Very loud humm from an amp - please help me fix it

[starfighter]

I have a Line 6 Spider III amp that makes a very loud humming/buzzing noise when I turn it on. It also does not amplify any input. The amp was working fine until I tried connecting an iPad in the aux-in port.

I want to repair the amp myself but have very little knowledge on how to do so. I am looking for some pointers from you kind people on what to look at first.

So far I have checked the output going into the speakers and it's about 20v (probably DC). I am not sure what to look for next. Please help.

[Enzo]

You look for the power IC with 11 or 15 legs, typically an LM3886 or TDA7293.  One or both are probably shorted.

[starfighter]

Thanks for your response. I will check as soon as I get home.

Do you think I can easily get new ICs? I will try replacing them if I can find them.

[Enzo]

Readily available from electronics parts suppliers.

[Jazz P Bass]

The Spyder III used a TDA2050 for the output section.

As far as I know that part is no longer manufactured.

You could try an LM1875 though.

[starfighter]

There is a reference point in the PCB after a LM317T IC that lists 3.3v as the reference voltage. I checked there and I see correct voltage.

I am not sure how to check if TDA2050 is malfunctioning. I want to make sure replacing that's where the problem is at. Do you guys know what I can do to test it?

Also, is LM1875 is a an equivalent replacement for TDA2050. From my limited knowledge, LM1875 seems to output less power? I am fine with that though. What I am interested is in if I replace TDA2050 with LM1875, do I need to change any other parts. Like the resistors or anything else?

[phatt]

If you have 20VDC across the speaker terminals then (as Enzo has already noted) the chip is likely fried.

Disconnect that speaker until you get zero volts on the output otherwise you could burn up the voice coil.

Check to see if the Zobel network on the chip output has signs of over heating, a sign there is instability,, possibly bad layout, circuit design.
If so then others here may know what is wrong.
The Zobel is the series resistor and cap across the output.  It should be as close as possible to the output terminals of the Power chip.
If there are signs of discoloration (overheating) then check that resistor is within tolerance.

To the best of my understanding the LM1875 will work,, though you may have to replace the zobel.
TDA2050 data shows 2R2/2Watt and 470nF,, while the data for LM1875 shows 1R and 220nF.

I just did a search on TDA2050 and spider 2 and 3 seem to have a habit of blowing.
so maybe bad design or maybe counterfeit chips. 
hope that helps,, Phil.

[starfighter]

I took a look around the PCB and everything looks normal (to my untrained eyes). There is a brownish layer on the guitar input plug metal and orange/brown color on the LM317T(see attachment). I am not sure if that is an issues.

I have ordered a few TDA2050 from ebay and am hoping to replace it soon. Hope what will fix my amp.

[starfighter]

Guess what folks? I replaced the TDA2050 today and my amplifier is working again! It’s a huge win for me. It’s almost magical. You replace a piece of silicon and everything is working again.

Thank you all for guiding me through this. You are all awesome.