No more 60 Hz hum in a PL2000 preamp.

BlueCrab said:
Ideally you'd like your source signal to be as free of harmonics & noise as possible. That's what the HP8903B does, i.e., it's output signal has very low noise and THD so that when measuring a DUT the readings are attributed to the DUT. That just makes everything easier. But you could measure your source and determine it's harmonics. When you apply that signal source to the DUT, you could calculate (knowing its gain) how much of the harmonics at the output was due to the harmonics in the input signal. This is complicated to do by hand, but a computer could do it easily.

But have a pure signal source is the best of all possibilities.
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Got it. Thanks Jim. Been doing some reading. Appears my set up of the software parameters for the sine wave generator is incorrect. I'm overdriving the generator and causing unwanted harmonics. Found a post about using the next model QA401 to measure distortion and they discussed settings that aren't documented in my QA400 manual, yet I believe are in the QA400 software.
Hopefully I'll find time this coming Monday to look at this.
Like you say, the sine wave should be spectrally pure.
The DUT produces the harmonics, and THD is the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Wish I had more free time to pursue this, I find it really interesting. Thanks!
 
Al, those dual pots I was cleaning are from my very first PL 2000 S2 that I was using to learn on. That one got the single 100k balance pot, and was then retired after I built two more 2000's based on what I learned on that first unit.
I kept the dual pots in these as the pots were good enough to use at that time.
The pots I just disassembled and cleaned, were bad, very bad, from that first unit.
So, how'd they turn out? The balance pot dual tracks consistently within 2k ohms of each other. The volume pot tracks within 3k ohms.
Not good enough. Going to investigate those 24 position attenuators on Ebay that are supposed to be within 1%.
 

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The original pots are 50k, and have a issue with out of spec carbon tracks. Whether they were made that way, or it's a aging issue, don't know.
The first balance dual pot I overhauled had carbon tracks with a 3k difference between them.
The second balance pot has a 7k difference between them.
I could have saved myself a lot of work by measuring the resistance of the 50k carbon tracks(the outer lugs) first.
Obviously, mismatched tracks will never be in perfect balance, though they will be equal in resistance at one point if testing at the properly wired pigtails. Then the pot can be rotated and tightened down, and said to be correctly indexed.
I'll wire them up with pigtails next weekend and see how they compare, though using a Bourns 100k single seems the best choice for the balance pot.
However, the volume pot is critical, and as Jim has said, those stepped attenuators with resistors are probably best. Will probably pursue those for the volume and perhaps the balance also.
 
George S. said:
The original pots are 50k, and have a issue with out of spec carbon tracks. Whether they were made that way, or it's a aging issue, don't know.
The first balance dual pot I overhauled had carbon tracks with a 3k difference between them.
The second balance pot has a 7k difference between them.
I could have saved myself a lot of work by measuring the resistance of the 50k carbon tracks(the outer lugs) first.
Obviously, mismatched tracks will never be in perfect balance, though they will be equal in resistance at one point if testing at the properly wired pigtails. Then the pot can be rotated and tightened down, and said to be correctly indexed.
I'll wire them up with pigtails next weekend and see how they compare, though using a Bourns 100k single seems the best choice for the balance pot.
However, the volume pot is critical, and as Jim has said, those stepped attenuators with resistors are probably best. Will probably pursue those for the volume and perhaps the balance also.
Click to expand...
That's why the 2000 has a balance pot George :-)
 
Joe, yes indeed, and I find it annoying that the values are so far off. I understand it can be dialed in, still, seems it should be more accurate. Anyhow, I have a couple of those cheap 24 step 50k attenuators with Dale resistors ordered from China. Will give those a try. Appears they even have the correct shaft and will fit. We'll see.
Work week begins tomorrow. Later!
 
Wouldnt take long at2.50 a pop for those IRC's to get a pricey attentuator. Course the last time I priced the Alps pot used in a Spec 1 it was north of 475.00....
 
Gepetto said:
That's why the 2000 has a balance pot George :)
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Joe, another pearl of wisdom from you!
I couldn't see it!
The balance pot fine tunes the volume pot!
So, I have a dual channel signal generator and a dual channel scope. On the bench, the preamp can be fine tuned for perfect output balance.
Then the balance pot can be tightened down for accurate indexing. That's why they mounted it with pigtails rather than directly to the board!
As long as the scope shows even waveforms at 1/4, 1/2, 3/4 volume, it's good. The carbon tracks of the dual pots don't need to be absolutely equal.
 
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George S. said:
Joe, another pearl of wisdom from you!
I couldn't see it!
The balance pot fine tunes the volume pot!
So, I have a dual channel signal generator and a dual channel scope. On the bench, the preamp can be fine tuned for perfect output balance.
Then the balance pot can be tightened down for accurate indexing. That's why the mounted it with pigtails rather than directly to the board!
As long as the scope shows even waveforms at 1/4, 1/2, 3/4 volume, it's good. The carbon tracks of the dual pots don't need to be absolutely equal.
Click to expand...
Glad that I connected some neurons for you George... :-)
 
laatsch55 said:
Wouldnt take long at2.50 a pop for those IRC's to get a pricey attentuator. Course the last time I priced the Alps pot used in a Spec 1 it was north of 475.00....
Click to expand...
Don't even wanna know the price of the pots for my Pioneer D23 if i can get them.
 

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So, what did I find really bad in the CTS balance and volume pots beside dirt and green copper (sulphate?) on the contacts? The wiper slip rings have deposits that alcohol will not remove, and they need to be absolutely clean before lubing them, the carbon tracks, the contacts, and shaft and barrel with DeoxIT DFG-213 grease.
Tried a Dremel nylon brush wheel, soft brass brush wheel, and settled on a well used steel brush wheel carefully used to polish the deposits off what appears to be silver plating.
Low speed setting, careful use in regard to plating and potentially snagging a contact, and they're clean and polished.
These were extra, unused pots. I'll do the pots in my two PL 2000 S2 preamps in use. Take noise measurements, then install the 24 step attenuators, and take additional noise measurements for comparison, and see if there's any difference.
 

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The 50K Ladder pots arrived. They look very good on the ohm meters in regard to resistance matching.
The "D" shaft stub will need shortened and the flat "flattened" slightly more. Should be a very good fit.
Going to be awhile before I get them installed.
 

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My 2000s2 Z3 went bad causing my hum recapped only way I could get rid of it was with that birddog thing you talked about stock z3 all had same problem so the birddog solved my hum.
 
GaryCooper said:
My 2000s2 Z3 went bad causing my hum recapped only way I could get rid of it was with that birddog thing you talked about stock z3 all had same problem so the birddog solved my hum.
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Ah yes, the BrownDog adapter. Thanks to Lee and Joe for turning me on to those, they're great!
Watch this thread as I have a pile of bypass caps to test on those BrownDogs. (If I ever get any free time!)
Gary, I'm glad your repairing and enjoying your PL gear. Excellent!
 
Going to open the Alps tone pots and clean and grease them. Then install the 50K volume and balance ladder pots.
 

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The 100K Alps tone potts have limited serviceability unfortunately. Staked construction and unable to disassemble them further.
Alcohol bath and careful greasing will be all I can do with these.
 

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Got the Alps tone pots done. The look really good on the ohm meter.
Going to disassemble, clean, and grease a extra switch pack I have. I'll soak strips of coffee filter paper in DeoxIT and pull them thru the contacts.
Will try to polish those chromed plastic buttons.
 

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