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NEW OLD STOCK...Should I worry?

Discussion in 'DIY' started by z-adamson, Jun 26, 2018.

  1. z-adamson

    z-adamson Super Member

    Messages:
    1,608
    So, I have a lead on some preamps that are new old stock.

    Date of manufacture is 10-15 years ago.

    They are in the oem boxes, untouched, never used, never plugged in.

    My concern.....

    The caps have never had a charge up. My basic understanding is that if an ecap sits for years on end without a charge, they go bad.

    The pots have never been turned back and forth. Could pots sitting for this long with no "excersice" go bad and get all scratchy?
     

     

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  2. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    You could use a Variac to slowly bring them to life, or a DBT with a small bulb.
     
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  3. z-adamson

    z-adamson Super Member

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    1,608
    I have a dbt and small bulbs.

    So, plug the device in with the bulb in series....then what?

    There are no adjustments or anything.

    Are you saying let the preamp idle with the bulb?
     
  4. Hipocrates

    Hipocrates Anti-Muppet Subscriber

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    Pretty much, yupe... Turn it up on a DBT, let it there for a little while, if everything is going fine test it. If pots dirty, faderlube and deoxit probably do the trick.
    I'm courious what units are.
     
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  5. Bill Ferris

    Bill Ferris Super Member

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    I`d let the preamp, after it passes the DBT, though I would prefer to bring it up on a variac slowly over an hour, then let it "acclimatize" to 120 volts over night DBT connected, of course, then hook up it up/test/check out for control/switches noise.
    Spray, if needed, enjoy.

    Last year in the early winter, my friend sold me his used Mac C 27 that he bought used over 18 years ago and had never hooked it up.

    Brought it up slowly on the variac as described above, while monitoring it`s current draw, on my variac`s selectable 2 amp scale amperemeter, for several hours, all the while, feeling all the main PS caps, rectifiers, regulator/s, and any high watt resistors for uncomfortably elevated temperatures while feeding it 120 volts, in this case, all was fine.

    After I was satisfied all was happy inside, I let the preamp run all night, ~24 hrs, then ran full circuit voltage and proof of performance test equipment measurement checks against the factory specifications..

    After it passed muster, into my sunroom`s stereo system it went, and it hasn`t been turned off since, along with it`s, now companion Mac MC 2205 power amp, and they play wonderfully all day long my ~10,000 WAV. ripped songs.

    Just my usual way of checking out long unpowered, or as in the OP`s case, and if it was brought to me, procedure, for never run since the factory(NIB), SS equipment.

    As you folks choose and see fit.

    Kind regards, OKB
     
  6. z-adamson

    z-adamson Super Member

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    1,608
    Thanks alot for the suggestions!

    I will move on them and do the DBT thing as suggested above.

    20 watt bulb OK?
     

     

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  7. Scott75s1974

    Scott75s1974 Active Member

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    I'd hesitate on the automatic deoxit. I don't see why a factory lubed pot would necessarily develop corrosion sitting unused/unmoved. No friction and even if the wiper has enough force to pressure the pad, deoxit wouldn't help that. I miss the smooth turning of my controls after a very necessary deoxit treatment.
     
    Bill Ferris likes this.
  8. KingBubba

    KingBubba "Too Much Stuff" Subscriber

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    Scott, following doing a Deoxit treatment, you should always follow up with an application of Caig Fader which will lube the pot and return that smooth feeling. In addition, an application of Caig Shield or Gold after the Fader will seal the contacts that will keep the corrosion from returning for a good long while. Deoxit Instructions.jpg
     
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  9. Bill Ferris

    Bill Ferris Super Member

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    1,687
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    Your most welcome.
    I would think 20 watt bulb would be fine, as most SS preamps tend to draw 20 watts, or less.

    The preamps electrical input consumption information, usually on the back of mains powered electronic equipment near the power cord/fuse would be helpful to determine, and or the owner`s manual under the specifications section.

    Good luck z-adamson.

    Regards, OKB
     
  10. Bill Ferris

    Bill Ferris Super Member

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    :thumbsup:
     
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  11. ConradH

    ConradH Addicted Member

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    I would not use a DBT, only a Variac. You just don't have good control over the voltage with the DBT, and if the caps have DC leakage they'll draw too much current unless you've sized the bulb just right. Even more iffy if you do more than one at a time. With the Variac you can simply set the voltage and soak them for a while. Wait a bit, raise it, wait a bit, raise it.
     
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  12. Bill Ferris

    Bill Ferris Super Member

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    My preferred method as well, Conrad
     
  13. z-adamson

    z-adamson Super Member

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    This would be nice, but dbt I have, variac I have not and I don't know that I could justify the price of a good variac just for this.

    I would rather size the bulb just right for the application at hand.

    Which brings me to my next question...

    How would I size it just right for this?

    Historically, I have used the dbt just for looking for shorts in the power supply in amps and receivers etc. Not for bringing something to life that has sat for a good while.
     
  14. petehall347

    petehall347 the brandy coffee man Subscriber

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    i would think start with a lamp that glows a little bit after its settled down .
     
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  15. z-adamson

    z-adamson Super Member

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    How does one calculate the bulb size to achieve this without trial and error?
     
  16. northpaw

    northpaw Active Member

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    You *could* do a calculation based on Ohm's law and a nominal resistance for the bulb (the latter calculated from the formula for power, using the nominal wattage of the bulb), but it would be meaningless because the resistance of the bulb varies greatly with current. If your unit is not shorting, the bulb will be working well under its rated wattage, and the unit under test will get near full line voltage, no matter what wattage bulb you use in a DBT (a bulb designed for full AC line voltage, that is).

    The bulb only dissipates its nominal wattage when full AC line voltage is across it. When a unit is wired in series with a DBT, each will get a fraction of the line voltage, the split depending on their relative resistances. If the unit under test is not shorting, it will have a large share of the resistance in the circuit and therefore will be limiting the current flowing in the circuit, and the voltage drop across the bulb will be a very small share of the line voltage. In this circumstance, the resistance of the bulb will be low. Only if the unit has a short will the full AC line voltage be across the bulb. In that case, the bulb's resistance will ramp up to the value calculated from its watt rating, and it will limit the current (and drop the voltage) passed onto the unit. This little dance with variable resistance, variable voltage drop, and variable current limiting provided by the bulb is the whole idea behind a DBT.

    So get a variac (the ones that handle 5A or 500VA aren't that expensive and should be enough for all but the highest power amps), or get a 600W rotary light dimmer (one that works for incandescent lamps) and wire it between the wall outlet and your DBT. They are only $10 or so home improvement places, plus the cost a wall box and wire/plug. I've not tried that for this purpose, but it should work.
     

     

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  17. Bill Ferris

    Bill Ferris Super Member

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    I wouldn`t try to use a dimmer as a poor man`s variac !
    Their output wave form looks pretty dirty and usually triangle shaped, IIRC. and super rich in harmonics and I`d think would be rough on a power transformer`s primary, whereas incandescent bulb`s resistive filaments don`t seem to mind, other than singing/buzzing at low dimmed levels.

    Just my thoughts.
     
    arts likes this.
  18. northpaw

    northpaw Active Member

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    Good to know. I've only used this type of control for heating duties (the 1500W ones), and never looked at their waveform.
     
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  19. dbxdx5

    dbxdx5 AK Subscriber Subscriber

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    You may have said and I missed it, but what brand and model preamps? Inquiring minds want to know. ;)
     
  20. z-adamson

    z-adamson Super Member

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    Parasound preamps
     
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