Testing Bad Filter Cap with a scope for 120HZ hum

[wsscott]

I've traced 120hz hum down to the reverb section of a ReVibe Mod that seems to be coming from the B+ C node, 30uf cap.  I think it may be in the cap because other components in the circuit pick up the hum at that point.

I hooked up my scope's probe to the terminals on that cap, and set the scope for 100mv/div with AC coupling.

Although I sporadically got an image of a sine wave of around 120hz, I did get voltage on the scope that reads about 750mVrms.

Is this methodology valid, and is this indicative of a bad cap?

[shooter]

Although I sporadically got an image of a sine wave of around 120hz, I did get voltage on the scope that reads about 750mVrms.

the voltage constant or sporadic also??


you can also use a Meter, set it to VAC, start at the 1st filter cap and work your way "in" to the last node.


typical would be something like;
2-4VAC at 1st tap, 100mV at 2nd tap, 50mV, 20mV.....
each tap should "kill" the AC more n more.


If.. the VAC "spikes up" at a tap, that node should be suspect.


a quick test, gator-clip a bigger 50-100uF cap in parallel with your 30uF (MAKE SURE voltage rating is equal or greater than what's there)


you should "hear" a decrease in background hum

[wsscott]

Interesting results:

ESR meter shows all 5 caps, Nodes A-E, have proper readings.

I don't have a 50-100uf cap to test, just a 20uf.

Node A:  This Node does not connect to any points in the circuit.  It just receives the power off the full wave rectifier, and passes the signal along to Node B, and on.

1.81 vRms--STEADY
120Hz Big sine wave skewed at top

Node B:
1.01 vRms-STEADY
120Hz large sine wave-pretty smooth-not as big as in Node A

Node C:
638-655 vRms- fairly steady
0 Hz--sometimes tiny sine wave appears-scope reads 0 Hz

Node D:
395 vRms-STEADY
0 Hz-no sine wave

Node E:
243 vRms-STEADY
0 Hz-no sine wave

[shooter]

Node E:243 vRms-STEADY0 Hz-no sine wave

most scopes should display 243mV, most nodes that far down the chain should be ~~~20-40mVrms
time to repeat with a good DVM

[tubeswell]

I don't have a 50-100uf cap to test, just a 20uf.

20uF will work fine for a test.

[wsscott]

Sorry, nodes C, D, and E are in mV.

[shooter]

figured they were, my scope will display down to ~~~ 20mV  so your scope might be sketchy, that's why I recommended a redo with a meter.

[BrianS]

I use a volt meter in the way Shooter described.

[wsscott]

I tried checking the VAC with my DMM on the filter caps, and got screwy numbers.  I don't have a DVM.  Maybe that's the issue.

The DMM has an AUTO and a Manual function for voltages.  I tried both methods and got these numbers that don't make any sense.

24.18, 23.65, 21.38, 16.47 and 12.24 for A-E respectively.

The DMM's manual shows the following AC ranges:  400mv, 4V, 40V, 400V, and 750V.

[wsscott]

I guess my DMM isn't sensitive enough.

[shooter]

i use DVM n DMM interchangeably


are those numbers millivolts, volts, kilovolts...?


on the 400mV scale that is the MAX reading for that setting, guessing IF...the volts are stable it should be able to see 40-50mVac fine.

[wsscott]

This seems crazy, but the DMM is in Manual mode and these numbers are all in Volts.

Its in the Manual 40V range since lower range gave me OL.

Doesn't make any sense.  Now these numbers probably are not true VRMS.  My scope's numbers were in VRMS.

Maybe that's the explanation.  But even if you multiply them by .7, they're still much higher than you predicted Shooter.

The DC voltages on the filter caps are 339, 337, 320, 288, and 262 respectively, and are right on with the schematic.

[shooter]

This seems crazy,

 


what I thought with your scope readings, hence meter, which still yields "crazy" so I suspect you either have equipment issues or ya got a gremlin that's all excited about being chased!!!

[wsscott]

I couldn't have said it better myself!

I think I've probably wasted enough of everyone's time on this.  Thanks to all for your input.  As always.

Happy Thanksgiving!

[wsscott]

This was my first project.  The circuit is designed for a full wave rectifier with a pair of diodes on each leg of the transformer's secondary going to the filter caps.  The transformer has a Center Tap.

Not knowing at the time the difference between a full wave rectifier and a bridge full wave rectifier, I bought a 3N259 bridge rectifier.  In the process of wiring it in, I learned that I should not connect the ground lead of the rectifier into the circuit because it would result in too much voltage with the CT also being used.  The CT is wired to the first filter cap.  So I was told to cut that lead off so I wouldn't screw up.  So I did.

And that's the way this has been wired. The voltages at the filter caps are spot on.  Its worked fine, but for this 120hz hum.

So my final thought on this:  Is that the cause for this 120hz hum?  Should I just remove that bridge rectifier and build a full wave with 4 diodes instead, and leave the CT connected as is to the first filter cap?

[shooter]

some lite reading;


take any good working tube thing you have, amp, PA.... and repeat your test of the taps with both your scope n meter.  POST both AC and DC volts.

[wsscott]

Well I'm totally baffled.  The voltages on the tube are spot on.  I've changed tubes, and no change in the hum.  The hum appears only when the 12AX7's pin 6 plate is connected to the 270K resistor coming off the Node C filter cap.  There's no 120hz signal from node C going into the 270K, and there's no 120hz signal after the 270K if it is NOT connected to Pin 6, plate.  There is only 120hz hum when the backside of the 270K resistor is connected to Pin 6.  The resistor reads within specs.

Will there be a Thanksgiving Miracle this year??

[shooter]

 


..."The ways of the Lord are mysterious..."


guessing you rolled a different tube to eliminate the tube
try temp'n in something between 500pF - .01uF cap across the plate R for that tube (Cap in parallel with resistor)

[wsscott]

Yes, the tube was the first thing I tried.

I'll give the other option a try.  Thanks for the input.

[wsscott]

I put a .01uf in parallel across the 270K plate resistor.

It shows over 340VDC with DMM and 1.5 VRMS with AC Coupling on scope, and 0Hz for the signal that was 120hz.

So that's good but the 340VDC is way too high.  It was, and should be, around 204 or so coming out of the resistor and into the plate.  Node C voltage into the resistor should be around 320VDC and it also went up to around 375VDC.

[shooter]

So that's good but the 340VDC is way too high.  It was, and should be, around 204 or so coming out of the resistor and into the plate.  Node C voltage into the resistor should be around 320VDC and it also went up to around 375VDC.

was "normal" before you added the cap???


IF SO....
measure VDC across the cathode R for the tube

[wsscott]

I decided to solder the cap across the resistor since the numbers were so screwy.

Interesting after I did that and turn on the amp, all of the Nodes were way high.  Then after about a minute they all came back down to where they're supposed to be.

As far as adding the cap, it made no difference.  Still have the 120hz signal.  I think the prior reading was because I didn't have good connections.  Also the VDC across the cathode resistor is 1.8VDC where it has always been.

So adding the cap made no difference and didn't drop the voltages or the Hum.

[wsscott]

For some reason the filaments on all the tubes have gotten really slow to warm up.  The power light comes on and measures fine.  Probably from screwing around with all this stuff.  When the tubes aren't lit, then no current is being drawn, and so the voltages on the filter caps and their connections are high--at least I think that's why they're high.  When lit, the voltages are normal.  Hopefully this quits acting up.

[shooter]

so the voltages on the filter caps and their connections are high

yep, I use the "high" voltage to spec the filter caps, I've even noted on the schematic the "high-time" 
the last amp was almost 30 seconds to drop down to normal operating VDC's


30 seconds at 400vdc across a cap rated at 350vdc might make a messy

[wsscott]

Shooter-when talking about VAC on the filter caps you said above:

"typical would be something like;
2-4VAC at 1st tap, 100mV at 2nd tap, 50mV, 20mV.....
each tap should "kill" the AC more n more.

Are these voltages RMS or PP?

[shooter]

I measure n post in RMS, but the "filtering" should follow the same VAC decrease PP or rms.  doesn't really matter which you use, just DON'T mix the metaphors!!!

[wsscott]

I ask because my nodes from A-E, in RMS, read, with my scope in AC coupling mode, 784mv, 715mv, 569mv, 329mv, and 207mv.  Both Nodes A and B have 120hz signal showing on them, and my scope doesn't/can't read any AC on the other nodes.  These seem much higher than you suggested they should be, and the B Node feeds the Reverb Driver circuit, which ultimately shows up in the C Node on the last 12AX7 where the Reverb Intensity pot comes into play.

I was thinking maybe the 120hz hum is actually coming from the B Node, 40uf filter cap, and being passed on through the circuit and amplified at the C Node plate on the last tube before going to the Output jack.

The cap seems ok, no visual leakage, and no ESR issues.  So that's what's got me still puzzled.

[shooter]

for testing you could probably move the "reverb tap"  downstream, VDC will be less downstream but there should be enough current for a couple extra AX7's to "make it work" n listen for the hum.


you're most likely chase'n a gremlin though, my sense is you have something mechanically poor, (sockets, solder, ground points...), or cross-coupling in the wires picking up the 120hz

[wsscott]

Yea, I never thought tracing something down like this would be so hard.  Thanks for your help.

[wsscott]

I was wondering if there would be any benefit in lowering the plate voltage provided by Node E on V7, Pin 1 Plate, down from the current 136VDC?  The specs call for about 122VDC. 

If so, what should I do?  Increase the dropping resistor between Nodes D and E, or increase the plate resistor at V7 and maybe V6 too?  If so, any suggestions on the size resistor?  Its currently a 10K dropping resistor connecting to a 1uf cap for Node E.

That Node E also serves the V1-Pin 6 Plate.  This half of the tube is receiving the signal from the Input jack.  The specs call for 72VDC, but it reads 161VDC.  The signal is clean when the Volume Pot is below half way, but gets dirty soon.  So maybe cooling that input would give more clean headroom out of the first preamp tube.

Node E at the filter cap is 261VDC, and the specs show it should be 212VDC.

So all of the Node E voltages on the plates are higher than spec.

All of the other VDC Nodes are in line with the specs of the schematic.

[wsscott]

Well I'm giving up on the last thoughts I had.

I feel like its related to the reverb Wet signal being amplified after it goes through the Reverb Intensity pot and connects to the Pin 7 grid.

The Dry Signal bypasses the reverb tank entirely and goes into the Cathode pin 8 of V7 and then through the Output Level pot and to the Output jack.  There is virtually no hum when the signal goes through the circuit this way.

If the Wet Signal is used, by either turning on the Reverb foot pedal or disconnecting the foot pedal, the hum appears. Its level increases with turning the Intensity Pot.  This pot is controlling the amount of signal going to the Grid vs. going to ground.

So I need to figure out some way to deal with that.  An older schematic on which this circuit is based, has the foot switch connecting between the .047 coupling cap and the input on the Reverb Intensity pot.  The main difference is that the Ground for the foot switch is before the pot and thus before the grid, vs. in my build the foot switch's Ground is on the output lug of the pot and thus after the grid.

[SEL49]

I'm pretty sure I know what circuit you are talking about, but I bet no one else does. Please post your as is schematic and layout so everyone can understand better what you are experiencing.

[wsscott]

Sluckey-sure happy to post your schematic.  I don't have a current Layout.  The components are still the same.  I finally traced the hum to a 120hz signal which comes off Node B, and then onto Node C but not as strong, and then not detectable by my scope on Nodes D or E.

[shooter]

figured that was what you were working on.


Is the hum there with no input, or only when you "add signal"

[wsscott]

Even with no input.

[wsscott]

I checked it again and I've got a 120hz signal at the Node B filter cap, and that signal also shows up on V4, Pins 1 and 6 plates which are the Vibrato Oscillator/Driver circuit, and V6, Pins 1 & 6 plates which are the Reverb Driver circuit.

Removing the RCA cable at the Reverb Input jack kills the hum.

It seems that it must be a bad Node B, 40uf, filter cap, since the signal seems to be originating from there, at the cap, and goes on out from that point through the wiring directly to V4 and V6.  Nothing is between the cap and the connection on those plates.

So I'll give that a try.

[shooter]

Removing the RCA cable at the Reverb Input jack kills the hum.

before you head for the cap;


remove the Ground-part of the cable but leave signal part connected, 120hz??

[wsscott]

The jack is isolated.  Do you want me just to take the jack’s ground lug out of the circuit by un-wiring it?

[wsscott]

Interesting, although I removed the ground wire connection on the RCA Reverb Input jack, I still have reverb and I can control it on/off with the foot pedal.  How is that?

As to the hum, with the Reverb On, the hum is louder than when it is turned OFF, but probably not as loud as before when the jack was still wired to the buss ground.

The Reverb Intensity pot increases the loudness of the hum, as well as the amount of Reverb.

I haven't checked the scope for 120hz signal readings, but I assume its still there in the same places as before, since that's the hum I'm hearing--I think.

So I would say its better, but I would like it to be quieter--if that's achievable with this circuit.

[shooter]

https://el34world.com/Forum/index.php?topic=32980.msg368863#new


both of you are probably chasing the same gremlin but from different mothers 

[wsscott]

So I checked the continuity of the jacks on the tank.

The Reverb Input jack is isolated from the chassis, and only the Input jack's Tip is connected to the jack.  The ground wire is now disconnected from the Input jack that is isolated on the chassis.

With both Input and Output plugs connected to their respective jacks on the chassis and on the tank, there is continuity between the sleeves of both jacks and also continuity between both sleeves and chassis.

So even though the Input jack on the chassis is not grounded, the cable that connects from it  to the tank is being grounded by the Output Jack which is grounded to chassis.

So the grounds on the jacks on the tank must share a common ground.  Either the tank's shell or their wiring.  Maybe these jacks are not insulated from the tank chassis, and that's how it's happening.

So would that create a ground loop hum?

[shooter]

would that create a ground loop hum?

yep