Make it (the buzz) stop... please

[idle_chatterbox]

The thread concerning the Crate amp that won't stop buzzing motivated me to post my own sad story. But first a question. Are the suggestions for troubleshooting that were given for the Crate pretty much standard, in the sense that they can be used for an SS amp that's acting up? I was especially interested in the tests of the transformer, and the little doodad involving a resistor and cap placed between the amp (I'm guessing output jack) and the speaker.

Now for my tale. I've had this old amp sitting around for some time, but never use it because it is just a noise generator, and a loud one. With nothing plugged into the input jacks, you can't turn the volume up past 1 without getting a very loud hissing sound. Turn past 5 and the hissing is accompanied by an even more ear-shattering buzz. Plug in a guitar and forget it. You can't even turn past .5 on the volume or the buzz just takes over. I'm not doing any of the obvious mistakes either. I don't play next to a flourescent light, not using high-gain overdrive pedal, and am not living next to a Tesla Coil.

I've taken the amp apart, but couldn't find anything loose or, bad solder, etc. The pots make no real noise, and everything SEEMS grounded ok. From the photo you can see that this really is a simple design; it's just what it looks like, a 4 channel amp with 4 identical amp circuits leading into one "master." The hum is the same no matter which amp I plug into. There's no headphone jack, just two 1/4 output jacks, wired together (for stereo?) in the back. Hum is the same no matter which I use to connect the speaker.

I'd really like to be able to use this amp. I've never seen another one like it, and judging from the way it puts out compared to my 30watt, it's much more powerful than that. Plus it's got the "I could plug in FOUR guitars at once if I wanted to" vibe, and you know how crucial that is....

Any thoughts? I truly need this thing to justify my love.  :trouble

[teemuk]

See if you can bypass the preamp and locate the noise source to either pre or power amp. After this you could go on locating it to certain stages. Is what you hearing hum or hiss? ...There's a difference between them. Hum has a frequency of either 50/60 Hz or 100/120 Hz. Same you'd hear from fluorescent lights. If it's hum then most likely the preamp is inducing an awful a lot of it (because it seems like you can control it's amplitude by volume control). If it's not hum it could be almost anything - for example oscillation triggered by any input signal. Locate the source - then it becomes easier to troubleshoot.

Edit: That "little doodad" is known as Zobel network and it is used to attenuate high frequencies (i.e. oscillation). It's actually not a cure for anything - (more like a solution of sweeping dirt under carpet) - and best would be to leave it totally away. Sometimes it's however the only thing keeping the amp stabile. If your amp doesn't have one then I'm pretty sure it shouldn't need it in the first place.

[idle_chatterbox]

Thanks for the suggestions.

Maybe this will help answer your questions.

1. It's hiss if I unplug the guitar and just crank the "master volume" (on the right in the photo). Loud hiss, no hum. Once I plug a guitar into one of the 4 amps (same with all of them, I tried), and I try to turn the master volume pot even a smidgen (i.e., less than "1"), I get a hum (it's a buzz to me, but if hum is the technical term, let's go with that) that's just so loud that you have to shut it back down. It's note the gentle hum that you get from, say, a tube amp. This is a "something is WRONG" hum.

2. Now, here's the weird part. Whenever I tinker around with amps, I always use my Pandora as a "signal injector." Plugging it in is a lot more convenient than plugging in a guitar when you want to test something. I just set it to play a drum fill, and connect it with the cord into the input jack. So just for the heck of it, I was plugging it into each of the four amps, into the Aux in jack, etc. Whatever orifice I found, I plugged Pandora into it and fired up the drum. No hum whatsoever. Still some heavy hissing if I crank it, but to be honest, at that level I'll get evicted from my apt before I care about the hiss. So i'm thinking, ok, that's cool if I only want to listen to a drum or metronome running through my amp, what about the guitar? I plug the guitar into the pandora (as it's meant to be, nothing fancy), then plug the pandora into one of the 4 amps. Hum is gone. It's like it was never there. Clearly the pandora is getting rid of the hum, but I'm not sure how. And I don't mean it's just adding noise or something to cover it up. Even cranked, the guitar makes no hum, and it does this regardless of which setting the Pandora is on.

So on one hand, I guess i can live life knowing that I can never play through this amp without Pandora acting as a mediator. On the other hand though, I'd like to know what's going on.

To the folks at Korg: great product!  :tu:

[Joe]

Some ideas:

-Make sure no ground screws are missing, that go through tabs on the PCB. If they are all there, make sure they are tight.

-This is a pain, but remove the PCB and check the solder connections, especially on the jacks/pots. You'll have to remove the knobs and washers on all the pots first.

-Sometimes due to age etc corrosion forms around part legs, and sometimes you can even pull them straight out if it's real bad. Resolder bad joints if you find them.

[joecool85]

Replace your power caps, if they are old, they will dry out and cause noises like this.

[teemuk]

Hmmm... This seems like a grounding problem: The Pandora is between the amp and guitar so it possibly breaks a ground loop or something. How are the inputs grounded?

[idle_chatterbox]

Thanks for all the advice guys. Ok,
I will tear into it again and look for bad joints.
Don't mind checking the pots, probably could use a cleaning.
The capacitors should be easy to replace. They are all out in the open (the amp is mostly empty space). All of them, or just the big egg-roll size guys?

Re: the pandora grounding theory, I think I understand, but the pandora's just a handheld box, it's not grounded to anything and runs on batteries. Wouldn't any grounding issues just transfer through it? 

[idle_chatterbox]

Here's some jpgs of the innards. The one photo shows the front panel pulled out, with the 4 amp modules and the 4 sets of input jacks. The other jpg is the back panel, not much there except the transformer and the power section, and two jacks for output to the speakers....

http://pg.photos.yahoo.com/ph/idle_chatterbox/my_photos



Thanks for all the suggestions. I'll get to work on it.  0:)

[joecool85]

Just replace the two big guys for starters, those are the power caps.

[idle_chatterbox]

Ok, will do. Thanks for the tip. For now I guess running through the pandora is an acceptable workaround. Still wish I could figure out what's going on though. Checked all the grounding points that I could think of checking inside, looked at the solder joints, and cleaned out all 22 (!) pots. Still hums, loudly, unless pandora is plugged into 'er.

Thanks for the advice guys. My ears thank you  8|

[teemuk]

My theory is based on an assumption that Pandora is a digital effects unit. Somewhere inside it the signal is converted from analog to digital and then back. At this point the ground path may also be "cut" since analog and digital never actually should share an equal ground plane (only one separate point truly at 0V potential). This might be enough to cut a ground loop.

You should focus your troubleshooting on the input and see if the input / preamp grounds share any other (high current grounds) - for example, by making a connection to chassis via non-insulated jacks (your amp seems to use those). Ideally they should not do this: The input grounds should be connected either at the ground of their preceeding gain stage or or straight to main ground point (usually located very near rectifying / filtering ssection). Consider all other grounding points than the latter as conductors with resistance. Any current that does not belong there will be heard some way. i.e. Speaker return currents in the input ground are definitely not wanted. You'd also may want to hunt down all points where a ground conductor is divided (i.e. ground via chassis and ground via wire).

The idea to tighten all screws and check for oxidization / corrosion etc. was good: for example, a screw tieing the pcb ground to ground plane formed by chassis may have become loose. Also, have you tried with any other signal source i.e. radio/mp3 player - do they suffer from the same problem? You should override the possibility that your guitar might be causing this. You have already narrowed the source to input, try to narrow it more.

Those capacitor changes, cleaning and tightening all pots, screws etc. are always a good way to start fixing any old equipment.

[idle_chatterbox]

Ok, thanks. I'll tear into it again (I'm thinking I need to attach the front panel with velcro!).

Once I order the big caps, I'll replace those too. Here's some additional thoughts. I've tried more than one guitar, and I've tried those guitars on other amps (solid state and tube). The humming only happens with this amp.

I tend to think, but could be wrong, that it's something to do with the "Master" section of the circuit, since the hum is present in all 4 amps, equally.

The comment about input and output ground made me wonder. Originally, the amp had the power switch (it is a 3 position switch, on-off-on) located on the back panel, right next to the transformer, fuse, etc. I thought this was a bit impractical, so I left that switch where it was, and spliced in another two throw switch and stuck that on the front panel. I was thinking that I had just done something pretty cool, since I now have a "standby" switch and a "power" switch. But did I do something really stupid? The new switch is now grounded on the front panel, but I guess I figured that the back and front panels were already grounded (they certainly were according to my multimeter) together anyway. Did I make a really amatuer mistake? I have to say, it's a lot more convenient to not have to reach to the back to turn the thing on.

Secondly, the problem is sometimes intermittant in the sense that it can be loud or louder (it never goes away completely). This really doesn't seem to reflect anything that I'm doing, i.e., one morning I'll fire it up, leaving the guitar and pandora just like it was last time, and it will be humming louder. Come back in a few hours and its just moderately loud (as opposed to "this things gonna blow" loud). If it was a ground problem, woudlnt' that stay the same?

Lastly, when it gets really loud, if I listen on the outside of the cabinent, there's a serious vibration inside. Tell me that doesn't mean the transformer is shot. I'm guessing a tran will cost more than I paid for the amp.

RE: the connections, corrosion, etc. I'll double check everything again. I didnt' think anything was loose or poorly connected, but who knows I might have missed something. Thanks for all the advice and the moral support 

[teemuk]

Remeber to replace the preamplifier filter capacitors too. If they are bad they can cause serious hum. Still I think this is not the case in your amplifier. If the problem would be the bad capacitors the amp would hum all the time - even with the Pandora plugged in. Actually, the amp should hum with nothing plugged in as well. Same thing with the "mixer" section: if the hum would be induced there you should hear it even without having to plug someting into the amp. If you hear hum only when you insert a guitar then there's a 99% chance that you have trouble with the ground connection.

If a bad grounding is not the problem then the easiest way to find the hum source would be to probe the signal output of the various preamp stages with a capacitor coupled wire feeding a high impedance amplifier plugged to small speaker.

However, this made me worried: You grounded a power switch to chassis... How can you ground a power switch? Did you connect "N" of the mains to chassis? You should never ever do that!!! The neutral wire is sure "0V" but when you reverse the power coord (at least where I live you can easily do that) you will have the chassis in mains potential. The only grounds you ever wish to appear on chassis are the safety ground (the "E" of three-prong coord systems) or the secondary ground (optional). Also: If you brought the mains wiring near inputs (sounds like you did that) it might induce hum. Remove your switching arrangement and see whether it makes any difference.

If you want to switch the amp on and off from the front panel then use a DPST switch (rated for proper voltage) that switches on/off both L and N. If you for some reason could switch only one of them then switch the L - (since it's obviously safer). Neither one of these wires should never make a contact to chassis! They should also be insulated so you never can touch them by accident. Keep this switch a safe distance away from the inputs of the amplifier. Actually keep it safely away from any part you could actually touch. Read: Make sure the mains wiring can never touch any part of the amp if it would ever come loose. ...In other words, wrap the wires to heatshrink tubing.

I couldn't see the inside pics in full res but it looked like all input jacks are non insulated from the front panel. If your inputs now share the same ground with the neutral wire (return currents of the whole amplifier) I wouldn't be surprised you have a hum problem. Also, you do not need a separate mixing stage to induce hum problems if the jacks share the same ground path (chassis) at the input.

Vibration you hear could be a bad tranny but I doubt it. In my opinion it isn't very easy to blow up a power transformer - most of them tend to tolerate even long term short circuits. Anyway, it's normal for transformers to vibrate slightly. If your amp now greatly amplifies the hum from the input stage it will be under relatively small load, during that time it's quite natural for the transformer windings to vibrate (more) and make noises. Even when amps runs normally I can hear noises from all transformers except toroids. Transformers make noise even when the amp is idling. Only, if you hear the transformer constantly emit a "loud" "howling"-like noise then suspect a short circuit somewhere.

[teemuk]

Switching should be done like this. If your amplifier switches the secondary side (some amps do this) then tear off the damn thing and rebuild like this. (Switching only secondary is both inefficient and dangerous). If your amp has a two prong coord (only L and N conductors) then do yourself a favour and replace it with a three-prong one.

Note: I haven't included the color codes for mains wiring since due to different variations (Europe/US & old/new) they may become quite confusing to some people who blindly follow instructions without thinking first. If one can not figure out which is which by him/herself then I suggest to stay away from mains wiring...

[idle_chatterbox]

Sorry, I didnt' mean to say that I grounded the main supply line to the chassis (I'm clueless, not crazy). I meant to say that I ran the line that had previously been going to the fuse through a DPDT switch and then back out (and back to the fuse). By making that wire longer, it can now reach to the front panel (where it's installed in a hole). So the only real modification, from the little electrons' perspective, is that they will now get to the fuse a few milliseconds later, because they have to go through a second switch that either makes or breaks the circuit going to the fuse.

When I said that the switch is grounded I meant that the metal part of the switch (the barrel, washer, nut) is of course touching the front panel where it's attached, and THAT panel is connected to ground (the green wire on the 3 prong plug). Sorry about the bad description earlier. In any event, just to satisfy my curiousity, I wired everything like it had been, with just the original switch. It didnt' make any difference.

Thanks for the diagram. I will re-mod my front-panel switch that way.