Ampeg SVT-6 Pro No Output

[ca7922303]

I looked at the website where you bought the MosFET's. The only thing I found there, which had to be toggled to get additional information on what they had to offer....they made NO MENTION of the parts being Matched for Transconduction...which translates to yielding equal Current sharing which is needed on these Ampeg SVT-series MosFET Output Stage amps. So, we have NO idea what to expect. That's why I wanted to have you slowly turn up the Bias so we could start seeing them conduct, and get readings without any going way high in current (voltage across the 0.47 ohm Source resistors) while waiting for those not yet turning on to see where we are. Not dissimilar to setting bias on power tubes, and matching them.

What we have instead is some of these that are turning on hard (334mV across the 0.47 ohms) or 5V across them). I have NO IDEA how you have this occurring. 5V across is 10A thru the MosFET, which may be dead/shorted now. With them mounted, best you can do is to unsolder/lift the 0.47 ohm Source Resistors up, unsolder/lift the Gate Resistors up, and also the 2.7k Current Sense Resistors up. That now has the MosFET only connected to the +/- 65VDC power supply busses. That has them isolated enough (with NO POWER APPLIED....disconnected from AC Mains) to THEN check each and ever one of them for shorted Drain/Source, Gate/Source, Gate/Drain.

Have you learned how to turn one on and off with your DMM in Diode Test mode? With your meter in this mode, we'll do a IRFP9240 P-Ch MosFET. Place your BLK probe on the gate, the RED probe on the Drain Then move the Blk probe over to the Source, leaving the RED probe on the Drain. It should measure something like 0.005V in either Drain-Source or Source-Drain. Then bias it the opposite way....RED probe to Gate, BLK probe to Drain. Move the RED probe to Drain, and BLK probe to Source. Should read around 0.51, and reverse....BLK to Drain, RED to Source, open circuit. For IRFP240 (N-Ch MosFET, reverse the probes and do the same thing. My Fluke has an 8V potential behind it's 1mA Current Source, so there's enough voltage behind the current source to turn on a MosFET like this. Can't say about yours.

You'll have to fully isolate your MosFETS (forget the fact that the Drain is soldered into the PCB....there's no circuit path to the other two leads having lifted the gate and source resistors, as well as the Current Sense resistors.

I hope you don't have shrapnel now installed instead of working MosFET's.

Before I remove all resistors, I wanted to take some measurements and see if anything stands out.
Lower Board- 9-5w source resistors@(.47 ohms)
1-5w source resistor@(7ohms)
2-sense resistors@(2.2k ohms)
5-sense resistors@(.55k ohms)
3-sense resistors@(.73k ohms)
10-gate resistors@(47 ohms)

Upper Board-10-5w source resistors @(.47 ohms)
1-sense resistor @(2.2k ohms)
4-sense resistors@(.55k ohms)
5-sense resistors @(.44k ohms)
10-gate resistors@(47 ohms)

[nevetslab]

I'm not sure if you have the service manual doc's, so I'm posting them here:

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The Power Amp PCB Schematic shows both the two output stage amplifier assemblies, as well as the AC Mains connection to the main power amp PCB assy, with wire colors and designation numbers. When I disassemble these and any like it, where there are quick disconnects and PCB designation numbers, I write those onto the connectors with a Sharpie so I don't loose track of where stuff goes. I see you did take photos, which is always wise! I looked at the photos I posted of both the SVT5-Pro and SVT6-Pro thread I had posted previously, and while I didn't see using any extender wires, I made myself a collection of male-female insulated 1/4" quick disconnects for when I need to remove, say, an SVT4-Pro from the chassis, where there, the power xfmr and mains wiring is NOT long enough to reach in my setup, I gain the distance using those. I'd have to go back to my service notes from when I took those photos to get further details. As I recall, the problem was associated with the power amp section, so I didn't need the preamp, apart from it needing to be powered. I inserted signal thru the Power Amp Input jack, which is on the rear panel, joined with the Preamp Output jack, and that gets disconnected when you patch into the power amp input jack.

Take a look at the Power Amp PCB Schematic. You'll find the measurements you're making, you are looking at a multitude of networks, all having effects of how you're measuring. I appreciate trying to take short cuts, but without having each resistor having one end lifted, you're going to get readings like you've listed. I'm not going to try and make sense of network readings. Unsolder one end....AT the MosFET end of them and lift them out.so you CAN then make proper readings and NOT have networks corrupt your readings. There is NO FAST WAY to bypass this.

[ca7922303]

I looked at the website where you bought the MosFET's. The only thing I found there, which had to be toggled to get additional information on what they had to offer....they made NO MENTION of the parts being Matched for Transconduction...which translates to yielding equal Current sharing which is needed on these Ampeg SVT-series MosFET Output Stage amps. So, we have NO idea what to expect. That's why I wanted to have you slowly turn up the Bias so we could start seeing them conduct, and get readings without any going way high in current (voltage across the 0.47 ohm Source resistors) while waiting for those not yet turning on to see where we are. Not dissimilar to setting bias on power tubes, and matching them.

What we have instead is some of these that are turning on hard (334mV across the 0.47 ohms) or 5V across them). I have NO IDEA how you have this occurring. 5V across is 10A thru the MosFET, which may be dead/shorted now. With them mounted, best you can do is to unsolder/lift the 0.47 ohm Source Resistors up, unsolder/lift the Gate Resistors up, and also the 2.7k Current Sense Resistors up. That now has the MosFET only connected to the +/- 65VDC power supply busses. That has them isolated enough (with NO POWER APPLIED....disconnected from AC Mains) to THEN check each and ever one of them for shorted Drain/Source, Gate/Source, Gate/Drain.

Have you learned how to turn one on and off with your DMM in Diode Test mode? With your meter in this mode, we'll do a IRFP9240 P-Ch MosFET. Place your BLK probe on the gate, the RED probe on the Drain Then move the Blk probe over to the Source, leaving the RED probe on the Drain. It should measure something like 0.005V in either Drain-Source or Source-Drain. Then bias it the opposite way....RED probe to Gate, BLK probe to Drain. Move the RED probe to Drain, and BLK probe to Source. Should read around 0.51, and reverse....BLK to Drain, RED to Source, open circuit. For IRFP240 (N-Ch MosFET, reverse the probes and do the same thing. My Fluke has an 8V potential behind it's 1mA Current Source, so there's enough voltage behind the current source to turn on a MosFET like this. Can't say about yours.

You'll have to fully isolate your MosFETS (forget the fact that the Drain is soldered into the PCB....there's no circuit path to the other two leads having lifted the gate and source resistors, as well as the Current Sense resistors.

I hope you don't have shrapnel now installed instead of working MosFET's.

I checked one of the sense resistors on the upper board that is reading (.55k ohms) removed and it reads (2.2k) out of circuit. I suspect that all of the resistors I pull will probably do the same. Could shorted mosfets be causing the resistors to read wrong or something else? Will pull all 3 resistors on the same MOSFET and post MOSFET measurements.

[ca7922303]

I checked one of the sense resistors on the upper board that is reading (.55k ohms) removed and it reads (2.2k) out of circuit. I suspect that all of the resistors I pull will probably do the same. Could shorted mosfets be causing the resistors to read wrong or something else? Will pull all 3 resistors on the same MOSFET and post MOSFET measurements.

Checked Q33, getting (.06v) diode test where you were looking for (.005v) and (0.6v) and (OL) otherwise.

[ca7922303]

Take a look at the Power Amp PCB Schematic. You'll find the measurements you're making, you are looking at a multitude of networks, all having effects of how you're measuring. I appreciate trying to take short cuts, but without having each resistor having one end lifted, you're going to get readings like you've listed. I'm not going to try and make sense of network readings. Unsolder one end....AT the MosFET end of them and lift them out.so you CAN then make proper readings and NOT have networks corrupt your readings. There is NO FAST WAY to bypass this.

All the 9240s on upper board are reading the same with resistor legs at MOSFET removed(0.6/0.6/OL) diode test.

[ca7922303]

Take a look at the Power Amp PCB Schematic. You'll find the measurements you're making, you are looking at a multitude of networks, all having effects of how you're measuring. I appreciate trying to take short cuts, but without having each resistor having one end lifted, you're going to get readings like you've listed. I'm not going to try and make sense of network readings. Unsolder one end....AT the MosFET end of them and lift them out.so you CAN then make proper readings and NOT have networks corrupt your readings. There is NO FAST WAY to bypass this.

Upper board 240s (0.5/OL/OL)red on source, black on drain(0.5v)

[ca7922303]

Lower board 9240s (0.5/0.5/OL)
240s(0.5/0.5/OL)

[ca7922303]

Lower board 9240s (0.5/0.5/OL)
240s(0.5/0.5/OL)

Got a 240 reading(0.0/0.0/0.0)

[ca7922303]

Got a 240 reading(0.0/0.0/0.0)

Replaced it, now reading (0.5/0.5/OL) diode test

[nevetslab]

Checked Q33, getting (.06v) diode test where you were looking for (.005v) and (0.6v) and (OL) otherwise.

DO YOU HAVE ANY of these MosFET's that have NOT been installed? Let's have you perform this test with your DMM so you can see one Turn ON and OFF with your DMM. Hopefully you have both a fresh non-shorted IRFP240 and an IRFP 9240.

We'll do a IRFP9240 P-Ch MosFET. Place your BLK probe on the gate, the RED probe on the Drain Then move the Blk probe over to the Source, leaving the RED probe on the Drain. It should measure something like 0.005V in either Drain-Source or Source-Drain. Then bias it the opposite way....RED probe to Gate, BLK probe to Drain. Move the RED probe to Drain, and BLK probe to Source. Should read around 0.51, and reverse....BLK to Drain, RED to Source, open circuit.

Can you do this test with your DMM, and get anything that resembles this? What we're doing it using the DMM in Diode Test to Turn the MosFET ON and OFF when applying gate voltage from your DMM. When you reverse the probes, you are NO LONGER applying the correct polarity, but instead you've turned the MosFET OFF. That's where you're reading a protection diode in the package in one direction across the Drain/Source, and when you reverse the probes, that protection diode is now reverse biased, and you read open circuit. Does this make any sense to you?

When you test an IRFP N-Ch MosFET, you're placing the RED probe on the Gate, and the BLK probe on the Drain. Then, moving the RED probe over to the Source, leaving the BLK probe on the Drain, and since you've turned the N-Ch MosFET ON, you're reading very low voltage across the Drain-Source in both directions. To turn it off, we verse the probes....BLK probe on the Gate, RED probe on the Drain. Looking at the Drain to Source, or Source to Drain, in one direction, you'll again read the protection diode across the Drain/Source, and in the other direction, it's open circuit.

Do you understand what your DMM is when it's in either Ohms mode or Diode Test mode? It's a current source. in Ohms mode, typically, you have a 2k range amongst the ranges on the meter. It provides a 1mA Constant Current in that mode. The voltage behind it varies with different mfgr's meters. If your DMM doesn't have a Diode Test mode, it actually does....the 2k Resistance mode. it will have at least 2V potential behind the 1mA Constant Current source, and you can test Semiconductors with it. It may be limited to 2VDC, so that's NOT enough to turn on a MosFET.

In Semiconductor Test mode, it's again a 1mA Constant Current Source with a DC Voltage source behind it. My Fluke 8060A has an 8VDC Source behind it, so devices that require around 4V gate potential, I can turn them on and off.....such as these IRFP 240's and IRFP9240's. It's possible your DMM doesn't have sufficient DC Voltage source behind it's 1mA Constant Current Source. To find out, if you have a second DMM, measure what the Voltage potential is in the DMM Diode Test mode. If no second DMM, use a scope to see what that DC potential is behind your constant current source.

This is basic Electronics, and learning what your meters and equipment can do and how to use them. Does this make sense to you?

IRFP240 N-Ch.pdf

I've attached a data sheet from International Rectifier....IRFP240. Look at the device symbol at the top of their page. There's what looks like a zener diode across the Drain to Source terminal. When you measure from Drain to Source in Diode Test mode on your DMM, in one direction, you will measure it's typical junction voltage, and in the other direction, it's open circuit. When you have Turned the MosFET off by applying the opposite Gate Voltage between Gate and Source, and measure between Drain and Source, in one direction, you're measuring this protection diode.

When you turn the part on, applying positive gate voltage between Gate and Source (needs to typically be in the 4V range, it will turn the device ON, and you will see VERY LOW Voltage between Drain and Source.

[ca7922303]

DO YOU HAVE ANY of these MosFET's that have NOT been installed? Let's have you perform this test with your DMM so you can see one Turn ON and OFF with your DMM. Hopefully you have both a fresh non-shorted IRFP240 and an IRFP 9240.

We'll do a IRFP9240 P-Ch MosFET. Place your BLK probe on the gate, the RED probe on the Drain Then move the Blk probe over to the Source, leaving the RED probe on the Drain. It should measure something like 0.005V in either Drain-Source or Source-Drain. Then bias it the opposite way....RED probe to Gate, BLK probe to Drain. Move the RED probe to Drain, and BLK probe to Source. Should read around 0.51, and reverse....BLK to Drain, RED to Source, open circuit.

Can you do this test with your DMM, and get anything that resembles this? What we're doing it using the DMM in Diode Test to Turn the MosFET ON and OFF when applying gate voltage from your DMM. When you reverse the probes, you are NO LONGER applying the correct polarity, but instead you've turned the MosFET OFF. That's where you're reading a protection diode in the package in one direction across the Drain/Source, and when you reverse the probes, that protection diode is now reverse biased, and you read open circuit. Does this make any sense to you?

When you test an IRFP N-Ch MosFET, you're placing the RED probe on the Gate, and the BLK probe on the Drain. Then, moving the RED probe over to the Source, leaving the BLK probe on the Drain, and since you've turned the N-Ch MosFET ON, you're reading very low voltage across the Drain-Source in both directions. To turn it off, we verse the probes....BLK probe on the Gate, RED probe on the Drain. Looking at the Drain to Source, or Source to Drain, in one direction, you'll again read the protection diode across the Drain/Source, and in the other direction, it's open circuit.

Do you understand what your DMM is when it's in either Ohms mode or Diode Test mode? It's a current source. in Ohms mode, typically, you have a 2k range amongst the ranges on the meter. It provides a 1mA Constant Current in that mode. The voltage behind it varies with different mfgr's meters. If your DMM doesn't have a Diode Test mode, it actually does....the 2k Resistance mode. it will have at least 2V potential behind the 1mA Constant Current source, and you can test Semiconductors with it. It may be limited to 2VDC, so that's NOT enough to turn on a MosFET.

In Semiconductor Test mode, it's again a 1mA Constant Current Source with a DC Voltage source behind it. My Fluke 8060A has an 8VDC Source behind it, so devices that require around 4V gate potential, I can turn them on and off.....such as these IRFP 240's and IRFP9240's. It's possible your DMM doesn't have sufficient DC Voltage source behind it's 1mA Constant Current Source. To find out, if you have a second DMM, measure what the Voltage potential is in the DMM Diode Test mode. If no second DMM, use a scope to see what that DC potential is behind your constant current source.

This is basic Electronics, and learning what your meters and equipment can do and how to use them. Does this make sense to you?

IRFP240 N-Ch.pdf

I've attached a data sheet from International Rectifier....IRFP240. Look at the device symbol at the top of their page. There's what looks like a zener diode across the Drain to Source terminal. When you measure from Drain to Source in Diode Test mode on your DMM, in one direction, you will measure it's typical junction voltage, and in the other direction, it's open circuit. When you have Turned the MosFET off by applying the opposite Gate Voltage between Gate and Source, and measure between Drain and Source, in one direction, you're measuring this protection diode.

When you turn the part on, applying positive gate voltage between Gate and Source (needs to typically be in the 4V range, it will turn the device ON, and you will see VERY LOW Voltage between Drain and Source.

Thanks for the information. Only have one 240 left. Used a 2nd dmm with diode test and got (100/500/OL). 1st dmm with diode test is only reading (.50) with red on source, black on drain.OL when reversed.

[ca7922303]

So replaced that one(1) 240 and resoldered all resistors back in place and now have about 3vdc on all the busses and 0.00mv across all gate and sense resistors with bias pots at full CCW and @ 110vac.

[nevetslab]

So replaced that one(1) 240 and resoldered all resistors back in place and now have about 3vdc on all the busses and 0.00mv across all gate and sense resistors with bias pots at full CCW and @ 110vac.

When you say about 3VDC on all the busses, is this just referring to +3VDC on the positive side Gate Busses and -3VDC on the negative side busses, same thing on the Current Sense Busses (where the 2.2k resistors feed to the current limiter clamps) and 3VDC on the output buss? Or just 3VDC on the Gate Busses, no polarity implied? 0VDC on the output buss that the MosFET's are feeding thru their Source resistors? You realize there IS a difference in what you said.

Also, what does 100/500/OL mean? Have you been able to turn a MosFET on and off yet with your DMM in Diode Test mode? Have you looked at it's manual to find out if it even CAN turn on a MosFET, which requires around 4VDC or -4VDC to turn it on or off?

I'm not willing to tell you all is fine, go ahead and with power applied, and NO DC Offset on the Output (sitting around 0V or within mV of that in either direction), and then, WHILE MONITORING Source Resistors, slowly turn up the Bias pot to start getting readings across the source resistors. As you have ten MosFET's that you have to monitor ON EACH OUTPUT STAGE, and the readings will no doubt vary wildly, as you DO NOT have matched MosFETs installed, its a critical step.

[ca7922303]

When you say about 3VDC on all the busses, is this just referring to +3VDC on the positive side Gate Busses and -3VDC on the negative side busses, same thing on the Current Sense Busses (where the 2.2k resistors feed to the current limiter clamps) and 3VDC on the output buss? Or just 3VDC on the Gate Busses, no polarity implied? 0VDC on the output buss that the MosFET's are feeding thru their Source resistors? You realize there IS a difference in what you said.

Also, what does 100/500/OL mean? Have you been able to turn a MosFET on and off yet with your DMM in Diode Test mode? Have you looked at it's manual to find out if it even CAN turn on a MosFET, which requires around 4VDC or -4VDC to turn it on or off?

I'm not willing to tell you all is fine, go ahead and with power applied, and NO DC Offset on the Output (sitting around 0V or within mV of that in either direction), and then, WHILE MONITORING Source Resistors, slowly turn up the Bias pot to start getting readings across the source resistors. As you have ten MosFET's that you have to monitor ON EACH OUTPUT STAGE, and the readings will no doubt vary wildly, as you DO NOT have matched MosFETs installed, its a critical step.

I'll check the busses again and post more accurate readings and I was able to get on a spare dmm a reading of 100 where you stated it should be .005v, 500 where you stated .50v and OL where you stated Open. The first dmm was showing .50/.50/OL at those points. So I assume MOSFET was turning on and off but most likely I need to get a better quality dmm, ones I have are probably in the $30-$40 range, not professional equipment obviously.

[nevetslab]

I'll check the busses again and post more accurate readings and I was able to get on a spare dmm a reading of 100 where you stated it should be .005v, 500 where you stated .50v and OL where you stated Open. The first dmm was showing .50/.50/OL at those points. So I assume MOSFET was turning on and off but most likely I need to get a better quality dmm, ones I have are probably in the $30-$40 range, not professional equipment obviously.

One clue you get when you've turned a MosFET on....you have the Drain and Source in conduction. You can remove the probes that turned it on, and it remains in that state until you reverse the probes and apply the opposite polarity gate voltage. Then that 'ON' State is gone. That's when in one direction, you can measure the protection diode junction, and in the opposite direction, it measures open circuit.

You still are not stating WHAT the DC Voltage source is of your DMM behind it's 1mA Constant Current Source. You can either read it in your instruction manual, or measure it with your second DMM in Voltage mode. Let's NOT Assume. Let's KNOW definitively.