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Old January 7th, 2011, 04:34 PM   #1 (permalink)
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Burned resistors on circuit board. Does this look fixable?

So my friend bought an me an amp the other day, and as I plugged it in, the thing started smoking.

Took the chassis out of the cab, and when the smoke had cleared, I saw some burned resistors and such on the circuit board:





The amp is a 60 watt Fender solid state BTW. Not sure what the specific model is, but I can check when I get home.

Looks simple enough to unsolder and replace the resistors in the burned area. But what would be the root cause and how would I find it? I guess I can test the effected parts for continuity and resistance, but what else should I do? What are those black squares with 3 lugs that screw into the heat sink? Grounding lugs? Can I even get those as replacement parts if they need to be replaced?

Thanks for the help, all. I've rewired several guitars, but haven't messed with amps much other than replacing an output jack or a speaker.

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Old January 7th, 2011, 04:59 PM   #2 (permalink)
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well, I'm pretty sure it's more than the resistors that are a problem, so you need to diagnose what may have cause the resistors to fail - one suspect issue is that transistor that's right next to the burnt spot.

Unless you know what you're doing - this may be a futile effort, but it's possible you can get lucky after replacing a few parts too.
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Old January 7th, 2011, 05:30 PM   #3 (permalink)
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Originally Posted by OpenG Capo4 View Post





Looks simple enough to unsolder and replace the resistors in the burned area. But what would be the root cause and how would I find it? I guess I can test the effected parts for continuity and resistance, but what else should I do? What are those black squares with 3 lugs that screw into the heat sink? Grounding lugs? Can I even get those as replacement parts if they need to be replaced?

.
Those black squares are power transistors, and the one next to the damage is probably shorted and the cause of all the damage.
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Old January 7th, 2011, 05:40 PM   #4 (permalink)
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It's almost certainly that to-220 transistor near the charred resistors. Until you find out what shorted the transistor, you'll almost just as certainly burn the new components up again. Did you do any mods to the amp? A 4 ohm speaker where there should be an 8 ohm speaker, etc.? Output transistors can just blow on occasion, but there is more often than not a logical reason why the outputs blow.
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Old January 7th, 2011, 05:58 PM   #5 (permalink)
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You definitely have a shorted output transistor. I got a Deluxe 112 solid-state Fender amp (65W, looks pretty similar to what I see in your photos) a couple of months back with a similar problem. Your photo of the underside of the board also shows a lifted pad on the rightmost output transistor...

When I got my amp, someone had already attempted to repair it, and had replaced diodes with resistors, made a mess of the soldering job, and gave up. I posted on here during the repair process, and will try to give any advice that I can to you now.

First, get a schematic for your amp. If fender doesn't have it on their website, send them an email--I got an owner's manual for the 112 within a couple of hours from them. I would carefully examine the entire output section and replace anything that might be even minimally suspicious. In particular, you will probably want to check the power resistors and diodes that run across the business end of the transistors very carefully.

Also, before doing any major work, I took the output transistors and shorted diodes out of circuit and made sure that the rest of the amp was okay. Once the misbehaving components are out, the amp shoud stop blowing fuses, and if it has a suitable output, you can run it into a slave amp. I used my old 15W Crate beater.

Anyhow, I accidentally ordered smaller transistors (TIP142 and TIP147), and figured, "what the hey" and played it until one shorted. It took two hours of continuous play at close to full volume in the gain channel to kill it. I got the first batch of parts from mouser, and had to go to digikey to get the larger footprint transistors for the second repair. For what it's worth, the speaker was blown when I put it together the first time, and was probably the culprit for the original failure.

Also, be careful when working on the amp, and when you do continuity checks on the transistors and diodes, be sure that your meter is set to check diodes, and also that values that you get in-circuit may differ than what you would get from out-of circuit.

I'll see if I can find a link to my old posts.
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Old January 7th, 2011, 05:59 PM   #6 (permalink)
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Here you go.

http://www.tdpri.com/forum/amp-centr...-packages.html
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Old January 21st, 2011, 01:50 PM   #7 (permalink)
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Thanks for the help. If I were to need the transistors or any of the other parts, would I be able to buy them from Fender? I've never dealt with their customer service before. I am assuming that for the other parts like caps, diodes, and resistors I can get them from the parts bins at Radio Shack?

How do I test diodes and capacitors? Pretty sure I can just set my multimeter to "ohms" to test the resistors, correct?

I guess I can also test the speaker by connecting it to another amp.
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Old January 21st, 2011, 03:40 PM   #8 (permalink)
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Originally Posted by OpenG Capo4 View Post
I am assuming that for the other parts like caps, diodes, and resistors I can get them from the parts bins at Radio Shack?
Wow. That's not a valid assumption any more. Ask around to find an actual electronics supply near you.
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Old January 22nd, 2011, 01:24 AM   #9 (permalink)
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At the risk of repeating what's already been said:

The fault relates to the high power output transistor, probably a TIP device. How and why it failed and how many components it took with it are open to your diagnosis and discovery. I can pretty well guarantee that you will be replacing a lot more than that transistor and the resistors that are visibly burned. In fact, you may well be replacing every black component associated with the power amp. Also check the big white 5W resistors.

This will be an extensive and expensive repair and probably not suited to a beginner. It's your call.

Now what may be a deal killer. That board is burned, there is black on it. That means carbon and it is conductive. Probably not enough to burn further, but enough to conduct between components. The first test is to see what is left when you scrape / scrub away the black stuff.
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Old January 22nd, 2011, 01:44 AM   #10 (permalink)
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I know the feeling.. I had a Crate V33H stop working on me while it was sitting on stand by, volume zero... a small pop...MMM?..

no tubes a glow, no sound..

pulled the board out for a quick scan.. saw a small blown resistor on the power board... and a blown circuit fuse...
I thought.. ahh easy fix here.. bought a 10c resistor and a fuse and soldered them in...
plugged it in power on POP!!.. blew the same ones again... oooooo not good..

off to the shop... semi conductor gone as well.. a mosfet unit...
none IC's of that type in OZ... source them myself from China to save shop some time... no probs... blew them trying to fix the power board...

got some more IC's from a cheaper source in China..... weeks go by... blew some of them.. gave up...

Shop rings... Mate.. whoever designed this power system needs to be shot, IT'S STUPID!!!... we can't get this amp going .. whatever we try... Voltages read good..plug in a guitar.. high voltages drop out.. goes again..
Come and collect it.... it was like picking up a dead dog from a vet...
4 shiny EL84's forever silent?... sigh

moral of the story... something blew the resistor in the first place... find that... a blown resistor is rarely the problem alone...

and avoid mosfet.... unless you have a rock solid no fixing or "replace the unit" money back,warranty..
. second hand?... shudder....

I hope you have better luck with yours...
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Old January 22nd, 2011, 02:25 AM   #11 (permalink)
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Skip Radio Shack for parts (unless it's for a quick test) because they are just not very good sounding and QC is not the best.

Am I seeing things, or does capacitor #56 have a bubble (convex) top?

You definitely have a shorted transistor as has been said, so start hunting and find the culprit of it's demise and you'll be back in business.

There is a vague possibility that the transistor failed on it's own and took the other components with it and those days are nice, but it is likely not that simple.

Still, nice of your friend to give you an amp and good luck!
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