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I'm not very well versed in solid-state amps, but I'm looking at a Line 6 Flextone for a friend. Seems the output transistor (LM3886FT) was smoked. I had seen other instances online with burnt transistors and they replaced them and everything worked. Well, I replaced it and I still have no signal coming out. The amp powers up and you can hear it thru the speaker, but after a few minutes, the LM3886tf gets hot and then shuts down. I have a signal back to the inputs on the op amps and resistors that appear to feed the pins 9 and 10 on the LM, but no signal(or next to none) at the pins on the LM. The voltage at V+ on pins 1 & 5 is 48v. I have no schematic for the amp and I know Line 6 is not good for giving them out so I am not sure what is going on circuit wise. Plus the components are so tiny I really can't even read the values on them. Like I said, the transistor stuff just isn't my cup of tea, but I'm trying to help out a friend. What should I be looking for and where? I can get some photos if that would help, just let me know. Any assistance would be appreciated.
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Here is a data sheet that might help. As you can see there should also be a negative 48 volts. With a normal operating power supply and a good IC there should be no DC on pin 3. -
Thanks Twist, Seems as there is -45v on pin 3 also. What would cause that? Bad trace on the board? Once you shut the amp down, the voltage on 5,4 & 1 all stay at 45(+ or-on 4) from the stored charge in the cap, but the voltage quickly drops off on 3. This is a brand new transistor.Comment
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I assume your saying you do have a negative 45 volts on pin 4. If you see any DC on pin 3 then you probably have blown another output IC. Make sure you have the heatsink tab on your IC insulated from the heat sink unless it's the isolated tab. When you replace it turn the amp on without a speaker load and make sure there's no DC on the speaker jack. You can run solid state amps without a speaker load for initial testing. You also need to inspect the copper pads where the IC solders in for breaks or debris across the pins.Comment
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The Lm3886tf has the tab to the heat sink insulated in the ISOLATED TO220 packaging according to National. So I mounted it as the original was mounted. I used heat sink compound at the junction also. I checked for voltages with the LM removed and found something like neg .175v at the pad for pin 3 and the pad for pin 9. That should just be from the V in- right? And the pad for pin 4 was neg 45-46v. Pin 1 and 5 were +45-46v. I don't see any crossover voltages anywhere. I'm not understanding what could have happened. According to National, this thing has Output protection from a short to ground or to the
supplies via internal current limiting circuitry and Output over-voltage protection against transients from inductive loads and all kinds of thermal protection. There must be something I'm missing here! It didn't pass a signal thru that ic from the get go. This could get expensive to keep replacing the LM plus shipping costs. Is the supply voltage too high? Sure would be nice to have a schematic with some voltages. I miss my old Fenders.Comment
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Anybody?Comment
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