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Loud Hum on Fender Twin Reverb 65 reissue when standby swith is off (play mode)

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  • #16
    Dear jalexquijano, we are trying to make you *FOCUS* on the problem.

    The tests we are asking you do, are exactly what we would be doing if we had that same amp open on the bench.

    *There is* a sequence of tests which must be performed to find and solve problems.

    You are NOT doing that, although clearly instructed to do so time and again, but instead are *guessing* about what the problem cause and solution might be.
    It simply does not work that way.
    Juan Manuel Fahey

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    • #17
      Last night i desoldered and soldered the pair of 220 uf 300v and 22 uf 500 volts and after assembly the massive hum was still present. I did noticed that the 1st preamp tube from right to left is microphonic. I inspected for any loose solder and did not found any.

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      • #18
        That's a start. You can measure the ripple voltage up on the main PCB in the middle of the chassis. Onr thing that hasn't been mentioned is the choke. If the choke is bad, the amp will have considerable hum. It connects with a pair of black wires, and sits between the O/T Primary CT connection and the Screen's tap. Look to see what the AC ripple voltage is on both sides of the choke. There should be a VERY significant reduction of ripple voltage on the output of the choke. If not, you have a bad choke. There will be a fair amount of ripple on the 1st filter stage, where the O/T Primary C/T is connected. I don't recall off hand what the typical ripple voltages are as you check at each decoupling stage, but can give you what I see on the Fender Deluxe Reverb amp is, as I have one in the service cradle at the moment: 2.9V RMS @ OT Pri C/T; 50mV RMS after choke, 40mV next stage, 10mV at final power supply stage (feeds the preamp tube plates). As you can see, there is a significant drop in ripple after the choke. It's on a pair of black wires. CP23 is the choke input, CP24 is the output The Fender Twin should be similar in trend on the amount of ripple

        Assusming you get similar readings we'd say it's not a capacitor issue.(unless C32 immediately following the choke is bad). That then points to connections on the main PCB....requiring it to be lifted up and throughly inspected.

        In all cases, before you go digging anywhere, the filter caps MUST BE DISCHARGED. Most of us doing service work have built our own discharge tool using a suitable power resistor with insulated probes connected to it for bleeding off stored charge. Mine is a 100 ohm/50W Dale resistor and old test lead probes attached. Or, in the case of no discharge tool, turn the amp off, and leave the S/B switch in Operate position. After about a minute, the high voltage will have dropped to arond 10VDC at the 1st filter stage, and after 10 minutes, it's down to mV level. Most current Fender amps will fully discharge, thogh not true of older units Always look first. And, of course unplug the amp from teh wall!
        Last edited by nevetslab; 12-01-2017, 05:34 PM.
        Logic is an organized way of going wrong with confidence

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        • #19
          First thing is to check that all of the grounds are good, including the output jacks - a number of the RI amps have a card stock insulator around the jacks, the lips of which were wrongly placed between the jacks and the chassis.

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          • #20
            mine also has that card board insulator

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            • #21
              Originally posted by Jazz P Bass View Post
              "What year is this amp from?"

              The first letters of the serial number (AC064286) indicate that the amplifier was manufactured in 1990 in the month of March.
              Just got the full electrolytic capacitor set for this amp! I will start the replacement of the caps on the power supply section and then move to the ones found on the pcb. Guess there are a lot of cables to disconnect on the middle pcb before removing.

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              • #22
                Use a small Sharpie pen to label each of the quick disconnect terminals as you remove them, so you know where they go when you put it back together. Or, even better, take close-up pictures of it. I label mine, but have also drawn myself a diagram of all the terminals, the PCB ID numbers, since there are some of the same color in the same vicinity (red, red, red...one is the O/T Center Tap, one is the 1st Filter stage in the cap compartment, two are the P/T HV Secondaries

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                You never replied regarding checking if the Choke was giving you correct power supply ripple reduction. Those Choke leads are the Black wires plugged into CP23 & CP24. If your Choke is bad, you will have loud hum, and installing new filter caps will NOT fix that!!

                Go back and re-read my post from 12/1/17 regarding the choke and the power supply ripple.
                Last edited by nevetslab; 12-13-2017, 07:33 PM.
                Logic is an organized way of going wrong with confidence

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                • #23
                  Originally posted by nevetslab View Post
                  Use a small Sharpie pen to label each of the quick disconnect terminals as you remove them, so you know where they go when you put it back together. Or, even better, take close-up pictures of it. I label mine, but have also drawn myself a diagram of all the terminals, the PCB ID numbers, since there are some of the same color in the same vicinity (red, red, red...one is the O/T Center Tap, one is the 1st Filter stage in the cap compartment, two are the P/T HV Secondaries

                  [ATTACH=CONFIG]46104[/ATTACH] [ATTACH=CONFIG]46105[/ATTACH] [ATTACH=CONFIG]46106[/ATTACH] [ATTACH=CONFIG]46107[/ATTACH]

                  You never replied regarding checking if the Choke was giving you correct power supply ripple reduction. Those Choke leads are the Black wires plugged into CP23 & CP24. If your Choke is bad, you will have loud hum, and installing new filter caps will NOT fix that!!

                  Go back and re-read my post from 12/1/17 regarding the choke and the power supply ripple.
                  Thanks. i changed all of the following capacitors:




                  2 F&T 220uF-300V (Main Filters)•3 F&T 22uF-500V (Decoupling Filters)
                  •1 Radial 100uF-100V (Bias Filter)
                  •1 Radial 22uF-100V (Bias Filter)
                  •1 Radial 10uF-100V (Footswitch Filter)




                  the only ones i did not change were:




                  (7) Radial 22uF-50V (Cathode Bypass)




                  Would changing these ones eliminate the hum or shall i review everything you mention above. Before replacing the caps, i tried a Brand new matched set of Groove tube 6L6GE and the hum also appeared. Where else shall i look?

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                  • #24
                    Quote Originally Posted by nevetslab View Post
                    Use a small Sharpie pen to label each of the quick disconnect terminals as you remove them, so you know where they go when you put it back together. Or, even better, take close-up pictures of it. I label mine, but have also drawn myself a diagram of all the terminals, the PCB ID numbers, since there are some of the same color in the same vicinity (red, red, red...one is the O/T Center Tap, one is the 1st Filter stage in the cap compartment, two are the P/T HV Secondaries

                    You never replied regarding checking if the Choke was giving you correct power supply ripple reduction. Those Choke leads are the Black wires plugged into CP23 & CP24. If your Choke is bad, you will have loud hum, and installing new filter caps will NOT fix that!!

                    Go back and re-read my post from 12/1/17 regarding the choke and the power supply ripple.
                    Originally posted by jalexquijano View Post
                    Thanks. i changed all of the following capacitors:




                    2 F&T 220uF-300V (Main Filters)•3 F&T 22uF-500V (Decoupling Filters)
                    •1 Radial 100uF-100V (Bias Filter)
                    •1 Radial 22uF-100V (Bias Filter)
                    •1 Radial 10uF-100V (Footswitch Filter)




                    the only ones i did not change were:




                    (7) Radial 22uF-50V (Cathode Bypass)




                    Would changing these ones eliminate the hum or shall i review everything you mention above. Before replacing the caps, i tried a Brand new matched set of Groove tube 6L6GE and the hum also appeared. Where else shall i look?
                    Is this a (bad) joke?

                    Why care to quote nevetslab excellent suggestions at all if you never ever followed them ?

                    Not even refer to or mention them.

                    As of:
                    Would changing these ones eliminate the hum
                    Is this a guessing game?
                    What´s the prize?
                    Juan Manuel Fahey

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                    • #25
                      Sorry for not following his instructions. I recently got the kit and though it was a good idea to replace the filter caps. Nevertheless, the problem is still present.

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                      • #26
                        Why am I not surprised.
                        Logic is an organized way of going wrong with confidence

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                        • #27
                          Originally posted by TimmyP1955 View Post
                          First thing is to check that all of the grounds are good, including the output jacks - a number of the RI amps have a card stock insulator around the jacks, the lips of which were wrongly placed between the jacks and the chassis.
                          Assuming you finally get around to checking the power supply choke, and find it IS doing it's job, reducing the power supply ripple. That points to grounding. One place to check, which hasn't yet been brought up, is the Hum Balance pot. It's the screwdriver control closest to the pilot light. 100 ohm value, connected across the heater winding, with it's wiper connected to chassis ground. That pot is known to fail, causing hum. Check to see if it is still an adjustable pot. If not, replace it.

                          Assuming the Hum balance pot is ok, and does function, changing the degree of hum heard, but remains high, what happens if you unplug V4 12AX7? That leaves you with just the power amp section (and the tremolo section). If you still have the same degree of hum, your problem is probably NOT in the preamp. You're still relying on all the ground wires getting to a common ground that gets to chassis from the power supply compartment, as well as off the main PCB where all the grounds collect. As stated before, you would still be looking for solder fractures in the grounding paths. And, you need to have TIGHT mechanical connections on the speaker jacks to the chassis, as well as on the front panel control bushings, as well as the four input jacks. There are grounds that come off the front panel PCB to the main PCB by way of the ribbon cables. Those solder joints as well as the IDC ribbon connections are also known to be faulty over time. We assume you've already checked to be certain the power transformer's HV C/T wire is tightly cinched down to the chassis (Gren/Yel wire).

                          Also, and perhaps what all of us have overlooked, was your statement that the hum goes away after a few minutes. That's suggesting the change is thermally induced. Some area that warms up and has poor contact until it warms up. I believe you've already replaced power tubes. If with V4 unplugged, that trend is still occuring, swap out V6 12AT7. I've had that driver tube cause such problems before.
                          Last edited by nevetslab; 12-15-2017, 01:52 PM.
                          Logic is an organized way of going wrong with confidence

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                          • #28
                            Hum is gone. I guess it was a ground issue as i tightened all the connections. Nevertheless, i still have to replace the following caps:


                            (7) Radial 22uF-50V (Cathode Bypass)

                            What is the purpose of these caps on the circuit of my Twin Reverb 65 reissue?

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                            • #29
                              Those bypass the cathode resistors in the tube gain stages, increasing the AC gain of the stage. You're probably fine with what's installed, as those aren't related to a hum issue.
                              Logic is an organized way of going wrong with confidence

                              Comment


                              • #30
                                Finally replaced all 7 cathode resistor. Nevertheless, while during the extraction of solder required to remove and replace the last one, the trace pad of one of the terminal was lifted and damaged. Is it fine if i just solder the leg of the cathode capacitor directly to the terminal of the resistor next in the circuit? or could this be a problem futher on?

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