Ampeg SVT-6 Pro No Output

[ca7922303]

That seems likely. Though R116 being a current limiter source resistor to Q5, which you already stated was bad & removed, have you verified all of the MosFET's are ok? With them out of circuit, I can turn them on and off with my Fluke 8060 in Diode Test Mode (has 1mA current source, and 8V potential behind it. If they have failed, they you normally see short circuit between Source/Drain and Gate. And, when they fail, the 0.47 ohm/5W Source Resistors often burn open. If these are reading 4.7k, then it sounds like they're open circuit, and you're reading something else across those. If they're good, you'll read around 0.47 ohms.

Photos of boards above and below heatsink.

[nevetslab]

You will need to lift one end of the gate resistor and source resistor at each MosFET to try this. Applying test current in Diode Test mode from my Fluke...RED lead to Gate, Blk Lead to Source, it will turn on, assuming this is a working MosFET, Then I move the leads to check if I have a low reading between Source and Drain. Reversing the leads from Source to Drain will turn it off, so there is no reading between Source and Drain. If the MosFET has failed short, then you read short ckt between Gate/Drain, Gate/Source, Drain/Source. Sometimes I can't get these to turn on/off, but with the IRFP240 N-Ch and IRFP9240 P-Ch parts, I've been able to test them. With the gate and source resistors connected, you can't turn them on and off. Shorted MosFETS....not so. Those shorts are permanent, and can be read

[ca7922303]

You will need to lift one end of the gate resistor and source resistor at each MosFET to try this. Applying test current in Diode Test mode from my Fluke...RED lead to Gate, Blk Lead to Source, it will turn on, assuming this is a working MosFET, Then I move the leads to check if I have a low reading between Source and Drain. Reversing the leads from Source to Drain will turn it off, so there is no reading between Source and Drain. If the MosFET has failed short, then you read short ckt between Gate/Drain, Gate/Source, Drain/Source. Sometimes I can't get these to turn on/off, but with the IRFP240 N-Ch and IRFP9240 P-Ch parts, I've been able to test them. With the gate and source resistors connected, you can't turn them on and off. Shorted MosFETS....not so. Those shorts are permanent, and can be read

Went ahead and removed R96(OL) and R90(OL) source and gate resistors for Q30. R123 had already burnt and getting all (0.00 diode test) on Q30. Hopefully these components are dead because of R123. Will check one without any burnt resistors nearby next.

[ca7922303]

You will need to lift one end of the gate resistor and source resistor at each MosFET to try this. Applying test current in Diode Test mode from my Fluke...RED lead to Gate, Blk Lead to Source, it will turn on, assuming this is a working MosFET, Then I move the leads to check if I have a low reading between Source and Drain. Reversing the leads from Source to Drain will turn it off, so there is no reading between Source and Drain. If the MosFET has failed short, then you read short ckt between Gate/Drain, Gate/Source, Drain/Source. Sometimes I can't get these to turn on/off, but with the IRFP240 N-Ch and IRFP9240 P-Ch parts, I've been able to test them. With the gate and source resistors connected, you can't turn them on and off. Shorted MosFETS....not so. Those shorts are permanent, and can be read

Q20-gate to drain(.018 diode test) gate/source(OL), drain/source(OL)
R122(2.2k)
R102(47r)
R86(OL)

[ca7922303]

Went ahead and removed R96(OL) and R90(OL) source and gate resistors for Q30. R123 had already burnt and getting all (0.00 diode test) on Q30. Hopefully these components are dead because of R123. Will check one without any burnt resistors nearby next.

Q10/R31/R38/R116(burnt) same readings as Q30 group

[nevetslab]

Went ahead and removed R96(OL) and R90(OL) source and gate resistors for Q30. R123 had already burnt and getting all (0.00 diode test) on Q30. Hopefully these components are dead because of R123. Will check one without any burnt resistors nearby next.

That looks to be shorted. Do you get that reading on any of the other MosFET's? Verify that shorting your dmm leads together, it reads the same 0.00

[ca7922303]

That looks to be shorted. Do you get that reading on any of the other MosFET's? Verify that shorting your dmm leads together, it reads the same 0.00

Dmm leads together(0.00). Have lifted 11 so far and 11-5w resistors read(OL), gate and source resistors have a couple that read(2.2k) or (47r) and 10 out of 11 MOSFETs are reading (0.00).

[ca7922303]

That looks to be shorted. Do you get that reading on any of the other MosFET's? Verify that shorting your dmm leads together, it reads the same 0.00

Looks like 19 out of 20 MOSFETs (0.00).
Q20(OL/.018/OL/OL).
All 20-5w resistors (OL)
A few source and gate resistors (2.2k mostly), some (47R).
Lots of burnt board under gate and source resistors.
No burn marks under 5w resistors or on the 5w resistor itself.
So I've got lots of components to order. Any thoughts on where else I should check for failures after all this occurred? Thanks

[nevetslab]

Looks like 19 out of 20 MOSFETs (0.00).
Q20(OL/.018/OL/OL).
All 20-5w resistors (OL)
A few source and gate resistors (2.2k mostly), some (47R).
Lots of burnt board under gate and source resistors.
No burn marks under 5w resistors or on the 5w resistor itself.
So I've got lots of components to order. Any thoughts on where else I should check for failures after all this occurred? Thanks

Well, what caused this.....hard to say. I killed one of these amps by grabbing the wrong bias pot and while waiting for the change in voltage across the Source resistor to occur as I diddled the bias pot, I was turning the wrong one, and found you CAN kill these by turning the bias up too high. You can also have catastrophic output stage failure under drive conditions.

You've already found some parts in the current limiter circuits (remember, you have two output stages in the Bridge-configured amplifier). I"m guessing you also have open gate resistors (47 ohms). I've been using 47 ohm 1W Small body Metal Oxide resistors for those, either Square cement-filled wire-wound 0.47 ohm/5W Source Resistors, or maybe 0.47 ohm/5W Metal Oxide Resistors like I used when I had rebuilt that SVT6-Pro in the photos.

The biggest problem is the IRFP240 and IRFP9240 N-Ch and P-Ch MosFET's need to be matched. They don't come that way out of distribution. I buy them in 50 or 100 pc quantities and go thru a procedure to measure their Vgs gate-source voltage, which I need to then set up the Curve Tracer to test each of the MosFET's purchased from the distributor (Mouser, Digi-Key, etc), and mark each of the parts with that value on the back side of their heat spreader plate, as well as log the readings in on my spread sheet. I end up with a range of N-Ch and P-Ch parts that can then be batched into working sets, which I then use in repairing the Ampeg SVT3-Pro, SVT4-Pro, SVT5-Pro, SVT6-Pro and other similar amps they made using these parts.

Buying the pre-matched parts from Ampeg is VERY EXPENSIVE. Not that owning the instrumentation that allows this to be done from Bulk-purchased parts isn't. That's how I've been getting by not paying Ampeg for their pre-matched parts.

[nevetslab]

I've attached this link to Sorting/Screening Bulk IRFP240 and IRFP9240 Power MosFET's, in the process of servicing Ampeg SVT4-Pro Amps. There are links within to add further insight into that procedure, and I have additional info for that.

Ampeg SVT 4 Pro 10V dc offset - Music Electronics Forum (music-electronics-forum.com)

[ca7922303]

I've attached this link to Sorting/Screening Bulk IRFP240 and IRFP9240 Power MosFET's, in the process of servicing Ampeg SVT4-Pro Amps. There are links within to add further insight into that procedure, and I have additional info for that.

Ampeg SVT 4 Pro 10V dc offset - Music Electronics Forum (music-electronics-forum.com)

What happens if I use 20 from digikey or mouser and install without any matching up?

[nevetslab]

What happens if I use 20 from digikey or mouser and install without any matching up?

Here's the test results of two tubes of 20 pcs each of IRFP240 and IRFP9240 MosFET's I had purchased from Mouser back in October, 2018. The min/max range on the N-Ch was 15.6mA thru 22mA, while the min/max range on the P-Ch was 35mA thru 54mA. I've attached two pdf documents showing the raw data of both N-Ch and P-Ch parts, sorted from min to max Id current. Then a second document installing parts in an SVT4-Pro. At idle, Q112 was showing much higher Vs voltage across the Source resistor and Ids Source Current at idle. Then shown under drive/load conditions from 12.5W thru 200W output into 8 ohms.

I have seen wider Id readings out of the 'tube' from the distributor, so you may be lucky and have a tight spread, or may be much wider, where there's not great current sharing under load. All it takes is one that's pulling way more current and doing most of the work under load, and if it fails, it takes many along with it..

Test Results of 20 ea N-Ch & P-Ch MosFETs.pdf

Test Results installed in SVT4-Pro-1.pdf

[nevetslab]

Here's a Set-up MosFET Testing/matching procedure document I put together and have on hand every time I have to start from scratch with new production parts from the distributor, to refresh my memory of how I was doing this operation. I had built a small box that lets me match MosFET's Vgs Gate-Source voltage at 50mA idle current on N-Ch and P-Ch parts used in the Ampeg SVT MosFET amps.

N-Ch & P-Ch MosFEET Matching Procedure.pdf

[ca7922303]

Here's a Set-up MosFET Testing/matching procedure document I put together and have on hand every time I have to start from scratch with new production parts from the distributor, to refresh my memory of how I was doing this operation. I had built a small box that lets me match MosFET's Vgs Gate-Source voltage at 50mA idle current on N-Ch and P-Ch parts used in the Ampeg SVT MosFET amps.

[ATTACH]n962784[/ATTACH]

What do you make of these?Look what I found on Etsy: https://www.etsy.com/listing/9586057...ef=share_v4_lx

[nevetslab]

What do you make of these?Look what I found on Etsy: https://www.etsy.com/listing/9586057...ef=share_v4_lx

That looks like a winner. Only I see it stated "1 left" Don't you need two sets?