Hello again Perry. I recieved this amp and it would not power on just pop the fuse. Some of the output fets were shorted so I replaced them all. Same thing pops the fuse. If I disconnect Q101, Q102, Q103, Q12 the amp will power up fine. Any thoughts??
I think this will be easiest to troubleshoot with DC.
Wire a pot (virtually any value 4.7k or greater will work) like the one shown in the attached photo. Connect it to the op-amp as shown in the other photo.
Remove R31 and R35 from the circuit. Set the pot to the center position before applying power. Rotate the pot about 90° each way from center (don't go to the end of rotation either direction).
When you do this, the DC voltage (measure with multimeter, not the scope) should go from positive 10v to negative 10v. Place the black probe on the source leg of the outputs and the red probe on the gate leg.
Check Q107 and Q104.
Wire a pot (virtually any value 4.7k or greater will work) like the one shown in the attached photo. Connect it to the op-amp as shown in the other photo.
Remove R31 and R35 from the circuit. Set the pot to the center position before applying power. Rotate the pot about 90° each way from center (don't go to the end of rotation either direction).
When you do this, the DC voltage (measure with multimeter, not the scope) should go from positive 10v to negative 10v. Place the black probe on the source leg of the outputs and the red probe on the gate leg.
Check Q107 and Q104.
Attachments
Ok my pots are 6 pin like the one you shown in pic but all in a straight line. Its a treble pot 5k for a DSM punch amp. How do I know which legs to use??
So which pair do I use? Should I use the A high A low and A wiper to the LM6171 or should I use the B high B low and B wiper to the LM6171. Not 100% sure which wires got to pins 2, 4, and 7 as you have shown on the LM6171.
To make it simple, use the yellow ones. They're in essentially the same order as the one with two rows of pins. The high and low are interchangeable. The wiper goes to pin 2 of the op-amp.
Hey Perry just got back from dinner. I built the test tool and installed it as requested and powered up the amp after removing R35 and R31.
Upon rotating the pot as you said checking Q104 and Q107 they both switch from positive to negative as you said. The DC voltage was 9.74 vdc.
What next?
Generally, this rules out all driver issues. Do you read infinite resistance (OL) if you measure from the gate leg of the 3415s that you have out of the circuit to the other legs of each individual 3415?
If you desolder the gate leg of the 6215s working with this group of 3415s, do you also read OL from their gate to each of the other legs of each individual transistor?
Did you check the gate resistors for this group of 8 transistors?
Did you replace this group of 3415s and 6215s?
If you desolder the gate leg of the 6215s working with this group of 3415s, do you also read OL from their gate to each of the other legs of each individual transistor?
Did you check the gate resistors for this group of 8 transistors?
Did you replace this group of 3415s and 6215s?
I dont have any of the output fets out of the circuit. Once I removed the 2 resistors you requested for the testing the amp powers up with them in the circuit.
I replaced all the IRF 6215 and IRF 3415 initially before we started trouble shooting. I replaced Q101, Q102, Q103, Q12 twice because I thought I might have gotten them to hot and thats why they were shorting.
Its just strange that when I disconnect those 4 transistors the amp powers up fine with R31 and R35 connected, then with those 4 transistors connected and R31 and R35 it wont power up. With the the 4 transistors connected and R31 and R35 disconnected the amp powers up.
If I test ohm readings with all output fets in circuit from the gate leg of one to the gate legs of the other 3 in each group I get 50 ohms all the way across.
Discharge the rail caps at the red and black wires.
Desolder the gate legs (or desolder the gate resistor for the FETs being tested) and check for leakage between the gate leg and each of the other legs for each transistor. All should read OL with your meter on ohms.
Do not apply power while the resistors or the gates legs are not connected.
If one (or more) is leaking, it could cause the problem to only show up when you drive it normally. Driving it with the pot produces much slower switching and slight leakage may not cause any problems.
Desolder the gate legs (or desolder the gate resistor for the FETs being tested) and check for leakage between the gate leg and each of the other legs for each transistor. All should read OL with your meter on ohms.
Do not apply power while the resistors or the gates legs are not connected.
If one (or more) is leaking, it could cause the problem to only show up when you drive it normally. Driving it with the pot produces much slower switching and slight leakage may not cause any problems.
Hey Perry should I test the 4 transistors that seem to make the problem or should I test them all. It seems only the 4 give me the problem. I was also wondering if I can remove the pot now or should I leave it hooked up?
Any thoughts??
Any thoughts??
If you haven't yet desoldered the gates, confirm that when the gate voltage switches (measured directly on the transistor) that the drain-source voltage also switches. For all outputs (only need to check one in each group of 4) the drain-source voltage should go from 0v to rail voltage. If that's what you have, you can remove the pot and reinstall the two resistors.
Check all 8 outputs in the group with the ones you had to remove to allow the amp to power up.
Check all 8 outputs in the group with the ones you had to remove to allow the amp to power up.
Hey Perry started checking things as you said, the drain was going to zero but the source was switching to + and - just like the gate. After checking a few the pot started clicking really fast then Q104 and Q25 exdploded with a huge blue fire ball, kinda scary.
Uhm I am at a loss, any thoughts ????
I dont know how or why, maybe it malfunctioned. Just started clicking by itself really quick for a few seconds then POW !!!!! The fets blew, and I think it blew alot of them not just the ones that exploded. I was hoping you had the answer. Maybe I was rotating it back and forth to fast ?? I dont know thought it was kinda wierd my self.
Well guess I should start by replacing all the ouput fets again???
I'm not sure what happened. If the pot started making a noise, maybe something failed internally and caused the problem. I would expect that pot to be rated for at least 0.25w so it should have been OK.
Could you have possibly shorted between the legs of the FETs when measuring the voltage?
Could you have possibly shorted between the legs of the FETs when measuring the voltage?
No I didnt short between legs, I am going to have to assume the pot failed internally, thats all I can think of. I guess I can try another pot but first I am going to have to replace the output fets. I think several of them shorted. I dont know what happened, everything was going good and then POW !!! Maybe since I was turning the pot fast it was switching to fast and the problem occured just dont know why it blew fets that were not even the problem to begin with. Maybe ther is other issues I dont know about. Any thoughts ???
Occasionally, if the pot is at the very center position, the FETs will switch on/off too quickly and the amp will go into protect but I've never seen them fail with the pot connected as shown. If you do the same thing with the op-amp removed (pot wiper connected to pad for pin 6), it's VERY easy to blow the outputs. That's why I recommend connecting it as you did.
While you have the outputs out, re-check the gate-source voltage to make sure it's still switching. If it's OK, you can remove the pot and re-connect the resistors. After tinning the backs of the transistors, check for leakage from the gates to the other legs (meter on ohms) to confirm that there is absolutely no leakage before soldering them to the MEHSA insulator.
While you have the outputs out, re-check the gate-source voltage to make sure it's still switching. If it's OK, you can remove the pot and re-connect the resistors. After tinning the backs of the transistors, check for leakage from the gates to the other legs (meter on ohms) to confirm that there is absolutely no leakage before soldering them to the MEHSA insulator.
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