What is needed to troubleshoot a amp?

[TEK]

Hi

I have a 5 channel amp where I think that one channel is gone as there is a "sparkling/scratching" noise from the speaker connected to that channel.
This is a THULE Space PA350B that is over 10 years old. If I should guess I would think that it is some condesator that has dryed out and needs to be replaced.

But what equipment must I minimum have to find a correct a problem of this kind?
(in addition to soldering iron).

[ergo]

If you have a multimeter (DMM) you could check the DC voltage at output. It should read close to zero or not more than + or - 20mV or so. If that's jumping around then it might mean bit worse problems with operating point of transistors rather than just a dried out cap.

Also one of the first things is to power off the amp and disconnect/reconnect any connectors reaching that board multiple times. Sometimes the connectors corrode and do not give a good contact any more.
Assuming you can see the solder side of the amp PCB you can also make a thural visible inspection of all the solder joints. Especially the voltage amp stage and final stage (bigger transistors/resistors) gets hot and after years of use the solder joint might form small circular cracks around the component pin. In such case re soldering all such connections can fix the issue.

If none of these simple steps work then you can of course
* just bulk change all the capacitors on amp PCB with new ones. There's usually less than 10 electrolyte ones, so it will not be all that expensive
* get an oscilloscope and signal generator (or use a CD player or PC for sine signal playback)... Oscilloscope will give you visibility of the signal, but without the schematic at hand it's usually still only half a win as you can easily identify only the input and output nodes.

Not sure if there is a way to get the schematic for Thule...

[TEK]

Thanks a lot for your great unput ergo.
I'm a bit unsure how much effort I will put into this as I'm playing eith the idea of making a hypex based 5 channel anp that can replace the Thule amp.
Probably corse of action is to run it using only tree channels until I get the new one up and running, and then put this on the bench for some troubleshooting.
Might be a good reason as any to get me a oscilloscope - guess I should have that anyway so that I might check my own builds as well.
Will probably go for a picoscope - just have to figure out what type is good enough without making me bancrupt...

Will drag out my multimeter just to check what kind of voltage i get on the terminals... that should be done in no time.

[JonMarsh]

Checking for DC offset or noise is definitely a good idea.

Usually I would say trouble shooting needs a copy of schematics, a scope, and knowledge of the components used. Sometimes what you describe could be a corroded connection or a failing solder joint- but also it could be a transistor starting to break down. The only kind of caps that dry out these days are electrolytic caps, which are usually only used in the power supply. And that would show up as more hum voltage on the power supply rails and output, not what you describe. Sometimes electrolytic caps are used for signal coupling, but generally not in good gear.

[TEK]

Thanks Jon!
Schematics for the amp is possible to get hold on.
At least if you are willing to use a couple on dollars on it.

A bit more information.
With my multimeter on 200m I'm getting a reading of approximate 2.6 (on speaker terminals).
The reading does however flux a bit and is not steady. I get the same on another channel without noise, but there the reading seems steady.
I also notice that the noises are a lot less noticable when the amp has been without power for some time first.

A totally different question. I also measured the voltage on my mains where I have two hypex power amps.
One read 3.0. The other one read 14.0.
Both dead steady and there is no noise what so ever from any of the speakers. Why am I getting different readings? May it indicate that there are something wrong with one of my mono amps, or may this just be natural differences?

A general question: does a reading of 1.0 with the multimeter set to 200m on V means that I'm reading 1mV?

[Stoney]

Yes. A reading of 1.0 on the 200mV range would indicate 1 mV.

[TEK]

Thanks.
Just ordered a Picoscope 2205A with probes. Just a simple tool so that I will be able to do at least some degree of error checking myself.

[ergo]

Depending on the design of the circuit the DC offset can also slowly change during the warmup... and ideally stabilize after some 30min or so when all the temps have reached an operating stability. But even after that there can be differences between channels. Something like +/-20mV is not considered a problem yet. It does make the midbass/bass unit to move in or out by 0,xx mm though, so the more there is the more offset your speaker unit gets. So a 100mV and up would start to show a clear move in cone when you connect the speaker to amp or remove it while no music is on.

Many amps will have a small multi turn pots. In case of class AB amps one would be to set the current amp bias current and the other for DC offset. Then one could adjust this to be closer to 0. I would not do this though unless you get the schematic and know what pot is what. Messing those just for trials would be a bad approach.

Class D amps have very different operating logic, so I don't know how hypex modules handle this. Anyhow, I'd first let the amps to be turned on for an hour + and check it then. Perhaps it gets closer to 0 after warm up.

If you do get the schematic then feel free to PM me. We are almost in same timezone so we could have a Skype call for example and I can probably think of some things you can try/measure. I did repair of HiFi stuff some years back... You do have a benefit in having 4 working channels to compare to... so thats a plus.

[TEK]

Thanks a lot ergo, that's a great offer and I will probably take you up on that.
But I think that will be sometime in the future, and that I for now will just leave it as it is - running 5.1 instead of 7.1.
Not to keen to disconnect the amp and only have a stereo setup - so I will build a new 5 channel amp before I start troubleshooting this one.
By experience I do know that if I first disassemble the amp it might suddenly be a long time before it is assembled again - as we all know, slow works takes time.
With 3 children, 2 cats, a dog a full time++ job as well as some additional studies the slow work might suddenly become very slow work and take a lot of time...
I'm trying not to start to many things at the same time.