Geovibe is buzzing--bad--in time with lamp

[celamm]

I have a stock build of a general guitar gadget/GEO neovibe, ie:
http://www.generalguitargadgets.com/index.php?option=content&task=view&id=136&Itemid=150

The build is stock--no mods yet at all--except substituted a 2N2222 for the 3904 that sinks the lamp since it was getting really hot (the 2222 gets hot too).  Using the GGG PCB and a BB case from Small Bear elect.

It has a horrible buzz in the audio out.  The buzz seems to get worse as I move the PCB closer to the pots and jacks in the enclosure but never fully goes away no matter what.  The buzz gets much worse as the bulb gets brightest, and fades again as the bulb fades.
The buzz sounds 60 cycle ish--low nasty buzz.  I also hear clicks sometimes depending on the positioin of the pcb.

The buzz is bad also when the depth is turned down. 

I am seeing between 27V to  29V DC at the + of the DB102 bridge rect. 

I am seeing 16V to 18V DC at the R40/R39/R41 junction (should be 11.7V according to docs)

I assume R41 should be a "solid" DC voltage?  Or is the voltage jumping in time to the lamp brightening normal?

I have checked, rechecked, and re-re checked grounds, resistors, and trannies.  All are OK as far as I can tell.

I have used a 18V DC 400mA wart, and 18V 1A AC wart.  No change.

I am seeing about a 1mV swing on the DC rail that powers the "phase array" (R14/Q4/C7/R13 etc etc).  Dc voltage here is 14.83V +/- 1mV.  Preamp section is the same.  Adding filter caps at these points makes no difference.

Other than the buzz the unit works--lamp lites, dims with depth control etc.

Note that if I put these on a scope I see DC rise and fall with the bulb throughout everything on the "LFO" side of the regulator.

Any ideas what to look at next?

[R.G.]

Did you by chance put a power jack into the box and power it from an external AC adapter? If so, does the jack let one side of the AC from the AC adapter touch the box?

This is one classic mistake that gets made over and over with the Neovibe - nonisolating power jack. When you put the thing into the box, the jacks connect signal ground to the box. The power ground connects one part of the low voltage AC to the box, and the two of them duke it out. The result is a big hum and possibly a smoking AC adapter.

That's not the only way you can get hum problems with this, but it is a common one.

[celamm]

No, I am using an insulated 2.1mm receptacle purchased from small bear.  I have checked and the wiring from the receptacle to the Bridge rectifier is "clean"--no shorts and not touching case, ground, or anything else.

[R.G.]

OK, not the simple problem then.

Has the unit ever worked well? Or is this the way it's always been?

Do you have fluorescent lighting near your workbench?

Some tests:
- clip your ohmmeter probe to ground on the PCB down at the power supply section. Probe the ground pads at the input, output and LFO section of the board to see if ground is continuous on the board. Then do the same for the off-board components which should be wired to ground.
- check to see if the hum voltage is present at the input and at the outputs of the third transistor with power applied and no signal in.

[celamm]

Thanks for the rapid responses.....

Unit has never worked well. It has always had this buzz.

Incan lighting near the workbench.  I have taken it to different spaces (studio, workbench) with same results.  No, no fluorescent lights, CRT computer monitors, etc etc, anywhere around my workspace.

I will retest ground continuity.  I have already done this and couldn't find anything amiss but this seemed like an obvious potential source of the problem to me too.

BTW did you see my question about the voltage on the LFO side?  Posts have indicated to me that I should be seeing more "DCish" readings around the R40/R41 voltage divider.  But I am not sure of this.  My voltages are much higher than what I see in the ggg pdf documentation--nearly 27VDC at the + terminal of the rectifier--and the entire LFO side sees quite a bit of voltage swing throughout the entire LFO section--on a scope I see DC that rises and falls in time with the lamp.   

Since it's a rising and falling voltage, and the resultant increasing and decreasing current, that makes the lamp brighter and darker, I would suspect this to be the case, and that this is normal operation, but I just wanted to make sure.   

I was just wondering if this was normal operation or not.

Again, thanks again for the rapid responses.

[R.G.]

Your DC at 27 to 29V is way high. That really ought to be under 24V, just for the safety of your lamp driver if nothing else. You don't use 25V electrolytics do you?

I haven't really probed the R40/R41 divider, and if it's oscillating, that's what it is supposed to do, so I'd solve the buzz first.

I would not be too concerned about the raw DC voltage waggling around with a heavy load like an incandescent lamp. The lamp getting brighter and darker is what you want, and when it gets brighter, it pulls down the raw DC voltage.

What are you powering it with to get 27Vdc?

[celamm]

It's an 18V DC 400mA wall wart that I got from All Electronics...surplus, so it may be suspect....I will measure it without load....yes, this seems unusually high to me as well.  However I tried a 18V AC wart as well and had the same issue.

This seems like a longshot but I wonder if some sort of wiring issue or other screw up has given me some sort of diode + cap voltage doubler.  Seems hard to believe though.  I will continue to test and post again....thanks!

[markphaser]

LFO section--on a scope I see DC that rises and falls in time with the lamp

I'm not sure if u should see rising and falling DC?

On the oscilloscope it should be DC offset on the output of a LFO with AC LFO waveform riding on top so the DC is a straight line not moveable but Positive DC offset is above the zero Reference with the waveform riding on top above the zero Reference

YOur using a 18V DC 400mA wall wart how are u getting 27 volts from a 18v wall wart ?

[Paul Perry (Frostwave)]

The buzz gettign worse as the lamp gets brighter, tells me that you are
sucking too much current for the transformer, so there is a LOT of ripple
at maximum current draw (and indeed even at minimum draw).
A different wart is called for.

[markphaser]

What causes a Lamp to buzz?

Is the buzz coming through the power supply going through the lamp causing it to buzz?

[celamm]

I like the idea of the wall wart not supplying enough current--I had a VCF that had the same problem, and upping the current the wart could source solved it.  The DC wart says 18V on its stick on label, but if I check the DC voltage unloaded it comes out 24V, so maybe there is something up with that. 

I will try a different wart and let everyone know....in the meantime this continues to drive me batty.....

[Mike Burgundy]

Did you measure the wallwart voltage under load? It's normal to get a higher voltage with no load, and a lower voltage under too much load, since the wart cannot supply enough current to maintain it's rated voltage. If you're measuring 24V *in circuit*, either the wart is shot, or it's waaay over-specced for the current draw, or you're simply not using enough - the vibe circuit is somehow wrong.
With 24V, I'd doubt there's an over-current situation on an 18V wart, IF the wart is okay. More like under-current.
Swap to another wart (perhaps the 18V DC one), make sure there's ample regulation inside the pedal (diode bridge, big cap will do) and see what that does. Voltage-wise, too.

[celamm]

FIXED! FIXED!  FIXED!  FIXED!  FIXED! 

The suggestion to use a heftier power supply FIXED THIS THING!!!

I ended up using a discarded 19V 2A DC wart from a broken laptop.  Buzz is completely gone, and the
stomp sounds GREAT!!!!!

Thanks to EVERYONE on this list who HELPED!  I was going to lose my mind over this one!!!!!

(OK put on your Telemundo sports announcer hats now!!!!)

          **Fijo!***       

(!!!!)

!!!!FFFFFFFFFFFFFFFFFFFFFIIIIIIIIIIIIIIIIIIIIIIIIIIJJJJJJJJJJJOOOOOOOOOOOOOOOO!!!!!
!!!!FFFFFFFFFFFFFFFFFFFFFIIIIIIIIIIIIIIIIIIIIIIIIIIJJJJJJJJJJJOOOOOOOOOOOOOOOO!!!!!
!!!!FFFFFFFFFFFFFFFFFFFFFIIIIIIIIIIIIIIIIIIIIIIIIIIJJJJJJJJJJJOOOOOOOOOOOOOOOO!!!!!

--CL