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70s Ampeg V4B Blows Fuses

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  • So are we saying that these two 47k resistors ate bad even though they measure correct? Like to understand why that is if that's the case? Thanks.

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    • Originally posted by vintagekiki View Post
      Compare the layout of the V4B in front of you with the schematics
      Look where all the wires are going from R52, R53
      I can see they go to R52,53, I'm just wondering if the cap they connect to could be bad because of the loud pop I heard the other day @ 120v.

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      • If they measure good, then they are probably good. Instead of mounting them on the circuit board, they run wires to them and mount them on that cap. Electronically it is the same. They run hot so it helps keep heat off the circuit board.
        The voltage from the power supply goes through a chain of resistors. Somewhere along the chain it broke, you heard the pop, and now the voltage isn't getting to some of the tubes any more. Each resistor in the chain will drop a bit of voltage, but somewhere along the line now it will all of a sudden go to zero. That will be the break. It could be a bad resistor or a bad connection.
        The order of the chain is R53,R52,R51.
        If you say you are measuring 570V at both sides of each of R53 and R52 in the photo, then check both sides of R51.
        "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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        • Originally posted by g1 View Post
          If they measure good, then they are probably good. Instead of mounting them on the circuit board, they run wires to them and mount them on that cap. Electronically it is the same. They run hot so it helps keep heat off the circuit board.
          The voltage from the power supply goes through a chain of resistors. Somewhere along the chain it broke, you heard the pop, and now the voltage isn't getting to some of the tubes any more. Each resistor in the chain will drop a bit of voltage, but somewhere along the line now it will all of a sudden go to zero. That will be the break. It could be a bad resistor or a bad connection.
          The order of the chain is R53,R52,R51.
          If you say you are measuring 570V at both sides of each of R53 and R52 in the photo, then check both sides of R51.
          R51(2.2k) resistance, 570v/570v.

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          • Looks t my eyes like they are wired directly to the main filter cap can right next to them.
            Education is what you're left with after you have forgotten what you have learned.

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            • Well it doesn't make much sense for the voltage to be staying so high all the way to the front like that, tubes along the way should still be running which would reduce the voltages somewhat.
              Could be that the break is along the ground line rather than the B+ line.
              Check diode D10 (edit typo, should be D5). Unit turned off, meter on diode check function, check then reverse probes and check the other way.
              Last edited by g1; 04-27-2021, 06:36 PM.
              "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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              • Originally posted by ca7922303 View Post
                I can see they go to R52,53, I'm just wondering if the cap they connect to could be bad because of the loud pop I heard the other day @ 120v.
                You asked "Can you explain further" #144
                We agreed "Let's go step by step"#145
                And you started the old way again.

                Dear friend. For these 155 posts in the story you spun "70s Ampeg V4B Blows Fuses", even a small child would have already learned something.
                But you are obviously bored.
                In addition to the good will to philosophize and spin the portal, do you have an idea of what you want from yourself and what you want from members who are trying to help you.
                Click image for larger version  Name:	'    Reminder.gif Views:	0 Size:	154 Bytes ID:	930047 If there's nothing to do, don't do it here.

                And return back to the post #145
                Inform about results of what you done about questions asked in post.
                Who does not know and knows that he does not know - teach him Confucius)
                Who knows and does not know that he knows - wake him Confucius)

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                • Originally posted by g1 View Post
                  Well it doesn't make much sense for the voltage to be staying so high all the way to the front like that, tubes along the way should still be running which would reduce the voltages somewhat.
                  Could be that the break is along the ground line rather than the B+ line.
                  Check diode D10. Unit turned off, meter on diode check function, check then reverse probes and check the other way.
                  Here is both sides of the board I have. Not seeing D10?

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                  • Originally posted by g1 View Post
                    Check diode D10. Unit turned off, meter on diode check function, check then reverse probes and check the other way.
                    This Ampeg V4B has no built-in Schottky diodes D7 - D10
                    Click image for larger version  Name:	fetch?photoid=930062.jpg Views:	0 Size:	2.74 MB ID:	930065 Click image for larger version

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                    Who does not know and knows that he does not know - teach him Confucius)
                    Who knows and does not know that he knows - wake him Confucius)

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                    • Originally posted by ca7922303 View Post

                      Here is both sides of the board I have. Not seeing D10?
                      Sorry, I meant D5.

                      "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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                      • Originally posted by g1 View Post

                        Sorry, I meant D5.
                        No problem. I know where D-5 is. I'll post the measurements this afternoon as soon as I get off work. Really appreciate your help.

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                        • Originally posted by g1 View Post

                          Sorry, I meant D5.
                          D-5(0.5/OL) diode test, (4.3m/Ol) Resistance, (10mv/0mv) vdc no power. D-6 (0.5v/OL) diode test, 133k/117k) resistance, (0mv/0mv) vdc no power, D1-D-4(0.5/OL) diode test, (0.6/0.9) resistance, (23v/23v) vdc no power.

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                          • Check both sides of D6 for DC volts with unit powered up and standby switch in 'play' position.
                            Does this amp have a 3 prong plug installed? Where do you connect your black probe as a ground when checking DC volts?
                            "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

                            Comment


                            • Originally posted by g1 View Post
                              Check both sides of D6 for DC volts with unit powered up and standby switch in 'play' position.
                              Does this amp have a 3 prong plug installed? Where do you connect your black probe as a ground when checking DC volts?
                              It does have a three(3) prong plug and negative probe goes to chassis for ground.(photo of location attached. D-6(-60/326mv), D-5(0mv/11.5v), D-1/D-2(556v/269v), D-3/D-4(10v/269v). Standby switch on, no power tubes installed, no variac used, 120v.

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                              • Originally posted by ca7922303 View Post
                                D-5(0mv/11.5v), D-1/D-2(556v/269v), D-3/D-4(10v/269v). Standby switch on, no power tubes installed, no variac used, 120v.
                                Under same conditions, measure DC volts at both sides of standby switch.

                                "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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