Make it (the buzz) stop... please

[idle_chatterbox]

I could email you a large scale photo if that would help.

Let me try to answer your questions.

1. I agree there is no "chassis" in the traditional sense. Like I said before, the amp head is mostly empty space, an empty box. On the front is a cheap loking piece of galvanized tin (?) that the long PCB with the input jacks and pots is screwed to. From the location of the screws, this seems to be where the panel is grounded, through the screws. Same with the back panel, which is also fairly thin sheet metal containing the PCB with the large egg roll caps. The transistor bolts to the sheet metal, just where the main cord comes in from the back. What ground there is between the front sheet and the back i'm not sure. I can try to figure that out. There are I believe a total of 5 wires coming from the back sheet to the front, but as I said in the last post, 2 of those (the green ones) are going to the indicator light. The other 3 I will have to check on.

2. yes, the little box is a spring reverb unit (which sounds terrible by the way). It has four very fine wires connected to it and going to the front PCB.

3. near the main power in and the transformer is a wire that is grounded to the back panel with a spade connection and a screw (that screw also holds the trans in place). I assume that's the main ground? Nothing else is connected to that screw.

4. QUOTE: " I think that grounding the front panel to a separate system could actually cure hum caused by this."

So connect a jumper wire from a front panel ground to another unit, like an effects board, that is grounded would work?

5. QUOTE:
"The section on the left (by left I mean the side housing that "spring reverb case" ) interests me and a better picture of it (or a set of them) would definitely help. I assume that this is the section which ties the circuits together."

Yes, that's the "master" section, with master volume, bass, treble, etc"

" Looks like three wires run out from it - can you identify them? "

I will try. No schematic, but I can try.


"The section at the far left looks like the switch you installed but there's something under it as well. Now, I saw some pictures of this amp while looking for the schematic and recall that this place holded direct inputs to power amp - is this true?"

Yep. Good memory! There were originally 3 jacks there. Monitor, aux in, and aux out. I took out the monitor jack when I installed the switch in its place. The aux in and out jacks are still there as they were.


"Looks like you lifted one jack off to house the switch.... make sure the jack can not short anything or that removing it has not ruined any important continuity."

The old jack is tied off and insulated so it can't short anything. And when I reinstalled the jack, it made no diference to the hum.


">If this indeed is a direct input to power amp the next thing is important: Do these jacks hum as well. If they do is the hum amplitude equal, smaller or higher. This might tell you a lot about where the hum source is located."

Let me try. I seem to rember that plugging my guitar into them did nothing. But let me try again tonight when I get home and I will let you know.

I'll have to wait to work on it when I get home... But I'll be back with the results.

Thanks again

[teemuk]

1) Yes, the panel is definitely "grounded" through the screws. You want to check them out for corrosion which could make this connection more resistive. What would that do - hard to say, probably nothing: The jacks are already grounded. A failed connection would ruin a large groundplane though. If the PCB design was bad (it seems to me that at least the ground routing is very bad) it might matter.
...The transistor? I assume you meant transformer.

3) "a wire that is grounded to the back panel with a spade connector" ...Where is the other end of this wire connected? Don't make quick assumptions of what this is. What you want to locate is the ground that the circuit uses as the reference. That must exists somewhere in the secondary circuit. The wiring of the transformer is pretty hard to visualize from the picture but those three wires connecting the PCB must be the secondary. That most likely means these should be two AC nodes and one center tap node. Besides these you would have two transformer nodes that connect to mains (there might be unconnected spare ones for other mains voltages). If there is a wire coming from transformer which connects straight to chassis it would most likely be a ground connection for the transformers casing.

4) "a jumper wire from a front panel ground to another unit, like an effects board, that is grounded would work?" ...Yes, this works. Your coord's "ground" wire has the same function. However, this wire is not a ground, it is a ground return with resistance and current through resistance means voltage. This again means that the ground voltage on the other end of the coord can not be the ideal 0 V. If the wire is very long you will surely have problems and the reason why you should route the ground returns inside the corresponding equipment is to avoid the problems caused by the resistivity of the ground return. Note: Every time you hookup the guitar wire to another system you connect the jack's ground return and the (another) system's ground return together.

5) I see, probably no direct in to power amplifier then but something which comes very close. The aux in is what you should test, it is probably designed for "line level" signals, which usually contain higher signal voltages than guitars output. Also the input impedance is most likely different. You should, hear something when you plug a guitar into it though. If not, try to amplify the signal input or try an external line level signal source. Remeber that by doing so you increase the signal to noise ratio - listen the same source through guitar inputs to have a reference of the amount of gain. The jack might be preceded by a gain stage before the signal is fed to power amp, however I suspect that the total gain of this auxliary channel is lower than of the guitar channels. If the hum source is the input you should hear this as a reduced amount of hum.

I don't know the difference between monitor and aux out. Monitor sounds like it could be a high current insert for monitor speaker (very unlikely) - aux out would be the same as preamp out. Plug this jack to a suitable system and test whether you have the same amount of hum that you hear through the power amp present there. Monitor could also be the direct connection to power amplifier - you need to trace this out to be sure. Better avoid plugging anything into it in case it truly is a jack for monitor speaker.

Higher resolution pictures will help me. My email address is teemukyttala[removespam]@gmail.com ....you know what to remove to make that a valid one.

[idle_chatterbox]

3) "a wire that is grounded to the back panel with a spade connector" ...Where is the other end of this wire connected?"

I was referring to the main ground wire coming into the back from the wall. It's the 3rd prong wire (green in the US).

"The aux in is what you should test, it is probably designed for "line level" signals, which usually contain higher signal voltages than guitars output. Also the input impedance is most likely different. You should, hear something when you plug a guitar into it though. If not, try to amplify the signal input or try an external line level signal source. Remeber that by doing so you increase the signal to noise ratio - listen the same source through guitar inputs to have a reference of the amount of gain."

Ok, I tried that. I first ran the guitar through the Aux in, and ran a cord from aux out to a small test amp (probably no more than 2.5 amps). The sound was reasonably quieter in terms of hum, but I'm not really sure if the Aux was really doing anything. It wasn't noticably louder than when I just plug into the test amp.

I then ran the pandora (no guitar) into the Aux in, and used the test amp as the aux out again. No hum whatsoever. But again, I don't know if it was any different in tone or volume than what I would have heard if I'd just have gone straight to the test amp.

I then ran the guitar into my Behringer pre-amp, ran that into Aux in, and ran a cord from aux out into the test amp. I'm not sure what that showed, to be honest. It produced a high pitch feedback type howl, so loud you couldn't hear the guitar at all.

In each case, no matter how I had things situated, I could turn the "Auxiliary" pot all the way up without really hearing a difference (is it possible I wasn't hooking up the right auxiliary?), but as before, as soon as I touch the "Master" volume, the hum is there again.

Overall, I'm not sure what I learned. In some ways, I'm not sure I heard anything different than I would have if the Aux in and Aux out was just a patchbay. I could make the signal louder if I cranked the Master volume, but that came at the cost of the hum.


"Higher resolution pictures will help me. My email address is teemukyttala[removespam]@gmail.com ....you know what to remove to make that a valid one."

they're on the way. Thanks

[teemuk]

Your tests did show very important points:

1) The master volume control introduces the hum regardless where you plug in the guitar.
2) The hum is not amplified by the early input stages of the preamplifier.
3) Only the signal heard through the power amplifier has a substantial amount of hum.

The only ground return path from the preamplifier to power amplifier seems to be that green wire. It is as well the only thing grounding together the front and rear panels. Your amp relies on use of large ground planes as conductors for return currents. Sometimes this is as effective technique as careful routing of the returns but what your amp fails to do is to make an adequate connection between the panels.

This causes some issues:
1) The ground return currents of both signal and supply are still forced to share the same path (wire). This ruins the idea of using large ground planes.
2) If front panel for some reason goes to mains potential the mains current has to flow through the preamp PCB traces, then through the green wire between the boards, then through the power amp PCB traces, not to mention all solder joints between. What you want is that fault currents flow straight to safety ground lug on the rear panel through a thickest wire possible. What you also want is that the voltage potential on the front panel (ideally zero) will not float according to amount of return currents and the resistance of that green wire connecting two boards together etc. - thingie. You want a solid ground connection which increases the size of the ground plane between these two panels.

Here's what I suggest - again: Hookup a very thick green wire under the screw holding up the upper left corner (seen from back) of the preamp PCB. This place seems to be reasonably close to both wiring in mains potential and what seems to me to be the point where the green wire leaves the board. Connect the other end of this wire to the safety ground lug, see if this removes the hum or weakens it. This should at least enlargen the ground plane not to mention improve the safety of the amplifier.

If this does not help try connecting to the upper right screw point holding together the power amp/supply PCB instead. This would be the (quite) ideal path to the secondary's main ground point which is somewhere in the area containing the speaker return, main filter capacitor return and the connection of the center tap wire (yel). However, I have a doubt this makes any difference if the hum was still present after the first procedure.

Even a better option would be to use a metal bottom plate in the case. However, you'd have to make sure it retains the electrical connection between both panels and mounting such a plate might prove much more complex than using a thick wire.

Thanks for the pics, they did help.

[teemuk]

Something like this...

[idle_chatterbox]

Thanks Teemuk.
I connected a large gauge wire from the ground on the back panel to the front panel. No change. Then, I tried connecting a number of jumper wires from ground points on the back panel to ground points on the front. No change. Wish it had worked, because now I'm wondering what it can be, if not a ground problem. 

[joecool85]

It could still be a ground problem.  My LM3886 amp had a bunch of noise no matter where I added ground wires, I then found it was due to one poorly placed wire near the power supply.

[idle_chatterbox]

Well I guess that's a little bit of encouragement JoeCool. Just out of curiousity, though, was the problem that you found one of a ground where it wasn't supposed to be, or the lack of a ground where one was supposed to be. The reason I'm wondering is i'm curious about what tipped you off. Was it something as simple as finally seeing a disconnected solder junction, or did you have to do a lot more analysis than that?

[teemuk]

Well, I guess the only thing left to do is to start probing the thing to find out where the hum is actually induced to the signal. You already eliminated the input stages as the source so plug straight into aux input (it's probably closest to the power amp in signal path) and work your way up there.

Before that, I suggest you try a complete disconnection of the preamp from the signal path to see if it cures the hum thus totally eliminating the possibility this is caused by sections in power amplifier.

You can do this fairly easily:
1. Connect the green wire from the power amp to the ground leg of a jack not connected to preamp in any way (i.e. hanging in the air). You either need a new guitar jack or have to remove one completely from the amp.
2. Connect the white wire from the power amp (I assume this is the signal) to "tip" lug of the jack.
3. Don't plug in yet. Power on and check if there exists any DC between the tip and the ground in the jack.
4. If you had DC you need to decouple the signal with a capacitor. At this point it is also worthwhile to check if the amplifier does the decoupling in either pre or power amp. If there's a decoupling cap in the power amp but you still have DC you have located a leaky capacitor.
5. Plug in a guitar (with it's volume control turned down). Raise the volume gradually and see if the amp still hums. It should not - you plug staight into the source which is always on and perfectly runs without hum. If you still have hum then for now on we can completely ignore the effect of preamplifier. Either the guitar is faulty or the bad return path exists in the power amplifier.

Next, (unless you already located the problem) put everything as it was and then connect the jack (preferably signal bypassed with a cap) in parallel with the power amp connecting wires. Once again you plug straight into power amp, this time with the addition of preamp grounds. See if you have hum. If the hum is gone it's most likely due to insulation of the jack from any other ground retuen paths than the one going straight to power amp. See if connecting the jack ground to chassis brings back the hum.

[joecool85]

Well I guess that's a little bit of encouragement JoeCool. Just out of curiousity, though, was the problem that you found one of a ground where it wasn't supposed to be, or the lack of a ground where one was supposed to be. The reason I'm wondering is i'm curious about what tipped you off. Was it something as simple as finally seeing a disconnected solder junction, or did you have to do a lot more analysis than that?

The ground was grounding what it was suppose to, but because the wire ran near the power supply, it was picking up noise.  I figured it out by doing a TON of research on making a quiet clone amp.

[joecool85]

I didn't get a chance to re-read the entire thread, but has anyone mentioned rectifying diodes?  They can go bad eventually, and it could cause some severe noises.

[teemuk]

I didn't get a chance to re-read the entire thread, but has anyone mentioned rectifying diodes?  They can go bad eventually, and it could cause some severe noises.

Yes, but in this case wouldn't the noise be heard constantly? I mean, he can plug in a guitar with Pandora and the amp works fine.

With my logic this indicates a grounding problem - something tricky is happening there - possibly in the way the return currents run to ground point. What causes this is another question: There might be a faulty component somewhere - something difficult to detect because the amp seems to work normally, except that it now generates a current flow to the signal ground returns. (With the term component - besides electronic parts - I mean as well solder joints, screws etc..) Somehow this effect is also canceled with some external effect units.

Edit: JoeCool85, As soon as I have more time I can start a thread about proper grounding methods, I think this is pretty important topic after all. ...Unless someone else is faster than I am of course. Then I will happily contribute the results of my research as well.

[joecool85]

You have a good point, it probably is a grounding issue.  I think a good grounding thread would be a great thing to have, maybe we can add it into the wiki as well.

[idle_chatterbox]

Just have a minute, but wanted to say thanks to you guys for all the time you're putting into curing my patient.

I'll try the stuff you suggested and get back with ya. By the way, last night saw Eric Johnson play. After the first song, when he was introducing the band members, he complained about "the hum" (while pointing to his marshall full stack). I was thinking, "dude, that's NOTHING...."   :lmao:

[joecool85]

Just have a minute, but wanted to say thanks to you guys for all the time you're putting into curing my patient.

I'll try the stuff you suggested and get back with ya. By the way, last night saw Eric Johnson play. After the first song, when he was introducing the band members, he complained about "the hum" (while pointing to his marshall full stack). I was thinking, "dude, that's NOTHING...."   :lmao:

You should have been thinking, "Dude, you can totally fix that!"  haha