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Input Hum - Yorkville Amplifier

gerryt

Feb 5, 2021
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I have a Yorkville Power Wedge Kw100 amplifier with two input channels. As the volume in channel 1 is increased there is a hum that gets louder. It does not sound like a 60 cycle hum. The humming problem is the same whether or not an instrument is plugged into the 1/4" input or a microphone is plugged into the XLR input. Channel two works fine and is quiet. As best as I can tell from the schematic both channel 1 and 2 get routed into the same output section of the amp. I am thinking that the problem is somewhere within the input section of channel 1. Wondering if there is a likely component culprit that might be causing this? I have attached the schematic. I am self taught in electronics and appreciate any help offered.
 

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Martaine2005

May 12, 2015
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Hi JerryT,
This is typical electrolytic capacitors in the power supply.
Furthermore, hum altering with volume. Again, input hum being amplified.
Should be a simple fix.

Martin
 

gerryt

Feb 5, 2021
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Hi JerryT,
This is typical electrolytic capacitors in the power supply.
Furthermore, hum altering with volume. Again, input hum being amplified.
Should be a simple fix.

Martin
Thanks Martin, I would be curious to know why it only hums on one channel.
 

Bluejets

Oct 5, 2014
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Thanks Martin, I would be curious to know why it only hums on one channel.
That's the channel with the dodgy component.
Try C6.
Look for usual signs of degradation.
Best to test with an appropriate meter.
 

danadak

Feb 19, 2021
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Hum can also be a poor ground connection in line plug.

Also a poor soldering joint that has created a high Z parasitic pickup point.
Connector joist as well on PCB. Helps to have an inspection scope or a magnifying
glass to do the inspection.

Additionally ground shield on audio cables no longer low Z to ground. Can occur
due to flex.....

Power supply rectifier diode crapped out transforming a FW rectified supply to 1/2 wave
creating huge ripple.....

Post hi res pics of both side of board, have to be well focused.....


Regards, Dana.
 

gerryt

Feb 5, 2021
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Hum can also be a poor ground connection in line plug.

Also a poor soldering joint that has created a high Z parasitic pickup point.
Connector joist as well on PCB. Helps to have an inspection scope or a magnifying
glass to do the inspection.

Additionally ground shield on audio cables no longer low Z to ground. Can occur
due to flex.....

Power supply rectifier diode crapped out transforming a FW rectified supply to 1/2 wave
creating huge ripple.....

Post hi res pics of both side of board, have to be well focused.....


Regards, Dana.
I replaced C6, no change. Channel Two is clean and Channel One hums, even with nothing plugged in to the inputs. I checked all of the ground circuits with a meter. Can't find or see any faults. Wondering if the op amp (U2) in channel one is the problem. Here are the pictures of the Input One section as well as the schematic. Thanks for your help.
 

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danadak

Feb 19, 2021
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Are the traces connected to their respective traces, lower left and right :

1689206256068.png

Is that C34 between the two inline horizontal; points ? Where is it in schematic ?

With inputs both channels grounded, what is the DC and AC value of the outputs of each of the
3 opamps show :

1689206689774.png

If you have a scope have you walked the signal path with it, starting at
output and walking back ?

Pot wipers a problem ? Have you used pot spray to clean them ?


Regards, Dana.
 

gerryt

Feb 5, 2021
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The traces are all connected. I cannot find C34 on the schematic (weird) but it does show up on the schematic with all of the traces shown. (see attached). I do not have a scope to test the signal path. I cleaned the pot, but I am thinking I will replace it just to be sure it is not the problem as it is an easy thing to do. I have another one of these units with exactly the same problem and sound. Channel two clean and channel One noisy. The noise increases as the pot is turned louder.
 

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gerryt

Feb 5, 2021
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I found C34 on the schematic, attached and circled.
 

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danadak

Feb 19, 2021
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If you do not have a scope you can get Zeitnitz freebe which turns your
PC into a scope, spectrum analyzer, tone generator. But you have to protect your sound card
inputs to make sure you dont blow up your PC audio channel.
Its good for audio freqs.

Also make up a 10:1 probe. Google "diy 10:1 probe"

Soundcard Scope

Software projects of Christian Zeitnitz
www.zeitnitz.eu

Daqarta - Sound Card Input Range and Limiter Circuits

Project 154

ESP Audio Projects - PC Oscilloscope interface.
sound-au.com

www.nutsvolts.com

Turn Your Computer’s Sound Card into a Scope

Use this free software tool to gain oscilloscope skills, analyze audio signals, and measure digital signals.
www.nutsvolts.com

You are pretty sure JB1 on channel 1, the contacts, solid contacts ? Use some denatured alcohol or contact
cleaner on them. Mechanically insert a male into them to get contact surfaces to move around.


Regards, Dana.
 
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gerryt

Feb 5, 2021
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Thanks for the lead about turning a laptop into a scope. I have an older spare laptop, I will give this a try. I tested the op amp on diode mode with my multimeter and it seems to be working fine. I also tested for other ground faults. The tip and ring tabs on my 1/4 input, and in the XLR input - when connected to ground with my multimeter both get a reading. It seems to me that there should not be any current travelling between the tip or the ring to ground. Wondering if this is the problem about why I get hum?
 

73's de Edd

Aug 21, 2015
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Sir gerryt . . . . . . (and that's being very heav-E E E E E E E E on the Teeeeeeeeeeeeeeeeeeee !)

How 'bouts us doing some super dooper uber- techno sniffing, using but a mere length of hook up wire and a 47 or 100'ish ufd @16(+) E-cap.
I have re-diddled-ed-ed your units schema of the sole area of interest and put a bit more of vertical spacing between its CHANNEL 1 and CHANNEL 2 sections.

LE REFERENCE . . . . . 'cest petite

1689297487674.png

FIRST STEP . . . .
Fire up the unit and get it making the CHANNEL 1's quite noticeable HUMMMMMM . . .chum.
Now, for just backing up your prior testing of grounding integrity.
Take said hook up wire . . . of such length to be able to span from GREEN SQUARE U1 Pin 5 to both U2 Pin5 and JB connector pin 1.
With our experiencing no fault with intregrity of GREEN SQUARE U1 Pin 5 and its connecting to its . . . SG . . .Signal Ground.
Lets confirm no excess rosin or conformal coating . . . to let us make a good connection to one bared end of hook up wire and span its other end up to connect to same the like condition of U2 pin 5 and its Signal Ground.
If no change in Hummmmmmmm lift up that just made connection and transfer up to JB2 pin 1.
If the hummmmmmm remains, our required grounds are O.K.
NEXT STEP . . . . .
Take the E-cap to be used and . . .likely it will be a radial unit . . . . and grip . . . . probably its insulated case . . . .between index finger and thumb and see if the touching of either one leads of the E-cap to pin 3 or 3 of the JB2 connector will over ride your hum, so as to be heard. If not use your other hand to tightly grasp around the insulated amps AC cord .
Thus your increased AC level injection into the amp . . . .a la body coupling.
Once you attain a noticeable level to make judgement with, test both caps leads to JB #2 or 3 to see if one lead is producing stronger hum.
If so, note that lead, as it needs to be solder tacked to that wire end and the other wire end stays at . . . and is solder tacked to U2 pin 5 the Sig Gnd or at U2 pin 5 Sig Gnd . . . . . take your pick.
Should you not be able to differentiate, just have the cap - going / soldering to the wire.
OBSERVATIONS . . . .
Note that Channel 1 and 2 outputs both meet at the RED and BLUE STARS to join at the small GREEN square to be co amplified by LARGE GREEN SQUARE U1.
Here you have no problem with CHANNEL 2 . . . . .at the BLUE STAR . . . . BUT . . . . the CHANNEL 1 RED STAR line is carrying in Hummmmm with it back from U2.

We are now going to take the E-cap and its extension ground wire and start at the U2 pin 7 and touch that free cap lead to it and expect an initial pop and then connect just long enough to see if the HUMMMM remains.
Lift E-cap and ground it to its other wire end to DC discharge it. ( Also, every test time . . .hereafter )
Touch E-cap lead to U2 #6 and check for presence or loss of that HUMMMMM.

THEN YOU SAY. . . .
As the volume in channel 1 is increased there is a hum that gets louder. It does not sound like a 60 cycle hum.
With a linear power supply and FWB . . . . 120~ hummmmmmm . . . . . . but lets just call it HUMMMMM for now

Now we're getting down to the U2 pin 6 and testing it . . . . noting results.
Then move to U2 pin 1 and testing it . . . . noting results.
Finally U2 pin 2 and then pin 3.
If you still had Hummmmmmmmmm at the last pins 2 and 3, I am wanting to suspect it being introduced from the SHARED power supply to U2.
The channel 2 has quite high input levels and would be MORE tolerant of power supply ripple.
NOW if you look at the CHANNEL 1 and the meager 2 and 10 mv input levels and the degree of required amplification, power supply ripple would be more significant in that CHANNEL 1 in its U2 supply.

REFERENCING . . .
The YELLOW SQUARE inset gives their shared power supply and the MC33078 snippet gives its power pin outs.

Don't be afraid to go on ahead and tack two good 470 ufds across the existing ones . . . . correct polarity observed . . . (vewy-vewy cawefuwwy! ) to test for Hummm elimination . . . . thus confirming it was timely decline of those caps..
Feed back the conditions at all of the tested U2 AF inputs and outputs . . . . . . . I be . . .
EatingPopcorn.jpg


Thaaaaaasssssit . . . . .

73's de Edd . . . . .

When I was a little boy, I was told that I had a disease that required me to eat dirt three times a day in order to survive...

It's certainly a good thing that my big brother told me about it !
.
 
Last edited:

danadak

Feb 19, 2021
783
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If you touch a cap to either OpAmp output or its inputs, the other side to ground
or supply V, good chance the OpAmp becomes an oscillator due to loss of phase
margin.


Regards, Dana.
 

gerryt

Feb 5, 2021
44
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Feb 5, 2021
Messages
44
Sir gerryt . . . . . . (and that's being very heav-E E E E E E E E on the Teeeeeeeeeeeeeeeeeeee !)

How 'bouts us doing some super dooper uber- techno sniffing, using but a mere length of hook up wire and a 47 or 100'ish ufd @16(+) E-cap.
I have re-diddled-ed-ed your units schema of the sole area of interest and put a bit more of vertical spacing between its CHANNEL 1 and CHANNEL 2 sections.

LE REFERENCE . . . . . 'cest petite

View attachment 59859

FIRST STEP . . . .
Fire up the unit and get it making the CHANNEL 1's quite noticeable HUMMMMMM . . .chum.
Now, for just backing up your prior testing of grounding integrity.
Take said hook up wire . . . of such length to be able to span from GREEN SQUARE U1 Pin 5 to both U2 Pin5 and JB connector pin 1.
With our experiencing no fault with intregrity of GREEN SQUARE U1 Pin 5 and its connecting to its . . . SG . . .Signal Ground.
Lets confirm no excess rosin or conformal coating . . . to let us make a good connection to one bared end of hook up wire and span its other end up to connect to same the like condition of U2 pin 5 and its Signal Ground.
If no change in Hummmmmmmm lift up that just made connection and transfer up to JB2 pin 1.
If the hummmmmmm remains, our required grounds are O.K.
NEXT STEP . . . . .
Take the E-cap to be used and . . .likely it will be a radial unit . . . . and grip . . . . probably its insulated case . . . .between index finger and thumb and see if the touching of either one leads of the E-cap to pin 3 or 3 of the JB2 connector will over ride your hum, so as to be heard. If not use your other hand to tightly grasp around the insulated amps AC cord .
Thus your increased AC level injection into the amp . . . .a la body coupling.
Once you attain a noticeable level to make judgement with, test both caps leads to JB #2 or 3 to see if one lead is producing stronger hum.
If so, note that lead, as it needs to be solder tacked to that wire end and the other wire end stays at . . . and is solder tacked to U2 pin 5 the Sig Gnd or at U2 pin 5 Sig Gnd . . . . . take your pick.
Should you not be able to differentiate, just have the cap - going / soldering to the wire.
OBSERVATIONS . . . .
Note that Channel 1 and 2 outputs both meet at the RED and BLUE STARS to join at the small GREEN square to be co amplified by LARGE GREEN SQUARE U1.
Here you have no problem with CHANNEL 2 . . . . .at the BLUE STAR . . . . BUT . . . . the CHANNEL 1 RED STAR line is carrying in Hummmmm with it back from U2.

We are now going to take the E-cap and its extension ground wire and start at the U2 pin 7 and touch that free cap lead to it and expect an initial pop and then connect just long enough to see if the HUMMMM remains.
Lift E-cap and ground it to its other wire end to DC discharge it. ( Also, every test time . . .hereafter )
Touch E-cap lead to U2 #6 and check for presence or loss of that HUMMMMM.

THEN YOU SAY. . . .
As the volume in channel 1 is increased there is a hum that gets louder. It does not sound like a 60 cycle hum.
With a linear power supply and FWB . . . . 120~ hummmmmmm . . . . . . but lets just call it HUMMMMM for now

Now we're getting down to the U2 pin 6 and testing it . . . . noting results.
Then move to U2 pin 1 and testing it . . . . noting results.
Finally U2 pin 2 and then pin 3.
If you still had Hummmmmmmmmm at the last pins 2 and 3, I am wanting to suspect it being introduced from the SHARED power supply to U2.
The channel 2 has quite high input levels and would be MORE tolerant of power supply ripple.
NOW if you look at the CHANNEL 1 and the meager 2 and 10 mv input levels and the degree of required amplification, power supply ripple would be more significant in that CHANNEL 1 in its U2 supply.

REFERENCING . . .
The YELLOW SQUARE inset gives their shared power supply and the MC33078 snippet gives its power pin outs.

Don't be afraid to go on ahead and tack two good 470 ufds across the existing ones . . . . correct polarity observed . . . (vewy-vewy cawefuwwy! ) to test for Hummm elimination . . . . thus confirming it was timely decline of those caps..
Feed back the conditions at all of the tested U2 AF inputs and outputs . . . . . . . I be . . .
EatingPopcorn.jpg


Thaaaaaasssssit . . . . .

73's de Edd . . . . .

When I was a little boy, I was told that I had a disease that required me to eat dirt three times a day in order to survive...

It's certainly a good thing that my big brother told me about it !
.
After running the various tests that you so kindly suggested I replaced the op amp. (U2) Problem solved!
 

gerryt

Feb 5, 2021
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Thanks everyone for your help on this problem. I replaced the op amp and the problem is resolved.
 

danadak

Feb 19, 2021
783
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783
Curious, was OpAmp socketed ? Usually they do not fail. So asking maybe input
pin was in poor contact with socket....


Regards, Dana.
 

gerryt

Feb 5, 2021
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Curious, was OpAmp socketed ? Usually they do not fail. So asking maybe input
pin was in poor contact with socket....


Regards, Dana.
The op amp was not socketed, pin mount to the board, I looked at all the contacts carefully and they seemed to be ok.
 

gerryt

Feb 5, 2021
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I am now starting to work on the second amp, it is exactly the same, a Yorkville 100KW. But this one has a different problem. Channel two has very low gain even at full volume. If I use the return input, there is lots of volume, so the power amp is working fine. Any thoughts as to what might be the obvious suspect to cause super low gain in the preamp of channel two? Visual inspection of the board does not show anything damaged.
 

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