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Post subject: 94 Twin Hum and volume won't go to 0
Posted: Thu Aug 07, 2014 4:02 pm
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Well this one ran me through the wringer. Maybe you can save some pulled out hair.
The hum was there all the time. Pulling V6 would stop it. I found that the hum was introduced at voltage divider R147 (130K) and R153(100K). They are at the end of the control board near the power supply. It's a high impedance circuit between C29 and C63. I just hung a 47K across each resistor to lower the hum to an acceptable level.
The other problem was that the volume would not go to 0 on either channel. Here I found crosstalk on the B+ from C43(22uf/500V). There is a tiny trace to all the plate resistors from it's connector. I hung a 10/450 cap on R5 to ground at the end of the control board near the input jack. This problem could cause a variety of symptoms.
If you have a better hum fix, let me know.....


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Thu Aug 07, 2014 4:11 pm
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TimsAudio wrote:
If you have a better hum fix, let me know.....


It sounds like you know your way around a Fender chassis, Tim.

Welcome to the forum.

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Fri Aug 08, 2014 9:12 am
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Each one is a new adventure. I haven't been into this twin model before, but in looking at the service bulletins, I knew it would be a difficult fix. All the mods were incorporated into this revision, but it still had the symptom. If Fender couldn't nail the hum down, what chance do I have?
I tried powering the V6 filament with 6VDC and the hum went away, but building a filtered supply for V6 wasn't an attractive prospect. It may come down to that if the customer still isn't happy.
I'm happy to be back on a fender forum again. I generally post my solutions as well as problems to help others out. I was kicked off the other fender forum a few years back. Apparently for trying to sell a homebuilt amp on Ebay that I had also posted on the forum. Go figure.


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Fri Aug 08, 2014 9:50 am
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TimsAudio wrote:
I tried powering the V6 filament with 6VDC and the hum went away, but building a filtered supply for V6 wasn't an attractive prospect. It may come down to that if the customer still isn't happy.


Is there even sufficient room within the chassis for some diodes and a couple of electrolytics?

My recollection of that amp is that it's already pretty crowded in there.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Mon Aug 11, 2014 11:26 am
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Oh it's crowded, but not crammed.
I spent the weekend trying out a few different things. First a rectifier and filters on the V6 filament. No luck. Then a separate transformer and rectifier and filters. Still no luck.
Then I reread the Fender bulletins. They said a noisy AC line causes it. So I tried a line filter. No change.
The funny thig was that the hum would increase when the reverb driver V5 was pulled. That seemed odd. It would also increase if the red wire to the reverb transformer was pulled when V5 was still in. That doesn't sound like a filament problem, it points to the B+. C44, a 22uf/500v feeds V5 and V4, so I hung a 47/450 across it HUM WAS GONE!
I took out the previous fix with the 47Ks, Still good.
It still seems odd that pulling V5 and reducing the load on the cap increased the hum. I would have expected it to go down. There still must be some evil lurking in this Twin, but I'm done exorcising.
This morning the customers ears were smilin when he picked it up and so was I


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Mon Aug 11, 2014 2:06 pm
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TimsAudio wrote:
This morning the customers ears were smilin when he picked it up and so was I


It doesn't get much better than that!

Congrats on a fine "exorcism", Father.

8)

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Mon Aug 11, 2014 5:35 pm
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" The funny thing was that the hum would increase when the reverb driver V5 was pulled. That seemed odd. It would also increase if the red wire to the reverb transformer was pulled when V5 was still in. That doesn't sound like a filament problem, it points to the B+. C44, a 22uf/500v feeds V5 and V4, so I hung a 47/450 across it HUM WAS GONE! "

Excellent diagnosis and fix. :mrgreen:

Yeah, sounds like the main PSU cap that directly feeds the reverb driver 12AT7 (V5) lost its filtering capacity. I'd suspect that pulling this tube would raise the voltage, and hum noise level, being fed to tubes downstream (like the input stage). Adding the 47/450 cap, in essence, fixed the "bad" stage's filtering issue. JMO.


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Fri Aug 15, 2014 8:19 am
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I didn't think to measure the filter cap at C44. I haven't seen any of these 22uf caps to go bad at this relatively young age. These fender caps are usually very reliable.
But I did try hanging the 47uf on C22. It is also a 22uf/500V cap that is just upstream in the power supply. It didn't make any difference in the hum. I concluded from that exercise that the hum was not coming from power supply ripple.
So it still baffles me as to where the 60Hz hum would come from...


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Mon Oct 13, 2014 8:26 am
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Rock Star, you had it right. That Twin came back this week with the same hum. The customer said it was OK, then he went to get a cup of coffee and it was humming when he came back.
Turns out it was a vibrational failure of the canter lead of the filter capacitors. On a whim, I tugged on the lead of the cap and it pulled right out. There was a blackened tip on the lead where it had been arcing internally. Then I pulled on the other filter caps and two more pulled out. Same thing.
Replacing the three caps allowed me to pull off the extra caps I had installed. I haven't run into vibration failures like this before, so rather than tinking it was a manufacturing defect, I'm thinking that this Mighty Twin has done some serious rockin in its day to shake them loose. I won't get fooled again....


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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Mon Oct 13, 2014 10:18 am
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Glad you found the original problem. Placing silicone sealer or glue gun application between the bodies of the caps and the circuit board can help reduce damage, due to vibrations.

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Post subject: Re: 94 Twin Hum and volume won't go to 0
Posted: Wed Oct 15, 2014 9:00 am
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These were glued down by the factory and still attached by the glue when I removed them.
I regularly use silicone, but I'm looking for something stiffer as the silicone still allows for some movement. If Elmers was only a little thicker....
I mostly use hot melt glue for solid state amps. I have seen it melt down in tube amps, making a mess. It's also good for stabilizing jacks on a circuit board.


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