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

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

    I have a Fender Twin Reverb reissue with series number AC064286 which produces a horrible hum when its cold and you switch it to play mode. After a couple of minutes the hum dissapears. Is this normal? I have never recapped this amp as i bought it used in 2010. Based on the above facts, i would like to know the following:

    1. What year is this amp from

    2. Should i consider changing all electrolytic caps? Which ones should i start with? Will this get rid of the hum when cold?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jalexquijano View Post
    I have a Fender Twin Reverb reissue with series number AC064286 which produces a horrible hum when its cold and you switch it to play mode. After a couple of minutes the hum dissapears. Is this normal? I have never recapped this amp as i bought it used in 2010. Based on the above facts, i would like to know the following:

    1. What year is this amp from

    2. Should i consider changing all electrolytic caps? Which ones should i start with? Will this get rid of the hum when cold?
    Is this a joke?
    You have started 21 (twenty-one) different threads asking about various very minor and normal "problems" regarding your Twins, both this very same one and the 1990 red knob one.
    Including this very same question about "hum after flipping Standby switch on, which disappears on its own after a few seconds"
    http://music-electronics-forum.com/t45283/

    plus:

    http://music-electronics-forum.com/t45448/

    http://music-electronics-forum.com/t45362/

    ***http://music-electronics-forum.com/t44973/***

    By the way, you have NOT answered your latest thread , where you were suggested some practical solutions to try: http://music-electronics-forum.com/t45487/

    Are you pulling our collective legs?

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    Juan Manuel Fahey

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    Supporting Member Jazz P Bass's Avatar
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    "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.

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    Thank you very much for your replies. I will buy some good quality electrolytic caps and recap this amp.

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    Stray Cap DrGonz78's Avatar
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    Quote Originally Posted by jalexquijano View Post
    Thank you very much for your replies. I will buy some good quality electrolytic caps and recap this amp.
    I would be curious as to what is causing the noise in such short duration at start up. Can you monitor AC voltage on the DC power supply while starting up the amp? At least by doing that test you will may see what you think is actually there.

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    Senior Member nevetslab's Avatar
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    Before you go out and replace all of the power supply caps, I'd suggest you first open up the lower power supply compartment, lift the board off (removed the 5 screws holding it in place) and look very close at the solder joints of the caps. I'd go as far as suggesting de-soldering and then re-soldering them. If the exposed plating is way smaller than the copper pattern, I'd recommend scraping away the rest of the green solder mask and tin that newly exposed copper, then clear the lead holes and re-insert the supply cap leads and fold them back over how they were before, THEN re-solder them. I'd also do the same on the main PCB. That takes a lot more effort, though, as all the AC Mains primary and secondary wiring is plugged in. Those will have to be disconnected (label each connector), and unplug the front panel PCB, so you can lift up the main PCB, folding it up, hinging on the socket wiring, and look closely at all the power supply caps soldering there as well. Look closely at the swagged male fast-on terminals that pick up all the grounds and cap connections from below, as well as from the P/T secondary wires.

    Your amp is old enough to have suffered one or several solder fractures, that can lead to the hum behavior you describe. If after repairing all of the soldering, yu still have the issues, AND after measuring the AC ripple on the power supply taps as DrGonz78 suggests, then I'd go for replacing the supply caps, if it's warranted.

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    The rear of the Fender twin reverb reissue says 4A SLO BLO 125VAC. How come i cant use a 7A SLO BLO 125V Fuse? Just blew the fuse while testing some power tubes i had lying around.

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    Lifetime Member Enzo's Avatar
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    Because it offers no protection to the thing.

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    Supporting Member Jazz P Bass's Avatar
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    Besides which, at idle (if that is where the fuse blew) the amp pulls less than an amp.

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    It would be very beneficial to build and use a light bulb limiter when doing this kind of thing.

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    Quote Originally Posted by nevetslab View Post
    Before you go out and replace all of the power supply caps, I'd suggest you first open up the lower power supply compartment, lift the board off (removed the 5 screws holding it in place) and look very close at the solder joints of the caps. I'd go as far as suggesting de-soldering and then re-soldering them. If the exposed plating is way smaller than the copper pattern, I'd recommend scraping away the rest of the green solder mask and tin that newly exposed copper, then clear the lead holes and re-insert the supply cap leads and fold them back over how they were before, THEN re-solder them. I'd also do the same on the main PCB. That takes a lot more effort, though, as all the AC Mains primary and secondary wiring is plugged in. Those will have to be disconnected (label each connector), and unplug the front panel PCB, so you can lift up the main PCB, folding it up, hinging on the socket wiring, and look closely at all the power supply caps soldering there as well. Look closely at the swagged male fast-on terminals that pick up all the grounds and cap connections from below, as well as from the P/T secondary wires.

    Your amp is old enough to have suffered one or several solder fractures, that can lead to the hum behavior you describe. If after repairing all of the soldering, yu still have the issues, AND after measuring the AC ripple on the power supply taps as DrGonz78 suggests, then I'd go for replacing the supply caps, if it's warranted.
    I would need at least 3 22uf 500v and 2 220uf 300 V axial electrolytic capacitors for this amp. Which Brand would you recommend? F & T or Illinois?

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    Old Timer J M Fahey's Avatar
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    If after repairing all of the soldering,
    which you didn´t
    you still have the issues, AND after measuring the AC ripple on the power supply taps as DrGonz78 suggests,
    which you didn´t
    then I'd go for replacing the supply caps, if it's warranted.
    which so far isn´t because you refuse to test and post results.
    All you want is "a Brand".

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    Quote Originally Posted by J M Fahey View Post
    which you didn´t
    which you didn´t

    which so far isn´t because you refuse to test and post results.
    All you want is "a Brand".
    I checked the PCB and those Illlinois Caps are the stock ones. They were never changed. THey are not that much expensive so why not try it?

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    Supporting Member The Dude's Avatar
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    Because it's easier and cheaper to simply check for ripple and see if they actually need replacing.

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    Senior Member nevetslab's Avatar
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    .....and, stating again, just replacing power supply buss caps, there's no guarantee that will solve your problem. Those of us who are trying to help you find the problem do this sort of thing for a living and have been at it for years. I cure Fender amps of this sort of problem on gear that makes it's living going out on tour in road cases, and they often come back in behaving like the amp you're trying to cure. It takes some effort to open it up and look. I cure fractured solder joints constantly, and it's probably what you're facing. it takes bright light, magnification during inspection and patience. If it's on the main PCB in the chassis, yes, there is some effort involved to get that board lifted up to look, and care is required so as NOT to break wire leads off in the process. Loads of quick-disconnects need to be removed, so you use a black sharpie to label the female push-on terminals so you can return them to their proper places when re-assemblying, after you've found all the solder fractures. If nothing else, it's a good exercise in maintenance work.

    That head light and loupes on my avatar (to the left of this text) is my surgical eyewear that I use in this process. It reveals all the bad solder joints clearly.

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    Old Timer J M Fahey's Avatar
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    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.

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    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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    Senior Member nevetslab's Avatar
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    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!

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    Last edited by nevetslab; 12-01-2017 at 07:34 PM.
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    Senior Member TimmyP1955's Avatar
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    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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    mine also has that card board insulator

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    Quote 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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    Senior Member nevetslab's Avatar
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    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.

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    Last edited by nevetslab; 12-13-2017 at 09:33 PM.
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    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

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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.
    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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    Old Timer J M Fahey's Avatar
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    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.
    Quote 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?

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    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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    Senior Member nevetslab's Avatar
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    Why am I not surprised.

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    Quote 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.

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    Last edited by nevetslab; 12-15-2017 at 03:52 PM.
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    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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    Senior Member nevetslab's Avatar
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    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.

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    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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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by jalexquijano View Post
    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?
    That can be done, or you can scrape off the solder mask of the trace where you lifted the pad, fold the resistor lead over and tack-solder it in place. Solder Pad adhesion failure on Fender's PCB's is way too common, even with the best soldering techniques.. You first have to get the solder off the folded leads, so you can then carefully break the bond between teh lead and solder pad with flush-cutting dikes, bend the leads up, and invariably, have to hit the joint again to get the part out.

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    I have an additional 22uf 50v radial cathode capacitor. Should i remove the one that is not soldered properly, insert this new one and solder one of the leads to the next pad where the resistor that continues the circuit is soldered?

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    Quote Originally Posted by jalexquijano View Post
    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?
    One more reason to not replace things you don't need to.

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