Really don't know what to sugest to you.
You are going to have to fit a new pot to eliminate or prove a problem with it.
Without seeing the amp and knowing all its history I would suggest you isolate the wire connecting points marked 17 and 11 on the diagram (volume control to C505) and just wire a pot from point 11 (the wiper of pot) and the other end of pot to ground of preamp (bottom of R515). Leave the other tag on the pot open and just see if that channel still makes a noise as the new pot is rotated.
You are going to have to fit a new pot to eliminate or prove a problem with it.
Without seeing the amp and knowing all its history I would suggest you isolate the wire connecting points marked 17 and 11 on the diagram (volume control to C505) and just wire a pot from point 11 (the wiper of pot) and the other end of pot to ground of preamp (bottom of R515). Leave the other tag on the pot open and just see if that channel still makes a noise as the new pot is rotated.
That wont be a problem, I kept original potentiometer, you can see that 30 year old, born in Japan, weighting only 24 grams in the picture below. However my wife wanted me to choose between solderless or wifeless x-mas, so I will have more news in two days. I beg of you not to loose interest in the meantime. What do you think of discrepancy between left and right channel resistors Mike suggested?
Merry xmas
Marko
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For some, a very easy decision.
For others, a very difficult decision.
For me, that decision never needs to be made. Life is so easy for a retired single person.
Merry Christmas Marko,
I guess you could always try one of these... probably be a wifeless 12 months if she found out.
http://www.diyaudio.com/forums/everything-else/203085-most-expensive-20k-carbon-pot-i-have-seen.html
What discrepancy do you mean ? It's normal for there to be small variations in discrete circuitry comparing left and right although things like final gain are absolutes determined by just a couple of components tolerances.
I guess you could always try one of these... probably be a wifeless 12 months if she found out.
http://www.diyaudio.com/forums/everything-else/203085-most-expensive-20k-carbon-pot-i-have-seen.html
What discrepancy do you mean ? It's normal for there to be small variations in discrete circuitry comparing left and right although things like final gain are absolutes determined by just a couple of components tolerances.
I was thinking about difference between R535 (one side 3.4mV other 3.2mV) and
R536 (one side 1mV other 17.3mV), R535 is the only resistor of those four that measured lower on the side on which all other resistors showed larger value. But I measured in circuit resistance an it was the same between R535 and R536, and R529 and R530, so that rules them out i suppose.
I measured DC on the pot and here are the results:
pin 1=earth
pin 2=signal out
pin 3=signal in
pin 3 fluctuating from 1.1. to 1.7
pin 2 steady at 1.5
across pins 1-3 fluctuating from -3.0 to +1.7
While measuring the pot I came to the interesting finding, if I place one hand on the pot and the other hand anywhere else on the chassis I get no more scratchiness while turning the pot up or down, I am not saying the the pot is completely silent but it is 10 times less noisy then. Does that indicates grounding problem?
Thanks
Marko
P.S. If I still have to try another pot for troubleshooting purposes, can I do that with TKD in place zero volume, and the old pot in parallel with him temporarily soldered to the pcb side?
The differences in voltage. One side of R535 and R536 both go to signal ground and so those points are taken to be are "zero" point. I suspect the tiny voltages you are measuring are down to method/equipment/leads etc. To read the voltage on those resistors try reading across each one rather than from ground to each end.
The two ends that go to ground should read zero between each.
Sounds very strange that the pot is less noisy when you touch it and the chassis.
Isn't the pot fitted all plastic bodied.
It won't harm to wire the pot in parallel but the results could be misleading. If it's quite then you have to isolate the old to further prove the theory 100%
The two ends that go to ground should read zero between each.
Sounds very strange that the pot is less noisy when you touch it and the chassis.
Isn't the pot fitted all plastic bodied.
It won't harm to wire the pot in parallel but the results could be misleading. If it's quite then you have to isolate the old to further prove the theory 100%
I will then measure across those resistors, I have two DMMs, will use them both.
It isn't all plastic, look at the picture.
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It is, that nut tightens it to the chassis. Andrew, I often remember your words "Life is so easy for a retired single person", I even showed your post to my friend who is also married and we both agree with you, he said that is something Jedi master Yoda would say probably.
Hi Marko, sorry to disappear, the holidays and workload haven't left much time to respond. Have you found the reason that touching the pot makes the noise go down? I'm assuming that you are touching some metal part of the pot. There is normally an internal wiper type contact that grounds the shaft that might not me working. Did you have a similar noise on the original control?
If you are just touching the plastic body, not the metal, it's more of a proximity thing and might indicate an oscillation. These tend to show up in multiple places and defy ground (metaphorically speaking).
Just curious. Mike
Measuring results, this time without dim bulb tester, I am feeling courageous:
across R535 1.5mV
across R536 14.2mV
across grounds of R535 and R536 0.00mV
Hi Mike, welcome back. I must explain my actions, I was rotating the volume potentiometer without the knob. I just found tonight that rotating the pot with knob or some other kind of isolator between my hand and the pot shaft makes rotating the pot completely silent, yes no more scratchiness
Is it allowed to touch that brass shaft directly, and if it is not why was the scratchiness in right channel and not in left also?Touching the plastic body with fingers produces a noise in headphones, like a slight buzzing which intensifies with fingers moving closer, but i think the noise does not come from the pot itself but from the wires that go from points 11 and 12 on the schematics to pot output pins. I was using pencil with graphite (probe of the poor) to trace that noise following the the leads to pcb.
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Hi Mike, welcome back. I must explain my actions, I was rotating the volume potentiometer without the knob. I just found tonight that rotating the pot with knob or some other kind of isolator between my hand and the pot shaft makes rotating the pot completely silent, yes no more scratchiness
Touching the plastic body with fingers produces a noise in headphones, like a slight buzzing which intensifies with fingers moving closer, but i think the noise does not come from the pot itself but from the wires that go from points 11 and 12 on the schematics to pot output pins. I was using pencil with graphite (probe of the poor) to trace that noise following the the leads to pcb.
As I suspected the ground on the volume control shaft is not working and your body is the source of the voltage being picked up by the control. Welcome to the world of electrostatics!
the cause is, in essence, arcing that occurs between the volume control shaft and the grounded mounting collar to be picked up and amplified by the gain stage following the control. Touch the chassis ground and there is no voltage difference between the shaft and the chassis so no noise. It's caused by the same effect that causes one to experience a shock when touching a doorknob after walking across a carpet on a day with low humidity. You'd be amazed at the normal voltage levels present on the human body and the things that cause them. It does point to a bad ground in the control though, and the answer to your question: yes you should be able to touch the control shaft with no noise. Why one channel is noisy and not the other is a combination of leakage paths, proximity effects and impedances that come into play that allows one channel to easily recieve this transmitted signal and the other to be unaffected.
The simple thought here is that the control is not right as most of these controls do have a ground wiper and a ground path for this reason.
The buzz when you put your finger near the control is a different issue but not uncommon and probably has no effect in normal operation.
There should be a clue in your observation that the pop is there when the filter switch is depressed more so the pressed but I'm to tired to think about it now.
Regards, Mike
Measuring results, this time without dim bulb tester, I am feeling courageous:
across R535 1.5mV
across R536 14.2mV
across grounds of R535 and R536 0.00mV
Across the grounds and a reading of 0.00 is absolutely correct.
Now listen carefully
The problem of the switch popping and the noisy volume control may or may not be related.
Ideally we need to put a scope on this and confirm that there is no unwanted AC of any kind present however,
lets take those readings of 1.5mv and particularly 14.2mv at face value. They should be zero. Look at the circuit. I'm think we might have done part of this but lets go over it again.
1. Remeasure and make sure 14mv or so is still present across R536
2. Now remove C518 and measure again. Is the the voltage zero or still 14mv ?
3. If still 14 mv now remove C528 and keep C518 disconnected. Any change ?
4. If still reading 14 mv keep all above disconnected and remove C522. Any change ?
5. If still reading 14 mv finally remove C524
All these steps isolate R536 and so at some point it should hopefully show where this 14mv is coming from. Do you see how isolating all those caps in sequence isolates the resistor. There is no path for any voltage to appear with those removed.
Although it may seem odd to lift caps to isolate a "DC" fault it's possible that the reading is caused by an AC signal of some kind.
If you still can not get the reading as zero then there has to be some leakage somewhere or some contamination on the PCB that is conductive as we are left with just a resistor connected to ground.
As I suspected the ground on the volume control shaft is not working and your body is the source of the voltage being picked up by the control. Welcome to the world of electrostatics!
Why one channel is noisy and not the other is a combination of leakage paths, proximity effects and impedances that come into play that allows one channel to easily recieve this transmitted signal and the other to be unaffected.
The simple thought here is that the control is not right as most of these controls do have a ground wiper and a ground path for this reason.
The buzz when you put your finger near the control is a different issue but not uncommon and probably has no effect in normal operation.
There should be a clue in your observation that the pop is there when the filter switch is depressed more so the pressed but I'm to tired to think about it now.
Regards, Mike
Hi Mike,
do you think ground on the volume control shaft not working is a bug or a feature? What I mean is did TKD pot came from the factory like this or something in my amp killed him?
Across the grounds and a reading of 0.00 is absolutely correct.
Now listen carefully
The problem of the switch popping and the noisy volume control may or may not be related.
Ideally we need to put a scope on this and confirm that there is no unwanted AC of any kind present however,
lets take those readings of 1.5mv and particularly 14.2mv at face value. They should be zero. Look at the circuit. I'm think we might have done part of this but lets go over it again.
1. Remeasure and make sure 14mv or so is still present across R536
2. Now remove C518 and measure again. Is the the voltage zero or still 14mv ?
3. If still 14 mv now remove C528 and keep C518 disconnected. Any change ?
4. If still reading 14 mv keep all above disconnected and remove C522. Any change ?
5. If still reading 14 mv finally remove C524
All these steps isolate R536 and so at some point it should hopefully show where this 14mv is coming from. Do you see how isolating all those caps in sequence isolates the resistor. There is no path for any voltage to appear with those removed.
Although it may seem odd to lift caps to isolate a "DC" fault it's possible that the reading is caused by an AC signal of some kind.
If you still can not get the reading as zero then there has to be some leakage somewhere or some contamination on the PCB that is conductive as we are left with just a resistor connected to ground.
Hi Mooly,
I am listening carefully
. Will do as instructed and get back with results.As for my DMM, I checked it against another we have at work place on a 9V battery, and they are only 1 mV apart, so I would say it is precise enough.
Those reading I took were made perhaps after a minute or two after powering on. The values were high at the start of measuring but kept dropping and settled to values I wrote here. I will remeasure again, no problem, how much time from powering on to measuring do you think it is advisable to wait?
Regards
Marko
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