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Post subject: Help 1970's Fender Twin Reverb Silverface Hum/Noise Problem
Posted: Tue Oct 17, 2017 1:35 pm
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Hey guys I was hoping to get some help on a Fender amp I just purchased. It's a 1970's Silverface Fender Twin Reverb with a master volume.

Problem: The amp hums when the amp is on. It only hums when it is fully on, not when it is on standby. The hum gets louder when the volume or master volume is increased with or without a guitar plugged in.

What I've tried:
-Different cable/guitar
-Different outlets
-Tested all of the tubes and tried other good working tubes from another Twin.
-Replaced the 3 prong power cable (the old one had a broken ground prong).
-Cleaned the tube sockets and connections.
-Replaced the 5 filter caps with new F&T caps.

Any other ideas on what could be causing this? Any advice/suggestions would be greatly appreciated!

Thanks!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Tue Oct 17, 2017 6:30 pm
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Check the wiring around the input and sockets. Look for loose or broken leads. Esp the braided hook-up wire from the input jacks to the input stage. Could you post a photo of your chassis wiring? This may help.

Kinda like this?

Image


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Wed Oct 18, 2017 5:53 am
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is it the same outlet you are plugging into? Maybe an issue there.
Clean the control pots. Check for proper hook up pf the reverb cables. Maybe disconnect them and see if the noise is still there.

pull one pre amp tube at a time. This could isolate the issue in a certain [art of the circuit.


Last edited by mxvin on Wed Oct 18, 2017 6:45 am, edited 1 time in total.

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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Wed Oct 18, 2017 6:31 am
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If it's the 100 watt master volume it should have a 100 ohm balance pot for the heaters (on the back) and a 10K balance pot for the power tubes. Have you adjusted them?

UN


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Wed Oct 18, 2017 9:04 am
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BMW2002Ti I'll the chasis out today and take some pictures and look for any loose/broken leads.

mxvin I've tried different outlets and the problem still persists. I did try unplugging the reverb cables as well with no luck. I went ahead and started pulling the pre amp tubes one at a time. I pulled out v6 (phase inverter) and the amp was dead quiet. So I'm guessing it might have something to do with that? I tried sticking in a NOS RCA 12AT7 and then a new JJ 5751 I had lying around but I still got the same humming problem whenever I increased the master volume/ch.1 volume/ch.2 volume.

I did get some improvement with v4 removed--there wasn't any extra noise when I moved the master volume or ch. 2 volume, but I did get extra noise when I moved ch. 1 volume.

upnorth2 I did try turning the hum balance pot in the back, but it was already in the quietest position. Where is the secondary balance pot located?

Thanks guys!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Thu Oct 19, 2017 7:41 am
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The other pot (tube balance) should be located on the bottom of the chassis in the area where a bias pot would be on other Fenders. It shows two pots on the CBS schematic but I have seen these amps with only one balance pot.

UN


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Thu Oct 19, 2017 11:19 am
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mydeadblues wrote:
BMW2002Ti I'll the chasis out today and take some pictures and look for any loose/broken leads.

mxvin I've tried different outlets and the problem still persists. I did try unplugging the reverb cables as well with no luck. I went ahead and started pulling the pre amp tubes one at a time. I pulled out v6 (phase inverter) and the amp was dead quiet. So I'm guessing it might have something to do with that? I tried sticking in a NOS RCA 12AT7 and then a new JJ 5751 I had lying around but I still got the same humming problem whenever I increased the master volume/ch.1 volume/ch.2 volume.

I did get some improvement with v4 removed--there wasn't any extra noise when I moved the master volume or ch. 2 volume, but I did get extra noise when I moved ch. 1 volume.

upnorth2 I did try turning the hum balance pot in the back, but it was already in the quietest position. Where is the secondary balance pot located?

Thanks guys!


If you still need to trouble shoot you can try "probing" the tag board while everything is up and running. Of course be very careful and use a wooden dowel for this. Sometimes you can reveal a cold solder joint or a wonky component. Also push on the tag board in different places with the dowel too. I have had "noises" being generated from underneath the tag board. If you do find a spot slip a wooden tongue depressor or smilar under that spot and see if it goes away.


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Fri Oct 20, 2017 12:00 pm
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I'm hoping to have time to pull the chasis out this weekend and look around some more. I'll be sure to look for loose connections and carefully poke around with a chop stick while the amp is on to see if there are any faults. I didn't see a second tube/bias pot underneath, but I'll double check once I have it on the bench. I mentioned earlier that the amp was dead quiet when the phase inverter preamp tube was removed. I'm not sure if that is something I should explore further or if that's normal not to get any sound when this tube isn't installed? I appreciated the help guys and will keep you updated on what I find. I'll try to get some pictures uploaded this weekend as well.

Thanks!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sat Oct 21, 2017 1:39 am
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No hum with no Phase Inverter tube ; hum is coming from or before Phase inverter circuit = lots of circuits.

1-Check and clean input jacks ground shorting contacts. No good contact to ground when no guitar cable = hum.

2- Check bias circuit filter cap.

3- Unmatched Output Power tubes. Too much difference = hum.

4- Noisy 12XX7 tubes, try know good one, tubes tester is not relaible

5- Check Power Transformer if heater center tap is working.

Post picture from inside your amp is a must


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sat Oct 21, 2017 5:57 pm
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Here are a few pictures with the chassis out:
Image
Image
Image
Image

**update** the problem appears to be more evident on channel 2 rather than channel 1. When the master volume is at 10 there is a slight hum. The hum increases when channel 2 volume is increased NOT channel 1 volume.


upnorth2 I did find the other bias pot and started probing around with a chopstick, but I did not find any loose wires/connections.

stratele52
1 I checked the ground on the input jacks, pots, and throughout the circuit.
2 I'm not sure how to test the bias circuit cap, but it does not have any dimples and appears to be in good shape. I cannot confirm with certainty that it is okay without testing it though.
3/4 I have another fender twin with known good tubes. I've swapped out all of the power and pre amp tubes and the problem still persists.
5 How do you go about testing the transformer?


I tried comparing the open chassis to the schematic and I did find a difference around the AC receptacle. The black and white wires connecting to the AC receptacle are reversed on my amp as you can see in the 2nd picture with the red arrows. I'm not sure if this was done when someone originally added the 3-prong cable? I don't know if this would have any affect.

Thanks for all the help guys!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sat Oct 21, 2017 6:12 pm
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One thing I forgot to ask... were the main filter caps, under the doghouse, replaced?

Thanks!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sat Oct 21, 2017 8:02 pm
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Check all the grounds running from the board to the brass plate under the pots. I've seen these come loose. The filament wires from V4, V5 and V6 aren't parallel but that shouldn't matter a whole lot on pre-amp tubes.

UN


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sun Oct 22, 2017 5:18 am
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stratele52
1 I checked the ground on the input jacks, pots, and throughout the circuit.
2 I'm not sure how to test the bias circuit cap, but it does not have any dimples and appears to be in good shape. I cannot confirm with certainty that it is okay without testing it though.
3/4 I have another fender twin with known good tubes. I've swapped out all of the power and pre amp tubes and the problem still persists.
5 How do you go about testing the transformer?


Answer;
1- I'm talking about jacks shorting contact to ground, not jack's ground.

Look here ; input shematic, you'll see contact;
https://robrobinette.com/How_Fender_Inp ... s_Work.htm

2- Hum in one channel ; it is not the bias filter cap wich is common on both channel.
5- Ohms reading between the center tap wire ( the yellow one solder to chassis to each side of heater wire, tubes and pilot light remove


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sun Oct 22, 2017 10:36 pm
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BMW2002Ti I already replaced the 5 filter caps underneath with new F&T caps.

upnorth2 I checked the grounds on the brass plate as well and they were all fine.

stratele52 I checked the jacks shorting contact to ground and they were all fine. I did not have a chance to check the transformer yet.

**Update** So I previously thought the problem was just on one channel, but it actually is on both channels. It seems that both the treble and volume pots on each channel increases the humming noise as they are turned up. I thought it was only on channel 2 because the treble pot was all the way day on channel 1 when I was messing with the channel 1 volume. So I'm guessing that I shouldn't rule out the bias circuit cap just yet. Is there a way to test it without removing the old cap such as alligator clipping another cap over the old cap?

Also, I noticed that the hum balance pot on the back of the amp looks pretty burnt. It seems to be working when I turn it, but something caused it to heat up/burn at some point.
Image

Thanks!


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Post subject: Re: Help 1970's Fender Twin Reverb Silverface Hum/Noise Prob
Posted: Sun Oct 22, 2017 11:51 pm
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Needs replacing, for sure. Most likely, majority of your hum issue.

Prolly, one tube's heater line shorted --- either internally or the wiring to the socket -- at one time.

NB: Check the fuse in your unit. I believe it's a 3 or 4-amp Slow Blow. Check the schematic.

:)

Good luck!

https://www.amplifiedparts.com/products ... er-control

https://www.tubesandmore.com/products/p ... er-control


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