Fresh Meat

[lam0go]

Hello I'm Levi and I'm an alcoholic, oh wait wrong group. I'm Levi and I'm a noob, that sounds better. I am just getting started in guitar and music in general. A friend is walking me through a Mel Bay book, but I am also interested in how my guitar works. I just bought a broken guitar that I needed to replace the volume dial and now I have an amp that has a bad buzz I want to fix. I think it is considered a Solid State since I don't see any tubes. I looked around on the Internet for a while and this site came up often so I thought I'd join and see if anyone can lend me a hand on my project. I'm an American now living in the Philippines going to school so buying parts isn't easy cause they are hard to find here and I've discovered no one has any idea what I am asking for when I go look. (No Radio Shack)

I am familiar with soldering but anything else dealing with circuitry is new to me. But let me describe my project. The Amp I am sure was manufactured here in the Philippines, so it may not be something standard like a Fender or Marshal amp. The case is broke because something fell on it when it was being moved. It still works but I am going to rebuild the case, and while I am at it hopefully get rid of the buzzing sound. I have an idea where the buzz comes from but would like to run it by someone first if someone wouldn't mind helping me.

Description of the Problem:
A constant buzz when the Amp is on, it gets louder when the volume is turned up, and also quieter when I touch and hold the strings, or the TRS jack. So I think it is a grounding problem. It may be due to the way the amp is built, so when I rebuild the cabinet I am thinking of modifying it. All the circuitry exists on a metal tray, I mean everything. I disconnected the speaker and powered on the amp and I can still here a hum coming from the large power supply, not sure what that is called. I think that same hum is what is being sent through the circuits. The large power supply is bolted on to the same metal tray with out any rubber bushings, metal to metal contact. I am considering mounting it to a new location in the new cabinet I'd like to build, on wood where it can act as a insulator and mount it with rubber bushings, would this help?

Also the amp has a lot or circuitry I don't understand. I looked and studied the amp yesterday for about an hour and have effectively managed to remove two of the circuit boards. (The amp has to sets of inputs, each having there own Low, Mid, High, and Volume knobs, which are fed into another board that also has a Low and High Knobs which then goes to a "Master Volume" knob, then to another board which connects to the speaker and some other things that might be acting as "tubes" from a Tube amp. This lead to a lot of excess wiring in the circuitry which made me think it was picking up lots of interference and since I will only need to use it for one guitar I only used one input circuit and cut the other one and the Mixer part out completely. This new setup works but the buzz is still there. The one circuit board that I am using also has a hot wire fed to it, but no ground, does this seem correct? I have also hooked my guitar up to the final circuit board, eliminating everything else, and used the volume and tone controls on my guitar and this worked, but the buzz is still there. I'm not sure why the input circuit board needs a hot wire, it won't work without it, but the final circuitry doesn't need it.

I hope someone understands and can help me. Thanks.

[pecefrogg]

Hi Levi!  I'm a noob here also, and I have nothing to offer regarding your amp issues, but I just wanted to say that I like the cut of your jib, sir.  You gave me a chuckle that damn near blew a mouthful of beer across the room.  Good luck with your project.   :dbtu:

[joecool85]

Welcome Levi.  I'd love to see some pictures of this amp, think you could get some posted up?  Also, does it have a brand name or is it a homemade type of thing?  It could be a ground problem, but more than likely it is either worn out power supply caps or power supply caps that are just plain old too small...or worse, in backwards (I've not done it on an amp, but I have on a wallwart project and it made a buzzing noise before leaking electrolytic fluid everywhere).

[lam0go]

Well thanks for the welcomes, and glad I got a chuckle out of some one. I am uploading a video to youtube.com as we speak, the URL is http://www.youtube.com/watch?v=9MtNzA-mw4E&feature=youtu.be. Let me know what you think and if there is anything you want a better photo of I'll take it and load it too. As mentioned before, there is a power wire fed into the preamps but there is no ground except what is used for the signal lead from the TRS jacks that feed all the way to the amp then to the speaker.

[phatt]

Yes sounds like you have lost a ground connection somewhere.
so now it's time to troubleshoot.

You have find the common path from PSU common rail all the way to the input socket.

Always tricky to explain but some **fairly good drawings** here might help you get your head around what needs to connect to what.

http://sound.westhost.com/project27.htm

Be careful with those bare terminals on the fuse holders. At least one will be mains wire and it will kill you fast. 8|
I would at least wrap them in electrical tape while working inside the amp.

And I hope the home made looking chassis IS GROUNDED??? :trouble

Phil.

[lam0go]

Thanks for the input Phatt but honestly you lost me after troubleshoot. What's a PSU? (please excuse my newness)

I have some additional info however.

The cabinet was broken so I built a new one today and I didn't exactly build it to the old one, but redesigned it. The amp really isn't a home build, but from a local company I guess. I wouldn't be surprised if it has had this hum since new, and it might also be a "replica" of some other popular amp. Being manufactured here in the Philippines all the measurements for the box are in inches not metric so it is possible this is some kind of clone.

My plan is to mount everything that I'll be using to the new wood cabinet, not this metal tray. Is it ok if I do that? And if so, can I mount it all inside the new cabinet without making a new hum or should it be shielded from the speaker?

Thanks for the help so far.

[joecool85]

Levi, you will want your electronics in a metal shell that is grounded.  If you put it all in a wooden box without that, not only would the humming be worse, but also it could be dangerous to operate.  Proper grounding can save your life.

Is the current metal "box" grounded?  Is the circuit grounded to that box?

[lam0go]

Thanks for the warning about the metal tray, I can still use the metal tray without any issues, actually it'll be easier than taking everything out and rewiring it. I am not sure what you mean when you ask if the chassis is grounded. I am familiar with automobiles and understand DC and having a chassis ground, but I am not sure about AC. If I understand correctly the amp takes AC current then converts it to DC and the whole amp uses DC, correct?I don't think the chassis can have an AC ground, here in the Philippines we use 220V and two prong outlets, hot and neutral.

Are you asking then if the amp chassis is grounded like from the DC power? I don't think it is... I was looking at the wiring schematic that Phatt posted yesterday and see there is a ground that is from the Input jack, that runs through the whole circuit to the speaker, can this be grounded to the chassis as well? I also noticed the pots are grounded to this, mine are not, unless it is on the circuit board but I'll have to unscrew it and flip it over to see for sure. Lastly, the schematic shows a positive and negative fed to the preamp, mine has one hot wire, no negative.

I think the main buzz is in the preamp, I can almost EQ it out if I have all the pots to zero. If I wanted to ground it, could I just ground the negative from the TRS jack to a ground on the PSU?

[joecool85]

Levi, I would recommend grounding the PSU to the metal chassis and also grounding your input and output to the metal chassis as well.

[lam0go]

Thank you for the help. I got it all finished and removed the buzz. I ran a loop of all the potentiometers, input jack, pre-amp, amp, and PSU and grounded it all to the metal tray.

[joecool85]

Thank you for the help. I got it all finished and removed the buzz. I ran a loop of all the potentiometers, input jack, pre-amp, amp, and PSU and grounded it all to the metal tray.

I figured that was the issue.  Glad you got it all working properly.