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Discussion Starter #1
Fender Sidekick Switcher; when I power it up the speaker starts to buzz and ramps up rather quickly and becomes very loud. I took the chassis out of the amp and checked the connections on the board and can't see any physical signs of possible problems. With the speaker on the bench you can see the cone slowly start to pull in and then oscillate very fast with a loud buzz. Any suggestions??
 

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Hey bRian; Like 60Hz buzz? Then it might be in the ps section, flt caps, rectifier diodes, that's where I would start, watch those volts!
Check all internal wiring closely, check components for overheating, could be a lot of things.

Good Luck!
 

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Check if you're reading a dc voltage on the speaker lines.

I suspect you've blown the output transistors. When they're good, the power supply voltages are balanced out and there's no dc on the speaker lines, just any AC signal.

If a transistor shorts the power supply voltage dumps through to the speaker line. You hear a very loud 120 hz hum, which is low enough in freq that you might be able to see the cone moving very fast.

Check and let us know.
 

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Discussion Starter #5
Bill when I initially power it up, the voltage goes a little over 6 vdc and drops to 4 vdc. A friend of mine owns the amp; he's just learning to play and didn't want to buy a new amp in case it doesn't work out. I told him I'd take a look to see if it was a cheap fix. This is not your typical 60 cycle hum, trust me it is freakin loud, sounds like a ripped cone in a high powered amp.
 

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bRian said:
Bill when I initially power it up, the voltage goes a little over 6 vdc and drops to 4 vdc. A friend of mine owns the amp; he's just learning to play and didn't want to buy a new amp in case it doesn't work out. I told him I'd take a look to see if it was a cheap fix. This is not your typical 60 cycle hum, trust me it is freakin loud, sounds like a ripped cone in a high powered amp.
bRian, I'm not talking about typical hum. I'm talking freakin' loud! if the finals are blown then essentially the power supply is feeding directly into the speaker.

You should have NO DC voltage on the speaker lines, period!

If it's the finals, it needs a real tech.
 

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Discussion Starter #7
Bill thanks for the advice.What's the laymen's term for finals?? Output transistors like Michele mentioned??
 

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bRian said:
Bill thanks for the advice.What's the laymen's term for finals?? Output transistors like Michele mentioned??
Yeah! I've only had one of those in and it was a while ago. I don't remember if it had discrete power transistors or if it used one of the high powered modules.

Whatever, it's rarely so simple as to just replace the transistors. Why did they blow? What else blew with them? If you simply replace the transistors without checking the rest of the circuit properly you may blow up the new ones as soon as you turn it on. This can be an expensive way to troubleshoot an amp! Unlike tube amps, transistor circuits are all directly coupled and when parts blow they tend to take others around them down with them.

Sorry, but I really think you'll need a tech. What's more, being a less expensive solid state amp you'll have to decide how much of a repair bill is worth it to you.
 

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Discussion Starter #9
Bill, I'd say it's headed for the recycle depot. I'm going to advise him to pick up another used practice amp; no shortage of them around here. Thanks to all for the advice.
 
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