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  • #76
    Originally posted by nevetslab View Post

    That looks like a winner. Only I see it stated "1 left" Don't you need two sets?
    He has another set of 12 for sale that is only a few dollars more.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #77
      Originally posted by g1 View Post

      He has another set of 12 for sale that is only a few dollars more.
      Appreciate the heads up on that. I went ahead and ordered 22 for $98.50(1-47.00, 1-51.50). Thanks.

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      • #78
        Originally posted by nevetslab View Post

        That looks like a winner. Only I see it stated "1 left" Don't you need two sets?
        Yeah, he sold one, but G1 pointed me e to 12-$51.50, same seller, so they are on the way. Thanks.

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        • #79
          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.
          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?

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          • #80
            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?
            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.
            Logic is an organized way of going wrong with confidence

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            • #81
              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.
              Finally have both boards repopulated. Any suggestions on anything before I put it back together for testing?

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              • #82
                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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                • #83
                  Originally posted by nevetslab View Post
                  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.
                  Checked all source resistors and they are all about the same about 2.0 vdc@110vac and bias pot @12:00.

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                  • #84
                    Nominal bias would be 25mV across each of the 0.47 ohm 5W Source Resistors
                    Logic is an organized way of going wrong with confidence

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                    • #85
                      Originally posted by nevetslab View Post
                      Nominal bias would be 25mV across each of the 0.47 ohm 5W Source Resistors
                      Getting 0.00mv across each of the 0.47 ohm 5w Source Resistors.(110vac/bias pot@12:00). All 5w resistors still reading 0.47 ohms resistance.

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                      • #86
                        Originally posted by ca7922303 View Post

                        Getting 0.00mv across each of the 0.47 ohm 5w Source Resistors.(110vac/bias pot@12:00). All 5w resistors still reading 0.47 ohms resistance.
                        Correction: found one(1) 5w 0.47 @ 0.40vdc. The other 19 @ 0.00mv.

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                        • #87
                          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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                          • #88
                            Originally posted by nevetslab View Post
                            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.
                            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?

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                            • #89
                              Originally posted by nevetslab View Post
                              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.
                              The two(2) spare 240s I have diode test@0.48v. The 240 connected to the 5w .47 resistor reading 0.40vdc, reads 0.48v diode test in circuit. Before I take this apart again, what do you make of this?

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                              • #90
                                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?
                                You want to have the Bias pots on both the Upper and Lower Power amp stages set to CCW. Forget where they were. Before applying AC Mains, lets double-check the Bias Pots and the do a diode-test of the Base-Emitter junction of Q2 and Q22 (bias xstrs of the two output stages) with the bias pots set to CCW. That should set the 10k bias pot so you're placing the 10k pot in parallel with the Base-Emitter junction. If it reads as a short, then the pot is set for Max Bias (Fully CW). That would have Q2/Q22 turned off, which lets the Gate Busses go to max voltage and turn the MosFET's on hard....which will kill the lot of them. I'm just going by what I see on the Power Amp Schematic to help get you oriented with this.

                                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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