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Old 08-17-2013, 03:06 AM   #1
reaper_falcon
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Amp problem

Hey everyone. Im just about to sell off my guitar amp, and just as I've gone to test it, it makes this horrible loud humming noise. Im pretty sure its never been there before in the years ive had it. All I can think of maybe somethings come loose when Ive moved house. Ive done a google search and taken it apart. What I can come up with is that its an earth problem. Ive got a Randall RG200 with 2 100W Celestion speakers. Its a great little amp but I just dont use it anymore and its taking up space. Ive swapped over power cables, used different power points around the house thinking the house is old and has a crap earth, tried only 1 speaker thinking a speaker might me damaged, cleaned the terminals on the back of the power port, checked the earth connection to the body. Im seriously out of ideas. Ive also checked the capacitors to see if any are bulging but all looks perfectly fine. Ive tightened the connectors on the plugs, made sure the plugs are all fine. Also tried bridging both earth points that the arrows are pointing to in the 2nd photo to see if one of the earths arent earthing properly. What about a blown fuse? PLEASE if anyone knows of an issue with these or has a much better idea than me, please let me know. Im tearing my hair out and wont have much left!


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Old 08-17-2013, 06:57 AM   #2
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if there is no cable plugged into the input is there still a hum?

Just turn your amp on with nothing but the speakers connected and turn the volume up to see if the noise is still there.

Spray some electrical contact cleaner in your guitar cable's jack and insert and remove a few times. Sometimes dust can cause your jack to become dirty and you will experience similar problems.

It also could be your guitar cable is messed up. Try another cable.

if all of this does not work, try a different guitar and see if you can try the amp at a friends house to make sure it's not your power in the house.
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Old 08-17-2013, 04:54 PM   #3
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If it still hums with no cable plugged in then the problem is the amp itself. How well is the amp shielded? Note that testing/operating the amp with the chassis outside the cabinet negates any shielding unless the entire chassis is a closed metal box and is grounded.

Also how old is the amp and how long has it sat unused? If it has sat unused for a relatively long period (5 years or more) then the filter caps could be hurting a bit. Try turning on the amp and leaving it on for a few hours and see if the humming is reduced or goes away.

Last edited by Invader Jim : 08-17-2013 at 04:58 PM.
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Old 08-19-2013, 06:25 AM   #4
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Hey guys thanks for the responses. Ive checked over all the contacts and solders all looks pretty good. I might leave the amp on for a few hours and see how that goes. Yeah it hums even without anything connected ie: guitar cable, and foot pedal. I dont know about the shielding. Its not a really well shielded amp, but paying $700 second hand you should think its good quality. I was thinking of putting a few feride cores somewhere or use aluminium foil to shield signal wires. The amp has sat around the house for about 6-8 months unused and is probably 3 years old.
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Old 08-19-2013, 05:22 PM   #5
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Pull the green/yellow ground wire's spade terminal off the AC socket in the amp and see if that helps. You can also try moving some of those wires around to see if lead dress is the problem. Lastly, you could try adding some shielding to the amp cabinet. They make copper tape with conductive adhesive backing but a staple gun and 3 layers of aluminum foil work too (it silenced my old Frontman 25R).

It's common for noise problems to crop up suddenly but diagnosing them is difficult, especially over the internet.
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Old 08-20-2013, 06:38 AM   #6
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I read on a few post which suggest putting a 3-2 cable adaptor in which eliminates that green/yellow wire. But ive got some copper shielding left over from my project guitar so ill have to give it a go.
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Old 08-20-2013, 06:57 AM   #7
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I tried removing the yellow/green wire but no luck. What it seems to be doing is once you turn it on, it makes a pop noise, then the buzz comes along. Which parts of the amp do you recommend shielding? Could the noise be caused by the radiation coming from the circuitry above the speakers?
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Old 08-20-2013, 12:35 PM   #8
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The pop noise is most likely just the switching circuits powering up. Basically all modern amps have some sort of electronic switching circuits for various features, especially Clean/Drive channel switching.

It's not likely that the noise is caused by the amp circuit acting on the speakers. The only way that would happen is if the speakers were in a strong AC magnetic field like from being right next to an unshielded power transformer. The AC magnetic field would induce a voltage in the speaker voice coil and that would cause the speaker itself to hum at the AC field's frequency. To test it, simply disconnect the speakers from the amp and turn the amp on. This amp has a solid-state power amplifier so no damage will occur. If the amp is on and the speakers hum while they are disconnected, then you know there's a stray magnetic field coming from somewhere...

Shield the entire head. Shielding tape should be applied to the cabinet where the head bolts into it. The tape goes where the head's chassis has exposed areas (the top and sides in this case). The idea is for the head to be totally encased so that it is in a metal "box" (which is made up of the chassis itseslf and the shielding tape along the top and sides). Make sure the shielding will have a good ground connection to the chassis when it bolts in. It should be fine if the tape contacts the mounting flanges.

Hope that made sense...
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I can't imagine a hand made Russian spec Big Muff would be that desirable. Maybe I'll just use both and crush people with my epic russian gain penis.

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Old 08-21-2013, 03:56 PM   #9
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Yeah mate thats what I would have thought. Iv had the amp on with the speakers disconnected with no noise. I will try to shield the head today and see how that goes
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Old 08-22-2013, 02:48 AM   #10
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You say it didn't hum like this before. Something has broken so shielding won't change that, it will only quieten a noisy amp, not fix a broken one.

It also seems unlikely to be an earth problem unless your tinkering has made a difference to the hum which you haven't reported. I'll get back to you on that though.

Firstly I'm worried about safety here. A 100W amp is running at least 70V DC which is enough to pick you up and throw you across the room (I speak from experience) and may stop your heart (I was lucky). Your questions indicate a lack of knowledge which could end up with you being hospitalised. If you live in the States then amp techs are cheaper than doctors.

Turn all the gains/volumes down. Does it still hum? The fault is in the output stages/power amp. If the hum is reduced it is in the pre-amp. The pre amp is the bit with all the controls on the power amp is the bit with the heatsink. In either case the problem could be in the power supply for the relevant bit of circuit. Almost always this is due to a failed smoothing capacitor. There may also be dropper resistors and a capacitor for particular parts of the pre amp.

It is just conceivable that you have a broken earth, either in the wires or the circuit tracks but more probably in the soldered joints. I don't think this is the most likely cause but you could fix a wire to the earth and try touching the free end to the other earth points one at a time to see if this removes the problem. this is potentially dangerous if you aren't completely clear about circuitry and you could ruin the amp if not yourself.

If you can solder, read a circuit diagram and use a multimeter then I am happy to advise. If not then you really should think about passing this problem on to someone else.
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Old 08-24-2013, 02:11 AM   #11
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Quote:
Originally Posted by Phil Starr
You say it didn't hum like this before. Something has broken so shielding won't change that, it will only quieten a noisy amp, not fix a broken one.

It also seems unlikely to be an earth problem unless your tinkering has made a difference to the hum which you haven't reported. I'll get back to you on that though.

Firstly I'm worried about safety here. A 100W amp is running at least 70V DC which is enough to pick you up and throw you across the room (I speak from experience) and may stop your heart (I was lucky). Your questions indicate a lack of knowledge which could end up with you being hospitalised. If you live in the States then amp techs are cheaper than doctors.

Turn all the gains/volumes down. Does it still hum? The fault is in the output stages/power amp. If the hum is reduced it is in the pre-amp. The pre amp is the bit with all the controls on the power amp is the bit with the heatsink. In either case the problem could be in the power supply for the relevant bit of circuit. Almost always this is due to a failed smoothing capacitor. There may also be dropper resistors and a capacitor for particular parts of the pre amp.

It is just conceivable that you have a broken earth, either in the wires or the circuit tracks but more probably in the soldered joints. I don't think this is the most likely cause but you could fix a wire to the earth and try touching the free end to the other earth points one at a time to see if this removes the problem. this is potentially dangerous if you aren't completely clear about circuitry and you could ruin the amp if not yourself.

If you can solder, read a circuit diagram and use a multimeter then I am happy to advise. If not then you really should think about passing this problem on to someone else.


Hey mate, I'm actually an auto electrician by trade and worked with many various photocopiers including Lexmark, Xerox, Kyosera and Oki for a few years so soldering, reading diagrams and multimering is definately not a problem. If I didnt know what I was doing i would have got someone else to do it. I just havent had much experience with guitar amps and from the things ive googled around the net it seems like a common problem so I was just curious to see how they work and how to conquer this problem. What ive done so far is shielded the amp head from the speakers and even shielded the speaker cables thinking there might be radiation causing the problem.

The next thing I was considering was to rip all the boards out, checking all the solders, possibly putting more solder on some of the contacts that look bad or in need, and cleaning all the contacts. Would dust build up cause this issue? Ive noticed it had quite a bit of dust on the board that has the headsink. I was going to brush it off and try to clean it up when I get it out.
Here is a picture of the amp head to, to give you guys a bit more of an idea.

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Old 08-24-2013, 02:12 AM   #12
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Also I'm not too sure if the noise might have been there before either. Im pretty sure it was there whenever I had the guitar too close but Ive only just turned back on recently and noticed the noise.
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Old 08-24-2013, 05:43 AM   #13
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Dust should be cleaned off. It acts like a blanket making it harder for heat to dissipate.

If you've done all this stuff and it still hums, that's as good as you're gonna get. All amps hum a bit, but when it becomes excessive/intolerable is when you'd want to start looking for possible problems.

In hindsight, a soundclip might have been helpful.
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I can't imagine a hand made Russian spec Big Muff would be that desirable. Maybe I'll just use both and crush people with my epic russian gain penis.

Last edited by Invader Jim : 08-24-2013 at 05:44 AM.
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Old 08-24-2013, 10:58 PM   #14
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Hi, fair enough you should be able to solve the problem but you've had over 200 views and I wouldn't want anyone poking inside a powerful amp without being aware of the safety issues. If you are aware of them that's great.

You haven't said if you isolated the problem to the amp or pre-amp as I suggested.

I'm also not clear if this is actually a fault that has occurred over time or a design fault which it has always had. As Jim has said all amps hum a little as the gain has to be fairly high for guitar. Are you dealing with an amp which makes an irritating noise which is only really noticeable when you aren't playing or is there definitely a fault? there's no point in giving you advice to locate a faulty component or joint if there isn't one, just a design that needs tweaking.
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Old 08-26-2013, 06:39 PM   #15
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Sorry guys. Ill try to see if I can get a video clip. The noise is present with everything disconnected, except the power cord of course all the knobs are all turned down even the master volume knob. As you turn it up the noise increases. Ive shielded the head from the speakers which has made a slight decrease in noise but its still there. I might take it to a mates place on the weekend and see if its still present and Ill get a video clip too
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Old 08-26-2013, 07:02 PM   #16
Invader Jim
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Usually when there's no plug on the input jack then the circuit input is grounded. A master volume is on the output of the preamp and kills the signal there so if the master volume is all the way down then the noise is coming from the power amp apparently.
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I can't imagine a hand made Russian spec Big Muff would be that desirable. Maybe I'll just use both and crush people with my epic russian gain penis.
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Old 08-26-2013, 11:35 PM   #17
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Assuming all the grounds are properly connected, Id attribute this problem to failed filter caps. a failed cap can look perfectly normal on the outside. a continuity test would confirm is the cap is bad or not. continuity test implies detatching the pcb to measure connections on the bottom, which is dangerous if you touch something bad. if you dont know what your doing, then take it to a tech or just give away/sell the amp as is and let the next guy take it to a tech.

caps deteriorate as the years go by and eventually fail. if the amp is old, then this may very well be the case.
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Old 08-27-2013, 12:47 AM   #18
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The amp is like 3 years old and has only been sitting for 8 months.

Continuity testing with a regular ohm meter is next to useless for measuring caps. The only reliable test it'll give is if the caps are shorted. If they were, you'd already know it...

The best way to test a cap is at full rated voltage with a cap tester. To test for intermittents and dielectric weakness a very low duty-cycle square wave pulse that goes to at least 150% of the cap's rated voltage is needed.

If you have an oscilloscope you can use the scope's square wave output to test an entire board full of caps without unsoldering anything. Connect the square wave output to a 1k resistor (depending on the capacity of the cap) and the cap from the resistor and ground and observe what it does to the waveform. A good cap will drag the waveform down almost to the baseline. An open cap will leave it unchanged. A high ESR cap will leave some of the wave but not all.
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I can't imagine a hand made Russian spec Big Muff would be that desirable. Maybe I'll just use both and crush people with my epic russian gain penis.
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Old 08-29-2013, 02:47 AM   #19
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A couple of things you might try.

Most amps will run for a few seconds after the power is disconnected on the charge stored in the power supply caps. Set yours up with something playing through it you don't need to touch. An iPod is great for this. Keep the volume right down to transistor radio levels so you can hear the hum and you don't use too much power and flatten the caps too quickly. With the amp running pull the mains plug out. Like I say the music should continue a few seconds and fade. If the hum is in the power rail then it should cease the moment you pull the plug. If it does then the main smoothing caps are suspect, either they have blown or they were too small to start with.

If you have an old fashioned analogue multimeter then you can check caps with this. Make sure the cap is fully discharged by shorting the terminals or you will destroy your meter. For anything over 200uF best to do this through a 1000ohm high current resistor. Set your meter on the ohms range. The resistance should start low and then appear to rise as the cap charges from the voltage produced by the meter, reverse the meter and the needle will jump as the cap discharges then the resistance will start to rise again as the cap reverse polarises. By waiting you can get an idea of how long it can hold a charge for. This isn't a very technical test but I've always managed to find the damaged cap this way and to sort duds from a fresh batch.
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