Ampeg SVT-6 Pro No Output

[g1]

Turning the bias all the way down won't do anything about bad Fets. They will still turn on hard and burn their associated components.
Please answer post #209.

[ca7922303]

Turning the bias all the way down won't do anything about bad Fets. They will still turn on hard and burn their associated components.
Please answer post #209.

This is how I tested Q1 per an article on internet: Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see “OL” (Over Limit).May 4, 2017.
Tested a N channel mosfet per post #115. Q1 has emitter/base/collector, mosfet has source, drain, gate if I'm looking at it correctly.

[g1]

This is how I tested Q1 per an article on internet: Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V.

This is one of 6 tests you need to do on a transistor, so a very incomplete explanation.
A transistor has 3 junctions, you measure each junction both ways (reverse probes for second reading).
So for that base-emitter junction reading, you would then reverse your probes and you should get 'OL' reading. Then you measure base-collector junction the same way, and should read .4 to .7 forward biased, 'OL' reverse biased. collector-emitter junction should read 'OL' both ways.

A fet gate/source/drain can be sort of like transistor parts but are quite different and you can't test them the same as transistors. Post #115 gives a complete test for Fets. If your meter won't turn the fet on and off you can't really test it completely. There are no shortcuts, aside from replacing parts with new ones if you suspect them. That will get expensive.

[ca7922303]

This is one of 6 tests you need to do on a transistor, so a very incomplete explanation.
A transistor has 3 junctions, you measure each junction both ways (reverse probes for second reading).
So for that base-emitter junction reading, you would then reverse your probes and you should get 'OL' reading. Then you measure base-collector junction the same way, and should read .4 to .7 forward biased, 'OL' reverse biased. collector-emitter junction should read 'OL' both ways.

A fet gate/source/drain can be sort of like transistor parts but are quite different and you can't test them the same as transistors. Post #115 gives a complete test for Fets. If your meter won't turn the fet on and off you can't really test it completely. There are no shortcuts, aside from replacing parts with new ones if you suspect them. That will get expensive.

Q1, per your instructions: (.63/OL/.63/OL/OL/OL).
N Channel mosfet per post #115: (OL/.011/.011/OL/OL/.53). It is turning mosfet on and off I believe. I have the upper board(Ch 1 - half per schematics) board has channel 2 stamped on it, with all consistent readings of all the mosfets(in circuit), other board has mosfets inconsistent readings. I'm going to pull all of the mosfets in the board and test out of circuit and replace I'm thinking.

[g1]

The transistor looks good.
I'm not sure what you are doing exactly with the Fet, but it seems it may be bad. Is it a new one? Do you have a new one to compare to? If it won't turn off (.011 from source to drain) it must be bad.
With the Fet testing you are charging the gate, and checking what happens between source and drain. The video below shows how to do it, and the second part shows a simple rig that could be used for fet matching.
The tests may or may not work when the fets are in circuit. The sequence of what probe you put where and when is important. N channel is shown, for P channel you would reverse the polarity (probes).
Here is also a written description, note that there is an important step before step '1)' : https://www.utm.edu/staff/leeb/mostest.htm

[ca7922303]

The transistor looks good.
I'm not sure what you are doing exactly with the Fet, but it seems it may be bad. Is it a new one? Do you have a new one to compare to? If it won't turn off (.011 from source to drain) it must be bad.
With the Fet testing you are charging the gate, and checking what happens between source and drain. The video below shows how to do it, and the second part shows a simple rig that could be used for fet matching.
The tests may or may not work when the fets are in circuit. The sequence of what probe you put where and when is important. N channel is shown, for P channel you would reverse the polarity (probes).
Here is also a written description, note that there is an important step before step '1)' : https://www.utm.edu/staff/leeb/mostest.htm

Thanks for all the great info. I am able to turn on and off the one spare N channel(240) mosfet I have spare. I will remove most all of the 10 mosfets on lower board(channel 1+) according to schematic, and test each one and replace as needed. Is it looking like bad mosfets are causing a load on variac? Thanks.

[g1]

Yes, the bad Fet's will most likely be shorted from source to drain and conducting hard as soon as you start applying voltage. So this will pull excess current from the variac.
The bad Fet's current will flow through their source resistors so that may account for the heat and smell. Check all associated resistors for any bad Fets.

[ca7922303]

Yes, the bad Fet's will most likely be shorted from source to drain and conducting hard as soon as you start applying voltage. So this will pull excess current from the variac.
The bad Fet's current will flow through their source resistors so that may account for the heat and smell. Check all associated resistors for any bad Fets.

Got it. What do you believe could have caused the new mosfets to go bad? I'm thinking it's only on the channel 1+ board that I here are bad mosfets. If that's true, what could cause mosfets to short on one board only?

[g1]

Many things will cause the Fet's to over-conduct and short out. Everything must be 100% right before any load or speaker is connected. Without a load, it is less likely that a fault will cause new Fet's to go bad.
It seems to me that you found other bad parts after you had connected the load and the new parts fried. It could be that they were the issue.

With all bad parts replaced, you want to bring up the variac slowly with no load on the amp. You monitor all the source resistors to make sure all Fet's are conducting equally, and check for DC at the output (pin4 of relay).

[ca7922303]

Many things will cause the Fet's to over-conduct and short out. Everything must be 100% right before any load or speaker is connected. Without a load, it is less likely that a fault will cause new Fet's to go bad.
It seems to me that you found other bad parts after you had connected the load and the new parts fried. It could be that they were the issue.

With all bad parts replaced, you want to bring up the variac slowly with no load on the amp. You monitor all the source resistors to make sure all Fet's are conducting equally, and check for DC at the output (pin4 of relay).

Removed all 10 mosfets on channel 1+ board. 6(3 P Ch, 3-N Ch) will not turn on. Pulled one leg on all 10 Sense resistors (2.2k). All good. Only 2 out of 10 read 2.2k in circuit so that's what made me check all of them. The gate resistors (47 ohms 1 Watt) and the (5w 0.47 ohm) cement resistors all read correct in circuit. Can I trust that they are good without pulling a leg on all 20 of those to test?

[ca7922303]

I'm going on the assumption that upper board(Ch 1-) mosfets are all good because they all read the same in circuit and the source resistors and gate resistors read what schematic shows in circuit like the lower board(Ch 1+) does. Sense resistors are reading different on both boards in most cases, so I'm thinking about not pulling one leg on the 20 resistors in question on (Ch 1+ board).

[g1]

I'm hoping the (1-) amp is ok as you said. Getting the (1+) amp readings into agreement with that is good strategy. You should not have to lift gate or source resistors if they read good in circuit.
If the sense resistors measure good but measure different than 1- amp when in circuit, look to parallel components. Previously I recall bad diodes causing the sense resistors to read off when in circuit.

[ca7922303]

I'm hoping the (1-) amp is ok as you said. Getting the (1+) amp readings into agreement with that is good strategy. You should not have to lift gate or source resistors if they read good in circuit.
If the sense resistors measure good but measure different than 1- amp when in circuit, look to parallel components. Previously I recall bad diodes causing the sense resistors to read off when in circuit.

Got 6 new mosfets that are a close match to remaining good 4. Starting to solder in now.

[g1]

Did you get all the sense resistors measuring same in circuit for both amps?

[ca7922303]

Did you get all the sense resistors measuring same in circuit for both amps?

Yes, all sense resistors are now reading (0.44k ohms) both amps. Bias pots are both on full CCW. Took variac up to 40vac and running normal. DC voltage readings are consistent on both boards.