knowledge and some electronics test equipment, I want to avoid swapping/unplugging/plugging the rest of the channels to test as it's not easily accessible?

I would like to start with the speaker (it's 12 years old and the lowest quality component by far).

I have a rarely used Rigol DS1102E and an Agilent U1272A as well as lots of electronics bits and bobs. Could I connect the speaker end of the speaker cables to my oscilloscope, start an AV test tone, what should I expect to see if the AV processor and amp are working correctly (also if this makes sense, which buttons should I press on the Rigol ?

Update: I disconnected the speaker and quickly tapped the speaker terminals with a 9v battery, noise comes out, i'm thinking that means that the speaker is not completely dead?

Update 2: I disconnected the amp from the setup and took it out and cabled it up, on the rear there are two LEDs, for 110v and 240v, I noticed that when first powered 240v was correctly lit but then a few seconds later it switched to 110v, i'm in AU so it's 200v+, so I peered inside, two blown fuses, after closer inspection they are T5AL250V, hopefully replacing them should fix it.

So a question, I don't have any surge protection on all this gear, which high quality protector (available in AU) would you recommend I use to protect it all, each item has it's own wall socket so individual protectors on each device would probably be best?

Thanks!

Richard

[alsetalokin4017]

Certainly replace the fuses but be prepared for them to blow again right away. Tracking down _why_ the fuses blow will probably require that you take the system apart to where you can get to all the connections, amplifier chassis etc.

The proper way to test the speaker would be to first disconnect it completely and probe the terminals with a DMM on resistance mode, to make sure that it is not shorted (it should have a low resistance but definitely not "zero"), then connect it to a known good audio source's speaker output and see (hear!) if it makes sound. Connecting it directly, even for a short time, to a 9v battery is probably not such a good idea as you could conceivably blow a good voicecoil that way.

Be very careful about connecting your scope directly to the audio output of your system. Be aware that the scope probe Ground reference is connected back through the Mains to ground, and thus will be connected to your AV system's Mains ground through your house wiring. Depending on the type of amplifier, the speaker output leads may not tolerate being shorted to ground by connecting one of them to the Scope Ground.  You could do some real damage to your amplifier by connecting the speaker output leads directly across the scope probe.  You could probably connect the scope probe Ground clip to the amplifier's chassis, which should be connected to Mains ground itself, and then probing the speaker outputs with the probe tip, one at a time. But before you even do this you should confirm that the amplifier's chassis is indeed connected to the Mains ground by its line cord.

From your question, probably the best "button to press" for you on your Rigol is going to be the "Auto" button........

[rthorntn]

Thanks.

I took the lid off the amp, the two fuses are right next to each other on the PSU pcb before it connects to a massive transformer.

I had just assumed the fuses blew because of a mains surge, all the other components on the pcb appear ok.

[Shock]

Thanks.
I took the lid off the amp, the two fuses are right next to each other on the PSU pcb before it connects to a massive transformer.
I had just assumed the fuses blew because of a mains surge, all the other components on the pcb appear ok.

They more likely blew because of a large increase in load (possibly caused by a short) in the power supply or attached circuit.

To answer your question, a multimeter and oscilloscope are the basic test equipment tools used to troubleshoot, another is a tone or signal generator but in a pinch you can use a CD, MP3 or other audio source. As mentioned connecting the oscilloscopes ground clip to a non floating circuit will create a short circuit.

I suggest if you need further assistance you start by taking some photos and listing the brand and model and clarify exactly you're working on. Start a new thread in the repair forum section if you need to.

[rthorntn]

Thanks.

It's this unit, Emotiva xpa100, just over a year old but I got it in the usa and it weighs a ton so shipping it back for repair is out of the question:

https://emotiva.com/products/amplifiers/xpa-100

The web page shows the internals and the manual mentions replacing the fuses.

I'm confused, prior to troubleshooting, should I try replacing the fuses or not?

Could someone please explain in a little more detail why an increase in load from the amp would cause a 250v 5a fuse to blow? The current goes up over 5a because of a short?  I think my lack of knowledge about fuses had me thinking that high voltage blew them!

[Shock]

Well if there was a storm or power surge then you know more than us, all you have said is one speaker was not working and now you have a dead amp.

If this is your first time around mains powered equipment then safety is a concern so if you don't know what you're doing or how to work on it safely, learn beforehand. Even when the amp is unplugged and removed from power there may be a residual charge in the capacitors.

As to the failure unless you can give us more info really anything can cause the fuses to blow, a shorted component can draw a lot of current. Normally you would look around the power supply area first and make sure the input and output of the supply are not shorted. Check the PTC, caps, diodes, filter caps, resistors, any burnt or melted tracks or components, transformer is still ok.

Then you would isolate the power supply from the rest of the circuit and replace the fuses and load test. Around this time is when the fuses normally reblow so buy spare fuses and of the right type/size.

If the supply works then you move on to test the rest of the circuit. Checking for shorts between ground and the supply rails and then lastly the output section of the amp. Most of the work requires just a multimeter.

Repairers avoid blowing fuses or damaging the circuit while they repair by using a variac or dim bulb tester (the latter being way cheaper - google). This allows current limiting to the whole device.

If the amp is in warranty and you didn't have a lightening strike or anything of that nature I would first be seeing where you have to send it for repair. In many countries you can do warranty returns to the retail store or local authorized service agents.

[bson]

It's probably too new to have a cap fail short.  More likely is one of the output drivers came off the heatsink ever so slightly, or its mounting was iffy to begin with, and overheated, failing short.  The easiest way to check for this is to disconnect everything including mains and check the resistance of each red speaker terminal against the rails.  If one is significantly lower than the others... there's your culprit.  The rail it's low against tells you which of the drivers is toast.

[rthorntn]

Thanks!

Its a mono amplifier so I don't think I have a second output driver to compare resistance with?

The output driver PCB is under a massive heatsink so I can't see anything, it feels secure, I have checked over all the visible PCBs and can see no signs of burst caps, I grabbed these.

http://www.jaycar.com.au/Passive-Components/Circuit-Protection/Cylindrical-Fuses-%26-Holders/5A-M205-Slow-Blow-Fuse/p/SF2178

I will try them tonight.

Cheers
Richard

[DrGeoff]

What was going on at the time the fuses popped?
Was it at switch on?
Was anything playing at the time?
What do the blown fuses look like (ie violently blown/blackened inside, or just a break in the wire)?

[rthorntn]

No idea when they popped (my wife sometimes switches on the amp when it's not required and on the front panel the blue LED still lights normally, so it appears to be working), one of the fuses is fairly violently blown.

What was going on at the time the fuses popped?
Was it at switch on?
Was anything playing at the time?
What do the blown fuses look like (ie violently blown/blackened inside, or just a break in the wire)?

[DrGeoff]

OK, I have seen amplifiers that have occasionally blown a mains fuse due to inrush current for a large toroid and capacitor bank, where there has been nothing at all wrong with the amplifier itself.

[rthorntn]

Thanks, so cross fingers and change fuses?

[DrGeoff]

In the absence of charring or burning smell (quite often shorted output transistors will smoke an emitter resistor or the driver preceding it) I'd look at the fuse replacement first, as it could have simply been inrush current.
If you have the service manual you may want to check that the bias has not drifted over the years too, but if you don't know how to do this then leave it alone.