Ampeg SVT-6 Pro No Output

[g1]

Ch 1+ and Ch 1- are not reading exactly the same, but fairly close. P Channels(.040v), N Channels(.23v) diode test.

Shorted Fet's ?

[ca7922303]

Shorted Fet's ?

I don't believe so. Let me ask you this see if you might know, what would be the correct voltage at the mosfets gates at full power. You have said that the source resistors should not exceed 25mv across source resistors. Thanks.

[ca7922303]

Shorted Fet's ?

Also, I did a comparison of capacitors associated with the two amps. Per schematic, each line corresponds with other amp. Trying to find the voltage difference @ 20vac on variac.

[ca7922303]

Somehow the N3440(Q18) is bad now. After removing it and measuring voltage it seems to me like the voltages are consistent between Ch1 +/Ch1 - now. I'm thinking that's were they're supposed to be at. Will order another N3440 and try to find where the issue is with the collector of Q18.

[ca7922303]

The collector of the 2n3440(Q18) does not have a good trace to J48 which connects to W8 on Ch 1 (-) board. Q19 collector has good trace to J47 which connects to W11 on Ch 1(-) board. Collector pad does have good trace to C34. Will solder in jumper wire when new 2n3440 delivers.

[ca7922303]

With Q18 removed.

[g1]

I don't believe so. Let me ask you this see if you might know, what would be the correct voltage at the mosfets gates at full power. You have said that the source resistors should not exceed 25mv across source resistors. Thanks.

Your source to drain readings in diode mode (.04) indicate fets are either turned on or blown. You should know how to turn them on or off with your meter now, if not go back and read it over.

With amp at full line voltage, gate busses should be around + and - 3.4 volts. This is only valid if source resistors are good and source voltages (output buss) is 0V.
Very little change in these voltages will cause the fets to blow. That is why you have to constantly monitor the source resistors as you bring up the line voltage.

[ca7922303]

Your source to drain readings in diode mode (.04) indicate fets are either turned on or blown. You should know how to turn them on or off with your meter now, if not go back and read it over.

With amp at full line voltage, gate busses should be around + and - 3.4 volts. This is only valid if source resistors are good and source voltages (output buss) is 0V.
Very little change in these voltages will cause the fets to blow. That is why you have to constantly monitor the source resistors as you bring up the line voltage.

I am measuring gate busses to ground @ 20vac and am getting (10-14vdc). All source resistors read good in circuit, and the output buss(measuring across 5w source resistors) is 0v.

[ca7922303]

Your source to drain readings in diode mode (.04) indicate fets are either turned on or blown. You should know how to turn them on or off with your meter now, if not go back and read it over.

With amp at full line voltage, gate busses should be around + and - 3.4 volts. This is only valid if source resistors are good and source voltages (output buss) is 0V.
Very little change in these voltages will cause the fets to blow. That is why you have to constantly monitor the source resistors as you bring up the line voltage.

Will source to drain readings be correct in circuit or do I need to lift one leg of the 2.2k resistors to check mosfets diode test?

[ca7922303]

Your source to drain readings in diode mode (.04) indicate fets are either turned on or blown. You should know how to turn them on or off with your meter now, if not go back and read it over.

With amp at full line voltage, gate busses should be around + and - 3.4 volts. This is only valid if source resistors are good and source voltages (output buss) is 0V.
Very little change in these voltages will cause the fets to blow. That is why you have to constantly monitor the source resistors as you bring up the line voltage.

Just pulled one leg of gate/source/sense resistors of mosfets Q11 and per post #115, have the following readings: black probe to gate/red probe to drain(OL)
red to drain/black to source(0.05v)
Red to gate/black to drain(OL)
Red-drain/black-source(0.8)
Black-drain/red-source(OL).

Also performed the test from post # 215
Red probe to source/black to gate(OL)
Red-source/black-drain(0.08v) Source to Gate(OL).

This mosfet is turning on/off but the numbers in post #115 are different. Would the numbers make this a bad mosfet?

I'm going to pull source resistor legs on one(1) P and one(1) N mosfet from both boards and post results.

[ca7922303]

Q31 mosfet red to gate/black to drain(OL)
Black-drain/red-source((0.01v)
Black-gate/red-drain(OL)
Black-drain/red-source(0.5v)
Red-drain/black-source(OL)

On/off
Black-source/red-gate(OL)
Black-source/red-drain(0.01) Source-gate(OL)

Those are from Ch 1(+). Pulled one(1) each P and N mosfet from Ch 1 (-) board and the readings are exactly the same. If the mosfets I tested are good, is 20%(4) out of 20 mosfets enough to trust the rest? The other 16 read the same in circuit, but not what the four(4) out of circuit.

[ca7922303]

With Q18 removed.

I'm looking at all these Q component readings and I'm seeing 10v-14v on about all of them. If these are supposed to be in the 3-4v range, what part(s) could be causing these to read too high? Let's assume all the mosfets check out good(20% have so far), where should I be looking?

[ca7922303]

Disconnected j30, 31, 37, 38, 40, 41, 47, 48 again and of course Ch1(-) has no readings, but Ch1(+) gates are now reading (+3.4/-0.5v). Source and drains show no readings.

[ca7922303]

Now have Ch 1(+) gates @ (+2/-2v) w/ Variac@(20vac). Thinking when Q18 delivers, Ch 1(-) should come down also. Changed Q5/6/15/17 to get Ch 1(+) gates to drop.

[ca7922303]

Your source to drain readings in diode mode (.04) indicate fets are either turned on or blown. You should know how to turn them on or off with your meter now, if not go back and read it over.

With amp at full line voltage, gate busses should be around + and - 3.4 volts. This is only valid if source resistors are good and source voltages (output buss) is 0V.
Very little change in these voltages will cause the fets to blow. That is why you have to constantly monitor the source resistors as you bring up the line voltage.

Installed new Q18(1n3440) now getting this on Qs

Q1(59/59/59v)
Q16(mv/mv/0.4v)
Q3(2/14/14v)
Q4(-2/-14/-15v)
Q18(-15/-15/-15v)
Q19(15/14/14v)
Q5(9mv/9mv/2v)
Q6(9mv/9mv/2v)
Q15(mv/-15mv/12v)
Q17(mv/-15nv/12v)
Q23(12/11/1.1v)
Q24(12/11/14v)
Q2 (-2v/-1.5v/-76mv)
Q22(13/13/14v)

Ch 1(+) gates are (+2/-2v)
Ch 1(-) gates are. (+13/+15v)