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

  1. #1
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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?

  2. #2
    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"
    Loud hum when turning the standby off on my 1990 Fender the twin "red knob" for 20sec

    plus:

    Fender red knob twin vs Fender twin reverb 65 reissue clean channel with Les Paul

    Twin Reverb 65 Reissue scratchy volume 1 and 2 potentiometers and other issues

    ***All Filter Caps swapped! Still some hum on Clean and Overdrive Channel of Red Knob!!!

    By the way, you have NOT answered your latest thread , where you were suggested some practical solutions to try: Connecting 4 ohms Twin Reverb Reissue 65 Speakers to Red Knob Twin for testing

    Are you pulling our collective legs?
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    Juan Manuel Fahey

  3. #3
    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.

  5. #5
    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.
    When the going gets weird... The weird turn pro!

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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.

  8. #8
    Senior Member Enzo's Avatar
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    Because it offers no protection to the thing.
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
    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?

  12. #12
    Old Timer J M Fahey's Avatar
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    If after repairing all of the soldering,
    which you didnīt
    you still have the issues, AND after measuring the AC ripple on the power supply taps as DrGonz78 suggests,
    which you didnīt
    then I'd go for replacing the supply caps, if it's warranted.
    which so far isnīt because you refuse to test and post results.
    All you want is "a Brand".
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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    which you didnīt
    which you didnīt

    which so far isnīt 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?

  14. #14
    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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    “Yeah, well, you know, that’s just, like, your opinion, man.”

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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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  16. #16
    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.
    Juan Manuel Fahey

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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.

  18. #18
    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!
    Last edited by nevetslab; 12-01-2017 at 07:34 PM.
    Logic is an organized way of going wrong with confidence

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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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