Is this the right area to ask about solid state amp repairs?

Daniel55

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Friend of mine has a friend who found an amp in a dumpster and gave it to him. It only made a loud hum noise. (None of the controls affect the hum noise at all by the way) He in turn gave it to me hoping i could get it working, so I opened her up and found a burnt resistor. I replaced that part. The resistors I replaced was the big ol square ceramic heat proof type. Afterwards I plugged it in. The loud hum wasnt there but the amp still didnt work. Later on i got out a bright light and decided to look at the parts on the board closer and found a burnt transistor. So I ordered that and replaced it. I thought, wow maybe we fixed her up cheaply with minimal effort. Well when i plugged it in after replacing the transistor, the resistor i originally replaced started glowing and another big ceramic resistor burnt out. There are 2 of these almost next to each other. I replaced that as well. Now when I plug it in the original resistor I replaced glows red. I cant leave it plugged in long or I'll burn that one out again. I only have one more left and I'll have to buy more. I found a blurry schematic online. Near bottom center in what looks like a speaker. On the left of that is where the 2 resistors are. R73 and R74. I'm not great at schematics yet and I still have a lot to learn about electronics. Any help or suggestions on where to look for the reason these keep burning up would be great . Thanks! crate_gx_65_sch.pdf_1.png
 

Pete Farrington

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Solid state amps are difficult to fix, most likely several parts are bad, and until all are replaced, it’ll carry on roasting stuff.
Google how to test transistors, at least one probably has a shorted junction.
If you’re getting tired of burning out parts, a light bulb limiter would be a really good idea.
 

Daniel55

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The sound it made just seems like something shorting out. I'll check out how to test transistors. Thanks for the starting point! We aren't worried if it gets fixed or not because it was free. I just wanted some more experience.
 

Jon Snell

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Quite straight forward; Replace both of the Darlington pair output transistors, NOT from fleabay! and R73 & 74 for the correct type of ceramic resistors.
Adjust AP2 for 7.5mv across R74 then connect a loud speaker to test.
 

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Daniel55

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I did buy 3 transistors when I ordered the other one. Not from Fleabay... lol I did replace R73 and R74 with like parts of the same value as the stock ones. The transistors you are talking about are the ones labeled 13 and 15 on opposite sides of the 2 ceramic 73,74 resistors? There is a second sheet with the board layout and listing of parts values. There is a small pot soldered to the board I remember. Thanks again for the reply. Any help is highly appreciated.
 

thetragichero

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i'm going to second bringing it up on a lightbulb limiter until it is no longer sick
when it comes to failure modes tube amps are relatively forgiving compared to solid state. until it's working right every time it's turned on while broken it is likely breaking more and more. certainly not for the easily discouraged or faint of heart
 

South Park

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When you have a bad transistor you have to replace the resisters that control the current to the transistors . If one goes both go You can test transistors it is not easy . you have to replace the power and ground resister
 

KraftyBob

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You have to test transistors out of circuit or you won’t get accurate results. Set your DMM to diode. When measuring from the base to the collector and base to emitter you should get somewhere around 600-700mv. Then measure the opposite way (e.g. collector to base and emitter to base) and you shouldn’t get a reading.

To know which leg is the collector and which is the emitter: the base to collector value will always be slightly less than the base to emitter. For example:
  • Base to Collector = .697v
  • Base to Emitter = .701v
 

Daniel55

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Learned how to build a dim bulb limiter. So I'll do that later after a stop at a hardware store. Good to have no matter what when working on this type of thing... After that I'll check the power and ground resistors. Thank you for the info on testing transistors!
 

Matthews Guitars

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A burned resistor is never the problem, it's a symptom of the problem.

The transistor you replaced that caused the resistor to get hot again, is biased to full conduction, so since it's new it's probably not bad,
but is being driven wide open by the driver transistor in front of it. It's more likely to be a bad driver transistor. And as was mentioned, this is a Darlington pair arrangement, so you replace both at once with quality, name brand components. NOT NTE branded substitutes.

The driver op-amp, IC3, is suspect. I'd be looking for DC on its output.
 

Daniel55

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Thanks Matt. I was able to get that dim bulb limiter built last night while watching the twisted sister biography lol , so the plan is to go over the amp this weekend. Thanks for the suggestions on what to check! I'm determined to get this dumpster amp working :dude:
 

alpha al

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Be VERY careful with the speaker you are using to test the amp. Blown solid state amps tend to put out DC, usually the full power supply rail voltage (could be positive or negative).

The hum you hear is AC, but the DC which you can't hear damages the speaker.
 

Daniel55

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I hooked up my meter to the speaker leads unplugged from the amp and get no reading at all. I’m thinking the speaker is junk now. I’ve replaced Q10 and Q11 transistors. Both had burnt leads. R68 and R71 should be 220 ohms but they aren’t reading correctly. I ordered some cuz I don’t have that size in my stock pile. I also tried to get a reading on the darlington transistors Q13 and Q15 power transistors and I don’t get anything at all no matter which order I hook the leads to them. I ordered a set of those too. i can’t see anything else burnt.

I don’t get any hum at all now, no sound what so ever. I did hook up and external speaker to the external speaker out as well and no sound. Guess I have a few weeks to wait for parts and try again.
 

PelliX

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While you're waiting for parts, go ahead and get yourself a (cheap?) component tester. Diodes are easy enough to check with an average multimeter, but transistors, MOSFETs, etc are a bit harder.

1654429693009.png

These testers can generally test NPN/PNP transistors, FETs, diodes, inductors, resistors of course, capacitance and ESR measurements and so on. At a small pricetag, they're well worth it, even if you hardly ever need them again.
 

william vogel

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The speaker could have been shorted to begin with and causing a low enough impedance to burn any of the output stage components.
 

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