good afternoon everybody,
I guess all of you know how important the proper functioning of a volume potentiometer is in a preamplifier, and of course many faced the problem of replacing old and scratchy pot with a new one, when there are absolutely no modern pots with the tap(s) at the right points available, there are too many solutions to mention, but there is one that I personally like the most, and feel that this is the right thing, a modern 47 step attenuator by Goldpoint Level Controls , very well made and smooth turning.
I`ve bought it because it is pretty easy to solder the tap(s) to any desired point(s), and I wouldn`t ask for help here, if I was 100% sure where these points were.
the original Harman Kardon Citation One preamplifier contains a dual 500K/500K potentiometer with 2 (two) taps, HK part # is RV3285193 (if anybody would like to sell an old used one, I would gladly buy it)
unfortunately, my unit came to me heavily molested, with not original "alps" pot of 100K (!) instead of original, 200K balance pot had also been replaced with 500K (?), so I can`t pull it out and measure, there is nothing to measure.
surely I wrote to HK, and surely they don`t know and don`t have the older papers.
then I wrote to the very famous and respectable gentlemen, who are indeed great masters in restoring HK apparatus, Jim McShane, Don Sachs and Sheldon Stokes, latter provided a fantastic replacement power supply kit, all of them say that if I`m to use a modern logarythmic pot,and would want to use loudness feature (I would), I should make only one tap at about 70K and solder the 27K resistors to it, as it was in original, and leave the other tap that went to 100K resistors not used at all.
they are probably quite right, and maybe this will work fine, but still I want to understand what the other tap they suggest not to use was for?
so I know now, (not 100% though) that the original RV3285193 had two taps at 70K and 250K, but was it logarythmic or linear? this is a crucial question, because I`ve already spent a month selecting zero tolerance resistors for the attenuator to make, but what if the original was linear, and that other 250K tap was just to make it pseudo-logarythmic?
summing all said, I would be very, very grateful for your opinion on the following questions:
if the original RV3285193 500K/500K pot with taps at 70K and 250K was LOGARYTHMIC, what was the second tap that went to 100K resistors for?
if the original pot was LINEAR, was that second tap to 100K resistors called upon to make it logaryhmic-like, just because they didn`t have the logarythmic pots available?
what will happen if I make a LOGARYTHMIC stepped attenuator of 500K/500K and use BOTH taps at 70K and 250K, soldering them to the original points?
would it be better (true to specs) if I make a LINEAR stepped attenuator of 500K/500K and use BOTH taps at 70K and 250K, soldering them to the original points?
I wouldn`t ask for help, but while searching for some replacement pot I found this guy:
Harman Kardon Citation I IV 1 4 loundess volume pot control potentiometer | eBay
he claims that he sells an EXACT COPY of original pot, but we know now, that the original has 2 taps at 70K and 250K, and this one he sells has only one tap at 150K. where is the truth?
it seems that the same questions can be asked about many other preamplifiers of the era with the same problems with scratchy pots, e.g. Scott-130, that I also own, there is again a 500K/500K potentiometer with TWO taps at 200K and 350K, was it linear or logarythmic?
and again that ebay guy claims he sells an "EXACT COPY" of the original, but what he sells again have only ONE pot at 250K :
Volume pot control power switch on/off Scott 299 299A 130 120-CP 120A 380 355 | eBay
I guess all of you know how important the proper functioning of a volume potentiometer is in a preamplifier, and of course many faced the problem of replacing old and scratchy pot with a new one, when there are absolutely no modern pots with the tap(s) at the right points available, there are too many solutions to mention, but there is one that I personally like the most, and feel that this is the right thing, a modern 47 step attenuator by Goldpoint Level Controls , very well made and smooth turning.
I`ve bought it because it is pretty easy to solder the tap(s) to any desired point(s), and I wouldn`t ask for help here, if I was 100% sure where these points were.
the original Harman Kardon Citation One preamplifier contains a dual 500K/500K potentiometer with 2 (two) taps, HK part # is RV3285193 (if anybody would like to sell an old used one, I would gladly buy it)
unfortunately, my unit came to me heavily molested, with not original "alps" pot of 100K (!) instead of original, 200K balance pot had also been replaced with 500K (?), so I can`t pull it out and measure, there is nothing to measure.
surely I wrote to HK, and surely they don`t know and don`t have the older papers.
then I wrote to the very famous and respectable gentlemen, who are indeed great masters in restoring HK apparatus, Jim McShane, Don Sachs and Sheldon Stokes, latter provided a fantastic replacement power supply kit, all of them say that if I`m to use a modern logarythmic pot,and would want to use loudness feature (I would), I should make only one tap at about 70K and solder the 27K resistors to it, as it was in original, and leave the other tap that went to 100K resistors not used at all.
they are probably quite right, and maybe this will work fine, but still I want to understand what the other tap they suggest not to use was for?
so I know now, (not 100% though) that the original RV3285193 had two taps at 70K and 250K, but was it logarythmic or linear? this is a crucial question, because I`ve already spent a month selecting zero tolerance resistors for the attenuator to make, but what if the original was linear, and that other 250K tap was just to make it pseudo-logarythmic?
summing all said, I would be very, very grateful for your opinion on the following questions:
if the original RV3285193 500K/500K pot with taps at 70K and 250K was LOGARYTHMIC, what was the second tap that went to 100K resistors for?
if the original pot was LINEAR, was that second tap to 100K resistors called upon to make it logaryhmic-like, just because they didn`t have the logarythmic pots available?
what will happen if I make a LOGARYTHMIC stepped attenuator of 500K/500K and use BOTH taps at 70K and 250K, soldering them to the original points?
would it be better (true to specs) if I make a LINEAR stepped attenuator of 500K/500K and use BOTH taps at 70K and 250K, soldering them to the original points?
I wouldn`t ask for help, but while searching for some replacement pot I found this guy:
Harman Kardon Citation I IV 1 4 loundess volume pot control potentiometer | eBay
he claims that he sells an EXACT COPY of original pot, but we know now, that the original has 2 taps at 70K and 250K, and this one he sells has only one tap at 150K. where is the truth?
it seems that the same questions can be asked about many other preamplifiers of the era with the same problems with scratchy pots, e.g. Scott-130, that I also own, there is again a 500K/500K potentiometer with TWO taps at 200K and 350K, was it linear or logarythmic?
and again that ebay guy claims he sells an "EXACT COPY" of the original, but what he sells again have only ONE pot at 250K :
Volume pot control power switch on/off Scott 299 299A 130 120-CP 120A 380 355 | eBay
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