Reading a schematic to track down a power issue

[thared33]

I'm a beginner, and I mean a real beginner. I can solder, and see if something is getting power with a multimeter, but that's about it. I have a Mackie HRS120 subwoofer whose power is intermittent, so the issue has to be in the power section. When it does come on, it works flawlessly. I think I've uncovered the schematic but I'm not sure how to read it and follow along to see where the issue lies. I don't have enough $$$ to just buy another, and it wouldn't be cheap to ship it to a tech either, so before I give up, I thought I'd post here.

I've got it disassembled and away from the speaker cabinet, but what I don't know is if it's ok to plug the power in and check anything while it's disconnected from the speaker cab (like a guitar amp?). If so I can proceed, but deciphering the schematic is like reading hieroglyphics to me (am I allowed to attach it here?). I actually once rebuilt a power supply with help from someone from this forum, so I'm not a complete ignoramus ether.

Any help or advice is appreciated.

[jwet]

Most here have been reading schematics so long they don't even think about it.  Its one of those skills that will become second nature.  A good book for a relative beginner is "Electronics for Inventors" or similar.  It covers this and goes on to give you a good math free grounding.  If you can't find this, your library has a lot of basic electronics texts that cover schematics and the basics.  Don't look at anything too old, schematics from earlier than the 80's used different conventions.  Consumer electronic schematic are usually Asian and be a little odd too.  The basics are all the same with small variations.

You can plug the sub in.  Usually sub woofers are activated by the audio signal from the main amp so they don't have to have a separate on off switch.  You might have to provide some audio for testing- this could be your phone audio out, etc.  This circuit can sometimes be the culprit- the sense circuit is not activating the power relay to the sub.  This is usually a simple circuit with an op-amp amplifier and perhaps a rectifier that drive a transistor to pull in a power relay.  They put in generous delays (30 seconds) it doesn't turn off on quiet passages our pauses.

Be careful probing around in anything line powered.  Be especially carefully connecting or probing with an AC powered scope, etc.  This equipment is often "floating" or might have a hot chassis and scope commons are grounded.  Dave had a video that explained this called "How not to blow up your Oscope".  A battery powered DMM is relatively safe but watch it if you slip off a node, etc.

You can take some measurements and post some hypothesis here or in the repair group.  There are some experienced and helpful guys that are happy to help.

[pcprogrammer]

To get more help it is best to supply as much information as possible and that includes the schematic. Pictures of the board with a clear view of the components can also help in detecting if something looks of and defective.

One advice is to be very careful if a direct mains connection to the speaker cabinet is involved. 110V (or higher depending on your location on the globe) can do serious harm when it goes through your body. In case of a transformer used in the speaker cabinet, the secondary low voltage side is usually safe to work with as long as it is below a couple of tens of volts.

When there is a switching power supply be sure to stay away from the primary side, because that has a high DC voltage that is more dangerous then the mains AC voltage. On the secondary side there might be some seeping through the Y capacitor between the primary and secondary side, that can cause a tingling sensation. Proper grounding can take care of that.