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??