Originally posted by nevetslab
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Ampeg SVT-6 Pro No Output
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Originally posted by nevetslab View Post
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.
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Originally posted by ca7922303 View Post
So, I need to upgrade the gate resistors from 1/4w to 1 watt, and source resistors to cement wire wound or metal oxide. What about the 2.2k resistors that are connected to the source? Any upgrade needed?
Logic is an organized way of going wrong with confidence
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Originally posted by nevetslab View Post
My change from the 1/2W gate resistors Ampeg uses (flameproof 1/2W parts) to that of small body 1W Metal Oxide, same or slightly larger body size, which are also flame proof, was my choice. The Source resistors, which have traditionally been 5W Cement filled square wire wound parts, I use them, as well as 5W Metal Oxide parts (depending on what I have on hand). The 2.2k resistors are not critical....they are just the source resistors feeding the current limiter xstrs.1 Photo
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Make sure the Bias level is set to keep the devices from turning on. I can't recall if their bias pot on these amps increases with CW rotation or Decreases with CW rotation. I'll have to check my files for that.
Having just looked at the schematics for the Power Amp circuit, they call out CW on the Bias pot. With the pot fully CCW, it has the Bias xstr turned on, which pulls the two Gate Busses towards each other, and the outputs are turned off. Turning the bias pots CW starts turning the bias xstr off, which then allows the MosFET's to start conducting. So, Clockwise from fully CCW is going to start turning the MosFET's on. You'll want to check multiple source resistors during this adjustment to make sure there aren't some MosFET's conducing harder than others. Should all be near same, as you did buy tightly matched parts.Last edited by nevetslab; 07-11-2022, 05:44 AM.Logic is an organized way of going wrong with confidence
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Originally posted by nevetslab View PostMake sure the Bias level is set to keep the devices from turning on. I can't recall if their bias pot on these amps increases with CW rotation or Decreases with CW rotation. I'll have to check my files for that.
Having just looked at the schematics for the Power Amp circuit, they call out CW on the Bias pot. With the pot fully CCW, it has the Bias xstr turned on, which pulls the two Gate Busses towards each other, and the outputs are turned off. Turning the bias pots CW starts turning the bias xstr off, which then allows the MosFET's to start conducting. So, Clockwise from fully CCW is going to start turning the MosFET's on. You'll want to check multiple source resistors during this adjustment to make sure there aren't some MosFET's conducing harder than others. Should all be near same, as you did buy tightly matched parts.
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That MosFET appears to be NOT matched with the others. Didn't I read that you had purchased more than 20 pcs total? If so, I'd replace that MosFET, as this one is pulling substantial current when you don't even have the bias turned up.Logic is an organized way of going wrong with confidence
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Originally posted by nevetslab View PostThat MosFET appears to be NOT matched with the others. Didn't I read that you had purchased more than 20 pcs total? If so, I'd replace that MosFET, as this one is pulling substantial current when you don't even have the bias turned up.
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Originally posted by nevetslab View PostThat MosFET appears to be NOT matched with the others. Didn't I read that you had purchased more than 20 pcs total? If so, I'd replace that MosFET, as this one is pulling substantial current when you don't even have the bias turned up.
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Originally posted by ca7922303 View Post
On the board that the .40v resistor is on has the bias pot set where it was when I took it apart at about 2:00. Do I run the risk of burning out mosfets, etc, etc by going beyond 2:00. The other board bias pot is set @ 12:00. Should I try to get it at 2:00?
I have NO Idea why one of your IRFP240's is reading 0.4VDC across it's Source resistor, when all the others are reading 0mV. I see Vgs readings written on the Drain slab of the back of each MosFET. Are you re-using the Bergquist Blue-grey insulator strips that you had to pry the blown MosFET's off of, or do you have fresh insulator strips in place on the heat sink....or instead Greased Mica washers to insulate each of the MosFET when you mount the amp assemblies to the heat sink?
I believe I had advised about the potential damage that occurs when you have to pry the MosFET's off of the heat sink, which happens from the Thermoset of the non-mica insulator, and what usually happens is part of the embedded insulation breaks away from the sheet insulator. When that happens, you no longer have viable insulation. Reusing that can lead to shorting the Drain slab thru that damaged portion of the insulator to Chassis Gnd on the heat sink. Any chance we're dealing this this?
That's the only place I can see where you might have an issue. Unless you have an open Gate resistor. Those are the 47 ohm Flame Proof 1/2W resistors (stock Ampeg parts). I've been using small-body 1W Metal Oxide parts in their place, also flame proof. I think you had mentioned having some or all of those damaged.
What else would cause just one of these to be giving you what sounds like this one IRFP240 is turning on hard, reading 0.40VDC across it's Source Resistor while the rest are reading 0V? Open traces? PCB damage that wasn't found in the rebuild?
Logic is an organized way of going wrong with confidence
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