Testing Bad Filter Cap with a scope for 120HZ hum

[wsscott]

Shooter, yes I disconnected the cap from the circuit on the downside and measured the voltage there.  My meter was set for auto-ranging. Maybe the jumping is because it’s in the oscillator circuit.

[SEL49]

Are y'all even looking at the schematic? What does the note in the gray call out box say?

[shooter]

 


I looked to see where I went wrong on the VDC readings, doesn't that count?? 

[wsscott]

Having strapped another cap across the Node B cap with no effect, I'm doubtful changing the cap will make a difference.

But before I do change the cap, I'd like your comments about how the reservoir and filter caps have been wired/grounded by me.

The CT from the PT is soldered to an isolated turret, which then connects via a 1" jumper wire (the CT wire was too short) to another isolated turret, where the Negative lead on the Node A reservoir cap is connected.

From that point a jumper wire is soldered to the ground buss that is only connected at the opposite end to the chassis through a Hum Loop Blocker circuit.

Each of the remaining 4 filter caps, Nodes B-E, have their Negative leads connected to separate points along the ground buss wire. Grounds from components are then wired to the grounding point of the ground buss where their Filter cap Node is wired. None of the filter caps are connected directly to one another, and none of them are connected to the Negative lead of the Node A reservoir cap. Their only "connection" with one another is through the ground buss.

Any helpful comments?

[shooter]

through a Hum Loop Blocker circuit.

been moving snow n wood all day so I mighta missed it in the schematic we're all using??

From that point a jumper wire is soldered to the ground buss that is only connected at the opposite end to the chassis

maybe for testing you remove the "jumper wire" the "loop-blocker" and just ground that point to the chassis

[wsscott]

Shooter-I just added the Hum Loop Blocker recently.  So it's not in the schematic.  It improved some other issues, but not the 120hz hum I'm trying to fix.

[AlNewman]

So you're saying if only V7 is connected, the hum is present?  And increasing the reverb intensity increases the hum?

If you ground the grid of V7A does the hum stop?

Perhaps you can put a jumper from the ground point of V7A and connect it to different parts of the bus wire to see if that makes a difference in the amount of hum?

[wsscott]

Thanks.  I’ll give both a try.

[wsscott]

AlNewman-I connected a jumper from V7A, pin 2, Grid to the chassis and the hum was much louder.

However, when I connected the jumper to the isolated ground buss, it is much quieter than when no jumper is connected.  So that is a definite improvement.  Maybe a level I can accept.  It is much better.

So the way that grid is currently wired, it connects via an insulated cable to the pin of the Isolated RCA Reverb Output Jack, and it's shield is only connected to the ground tab of that jack, which then goes to the isolated ground buss for its ground.

There is a 47K resistor between that jack's tip and ground tab.  That ground tab is sharing the ground for the Reverb Output Jack and the resistor.

So, given this info, what do you suggest I do.  Is that 47k resistor part of the problem?

[shooter]

Is that 47k resistor part of the problem?

IF.. it's the one circled;
It's typically soldered to the sockets grid-pin at one end, the ground Buss at the other.

[wsscott]

Shooter, that's the resistor I'm talking about.

I realized that when I connect a jumper from the grid to the ground buss, it cuts off the reverb, so that's why its quieter.

I should add that the Reverb Tank's Output Jack is grounded to the tank's chassis.  Its Input Jack is isolated from the tank's chassis.  Both of the jacks are isolated from the amp's chassis using isolation washers.

So the tank's Output Jack is grounded to the tank's chassis, and its also grounded to the isolated ground buss.  Maybe that's part of the problem. But it needs that connection the buss ground for the Reverb to function.

The resistor is connecting at the RCA Reverb Output Jack's pin, and also I've got the shielded cable with its isolated lead connected to that point and running to the grid pin on the tube socket.  The shield for that cable is connected to the same jack's ground lug which is connected to the isolated ground buss.

[shooter]

it's shield is only connected to the ground tab of that jack, which then goes to the isolated ground buss for its ground.

temp the ground wire to the chassis n off the isolated ground buss, better, worse?


FWIW I flunked story problems in math, but the ones with pictures I did ok in 

[AlNewman]

FWIW I flunked story problems in math, but the ones with pictures I did ok in 

As the saying goes, a picture's worth 1000 words. 

[wsscott]

I've tried sending a photo, and its only 1.6megs, but having an issue.

[wsscott]

No luck.  I've tried 4 times but it won't upload.  Don't know why.  Sorry.

[wsscott]

Shooter, the hum is lowest when the Output Jack is grounded to the ground buss.

There's really no difference in the level of hum when I disconnect the shield's ground totally.  So I don't think shielding the cable going from the Reverb Tank's Output Jack to the grid makes any difference. Surprising. It has the same level of hum regardless of whether it's grounded or not.

So when the Reverb is Off the hum is barely there.  When it is on, the hum is a bit louder and increases some with the Intensity level of the Reverb pot.  It is at a low level though, so maybe I'm just being too picky.

[shooter]

so maybe I'm just being too picky.

the sub-circuit you chased the gremlin to is the most sensitive of the whole circuit, not a reverb guy so just guessing the recovery stage will have a signal feeding the grid in the small mV range.  any "noise" in that area approaching signal level is gonna be a "problem"


my "too picky test";
put a signal into the box, if I can make the background "noise" go away with "volume/gain" levels at 1 or 2 on the dial will "pass"
If it's still annoying at 3-4, "failed" then i'm hunting or scaping the sub-circuit or whole box 

[wsscott]

Yea, I don't understand how I hear 120 hz hum on my amp, but don't see a signal for it at the Output jack of the "Revibe".  It would seem that if I can hear it coming through the amp that the Revibe is connected to, and that amp on its own doesn't have any hum, that I should see its signal at the Revibe's Output that sends the signal on to the amp.

[shooter]

I should see its signal at the Revibe's Output that sends the signal on to the amp.

by "signal" you mean 120hz hum?????
if that's the case "no 120hz at amp input", look at amp speaker n see if it's there


does either the Revibe or the amp "float ground" by using a 2 prone adaptor?

[wsscott]

I've tried the 2-prong adapter for the plug, and the 120hz hum is still there.

What I was trying to say is that when there is no Input signal going into the Revibe, and the Revibe is turned on, and I check the Output Jack on the Revibe that goes to the Input on the amp, I can't read any 120hz or other signal at that point, but I can hear the hum when the amp is turned on.  If I turn off the amp, then all is quiet.

It gets louder when I turn the Output Level pot CW.

I guess maybe the 120hz signal is so weak that its not audible, or measurable, until its amplified by the guitar amp.

I think the best practical solution at this point is just to keep the Output Level setting on the Revibe low.

[shooter]

the 120hz signal is so weak that its not audible, or measurable, until its amplified by the guitar amp.

as a field engineer; "IF I can't measure it, it doesn't exist" 

[wsscott]

Probably a good philosophy to have!

[wsscott]

I thought maybe 2026 would be the year I solved this issue, but not so.

I decided to give it one more try, and here's what I did with no success:

I previously put in another 12AX7 for V7, and no change.

I jumped in parallel another 50uf/400V cap across the existing 30uf Node C cap.  No change.

With reference to the partial schematic shown in Post 59 above:

I disconnected the grid input lead at Pin 7 on V7 coming from the Reverb Intensity pot. No change. Reinstalled.

I disconnected the cathode input lead at Pin 8 on V7 carrying the dry signal from the cathodes of V6. No change. Reinstalled.

So everything going into V7B has been tested.  The tube is good, the Node C power supply is good, the wet input signal is good, and the dry input signal is good.

So I'm left with the .047uf cap, the 1M resistor, the 100KA pot, and the output jack.  All of which test good.

So I'm finished on this one. Maybe one day I'll build a new one using the same schematic, but with a better layout and better focus on grounding and lead dress.  This was my first build from scratch, so I'm still very pleased with the result.

Thanks again for your help and patience with me.

Happy 2026!

[wsscott]

Just to prove I'm not hearing things or imagining the hum, I've attached a screenshot of my scope's image of the "Hum".

The setup is as follows:

No input going into the Revibe unit.
Output jack from Revibe has a 1/4" plug, with scope probes attached to tip and sleeve connection points.
Scope is set to 10ms/50mV.  50mV is the most sensitive my scope will go.
Output Volume pot is FULL CW.  Anything less and my scope can't detect it.
Input Volume adjustment has no effect on the image or the hum.
As you can see it's a 120hz sine wave, of 9mVrms or 32mVPP.

So since there's no Input signal from an external source, its being generated internally, and when plugged into an amplifier, the amplitude of the 120hz signal is greatly increased by the gain of the amplifier and is audible.

So at least I can see the hum now.  Still no solution, but I just wanted to let anyone who's interested see what's going on.

[shooter]

since you can see it, it IS an issue 

scope probes attached to tip and sleeve connection points.

if you get bored;
move the ground probe of the scope to various grounds within your box
so start at the input jack ground, or the PS mains ground to chassis, recheck 120hz amplitude
every ground point you use should produce about the same amplitude 120hz at the scope, I'm only interested in deviations in amplitude <> 25% of your "test" amplitude.

[wsscott]

I'll let you know what I find out.  I need to borrow a buddy's more sensitive scope that will hold the readings better. Mine is pushing the limit.

[shooter]

As you can see it's a 120hz sine wave, of 9mVrms or 32mVPP.

or not, the 9mVrms = 32mVpp, techs like rms, musicians get excited when the salesman quotes Power in pp


stick with 9, is it "stable" on your scope?


add a 480hz, 50mVrms signal to the input


what you want to measure is the ratio....  47mVrms(480hz): 9mVrms(120hz..."noise")
since 480 is 4X 120, they should both be visible on the scope.


I'm not sure what a "good number" is for S/N in a preamp, maybe one of the sharper knives??

[wsscott]

Ok, I got a more sensitive scope to use and it's much better.  I'm reading the same numbers but they're steady and I'm not losing the sine wave.

I also injected a 480hz signal and I can see both sine waves.

Without the 480hz signal, ie. no input signal, it's reading 8mVrms.  With the 480hz signal injected its reading 84mVrms.  So 10:1 ratio.

I checked all the grounds as you suggested and they're all the same.  All of the Neg/Ground leads from the 5 filter caps all read the same as it reads at the Output jack. And the connections on the buss wire are all the same.

So no changes.

What does this tell us?

[shooter]

they're all the same.

tells me there are no "resistive" grounds where a Vdrop could "let in" the 120hz


what happens at the speaker of an amp, if you "open" the ground at the output of the pre, so that only the signal gets to the amp, does the amp still "hum"?  (I would make up a patch cable with no ground wire, just signal at tip.)

[wsscott]

I decided to check the Output Level pot for 120hz hum, since the center wiper connects to the Tip on the Output Jack which I have isolated from the chassis.

At both the Center Wiper and the Input Lug going into that pot, it reads 60hz at 8mVrms.  The Center Wiper lug connects through shielded cable to the Output Jack's tip lug.  It is a wire about 3" long.  Its shield is grounded at the pot end of the wire that I have connected to the Ground buss.  When I read the voltage at the Tip connection on the Output Jack it reads 120hz at 8mVrms!

Now why is that?  Its leaving the pot at 60hz, but it reads 120hz on the output jack.

[wsscott]

I checked the B node filter cap, which is a 40uf cap, and at the Pos lead on that cap it is putting out a 120hz signal at 6VAC.

That doesn't seem right to me.  And that Node feeds the Vibrato Oscillator/Driver circuit, and the Reverb Driver circuit.

So maybe that 120hz is following the Dry Path from the Reverb Driver over to the V7, Pin 8 Cathode, and there it gets into the tube, and then flows out the Pin 6 Plate on to the Output Level pot and to the Output Jack.

[Willabe]

I checked the B node filter cap, which is a 40uf cap, and at the Pos lead on that cap it is putting out a 120hz signal at 6VAC.

What does the A node filter cap read?

And what do the the rest of the nodes read? C is what I'd be most interested in. 

[shooter]

Its leaving the pot at 60hz, but it reads 120hz on the output jack.

that with a scope?  can you CH1 the 60, CH2 the 120 points n grab a pic?


what is your scope's sync source? int, ch1, line?

[wsscott]

Shooter, the scope’s trigger is set to Ch 1 Auto.  That’s the channel I’ve been using for my probe.

Question, when I read AC voltage on the filter caps’ Positive leads, it shows as a straight line, not a sine wave. Is that the way it should be?

[shooter]

should show a sin wave if you're AC coupled, DC coupled should show the DC voltage with sinwave riding on that DC voltage (NOTE: dc coupled can n will damage equipment if you're not sure)

[wsscott]

Shooter-How do you suggest I setup the scope in order for me to check the Filter Caps for AC ripple?  When I try a DMM I don't get any AC readings. My meter may not be sensitive enough. So I'm hoping there's little or none there.  But, it's getting to the Output Jack from somewhere.

I just want to eliminate the caps as a source.

[shooter]

the DCV at your cap rail is typically higher than your scopes probes rating (usually ~300VDC)  so that's where I start.  my 1x/10x probe is good to 350vdc, my 100x is 600vdc.


a scope may also be limited, but that limit is at the probe input, so if you're measuring 400VDC with a 10X probe, the input will only "see" 40vdc


I'd check with the scopes owner n see if maybe they have a DMM, 1st, or VERIFY your probes can handle ~~400vdc since your 1st node is ~~380vdc

[wsscott]

The scope is good for 400V.

But I'm measuring AC.

[wsscott]

Willabe-I set the scope to AC coupling and checked the readings on the filter caps.

Nodes A and B each showed a Sine wave.  A reads 120hz/5.36VRMS. B reads 120hz/4.6VRMS.

Nodes C, D and E each showed only a straight line.  C reads 1.4KHz/3.63VRMS; D reads 1.1KHz/2.16VRMS; and E reads 1.2KHz/1.32VRMS.

Note that all RMS voltages were in Volts not MV.

[Willabe]

I think their all a little high, but wait and see what the others say. 

[Willabe]

Can you post some good pics? Including the back side of the chassis.

I don't see any in the entire thread.

[shooter]

when I read AC voltage on the filter caps’ Positive leads, it shows as a straight line, not a sine wave

A and B each showed a Sine wave.  A reads 120hz/5.36VRMS. B reads 120hz/4.6VRMS.

??


with your rectifier i would expect about 5VAC at tap 1, 1-2VAC ar 2, <500mVac at 3, <50mV at last tap, so those are kinda in-line with what I expect

did you make up a signal only cable to connect your problem child to an amp?  the reasoning;  sorta "forcing" each device to rely on it's individual grounds instead of a "common" system ground. 

what happened at the output jack with 69hz n 120hz on the pot??

[wsscott]

I just tried to send 2 photos, jpg format and each less than 1.5 megs.  They still won't go through.  I don't know why.  Is there something special I now need to do to upload photos?  I've done it in the past without an issue.

I also responded to Shooter about the cable and not having done it yet since the 120hz is measured at the tip and sleeve of the Output jack and its not even connected to an amp, and also that the different voltages at the pot and the jack have cleared up to just 120hz at both locations for some reason.

[shooter]

since the 120hz is measured at the tip and sleeve of the Output jack and its not even connected to an amp

the purpose for the test is to hopefully find a "work-a-round" to what I've believed for awhile is a problem "baked-into"  your build.  you've chased the gremlin well, pour a good single malt, sit in your comfortable chair n thank God you didn't have to do this for a living   
 

[wsscott]

I think that’s some great advice.  And thank goodness I’m retired!

[shooter]

since the 120hz is measured at the tip and sleeve of the Output jack

this doesn't keep me up at night, but it is stuck in the "something's off" part of my brain 

[wsscott]

I think my phrasing was poor.  I meant to say that I read 120hz when the probe of the scope is connected to the tip and ground/sleeve connection of the Output jack.  Not at each point separately.

[wsscott]

Shooter-I just tried the cable trick, but no luck.  I disconnected the ground lead from 1 end of a guitar cable's plug, inserted that end into the Output jack of the ReVibe and the other end of the cable into an amp.  There is no Input signal plugged in at the ReVibe's input jack.  Still get the same hum.

[wsscott]

So this morning I did 2 things. I tested it with a guitar plugged in and also with no input signal. Then I have a guitar cable from the ReVibe's Output Jack to the amp.  The results were the same as shown below.

First, I disconnected the ground lead from the Output Volume pot to the ground buss.  The hum is still there, but it is at maximum volume and isn't controlled by turning the pot.  I then re-connected that ground lead to the buss.

Second, I disconnected the Output Jack's ground/sleeve connection to the ground buss.  There is a noticeable decrease in hum, but it's still there, and the volume of the hum is now controllable once again with the Output Volume pot.

So that's good, BUT how can I be getting any sound at all if that ground from the Output jack is not connected to the buss. That Output Jack is isolated from the chassis with isolation washers, so there's no ground connection between the jack and the chassis--one would think.  It is a Switched type of jack, but nothing is connected on the Switch lug of the jack.  In this configuration the only connection to the jack from the circuit is from the Output Volume Level pot's wiper lug.   I even get sound from the guitar when its plugged into the ReVibe. 

[wsscott]

Ok, I found the answer to my question.

It's getting the ground from the cable connecting from the Output Volume Jack to the amp.  It's getting the ground from the amp.  When I remove the cable from the jack, there's no continuity between the chassis and the ground/sleeve on that jack.  But when the cable is connected to that jack and the amp, then there is continuity.

Also, when I connect the scope to the Output Volume jack it now shows a 60hz fuzzy signal whose amplitude doesn't change with adjustments to the pot.  Remember the jack is not grounded now.  When that jack was grounded it showed a pretty clean 120hz signal.

I think we're getting close.