Volume potentiometer?

Discussion in 'The Projects Forum' started by redrooster01, Aug 8, 2014.

  1. redrooster01

    Thread Starter Member

    Jul 15, 2014
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    I'm restoring a vintage Astor radiogram.The volume pot is unique in that it is 250k ohms dual gang with 2 x 25k ohm tags so I cant just buy another one? The front channel of the pot measures 166k ohms in circuit and the back channel 183k ohms,one channel works OK but the other is playing the radio at approx half volume at fully anticlockwise position? The only way to control that ''bad'' channel is to use the balance pot,it acts as a volume pot? Is there someway to fix this, maybe by putting resistors in series in the circuit or by somehow fixing 25k ohm tags to another 250k ohm volume pot?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I'd look for a replacement pot, but you might benefit from the links in the 2nd post here.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The tapped pot and RC network apparently provides a roll-off of the high frequencies relative to the bass as the voltage is increased (perhaps some sort of bass-boost for a simple Fletcher-Munson correction). You can likely reasonably simulate that by just connecting the RC network to the wiper of a standard 250kΩ standard dual logarithmic (volume control) pot. You may need to adjust the RC values to obtain the desired effect.

    Alternately you could just do away with the frequency change versus volume position, as is done in most modern amps, and just use the dual-pot by itself without the RC networks on the taps.
     
  4. vk6zgo

    Active Member

    Jul 21, 2012
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    These things were a commercially available part in Oz, back in the late 1950s,early '60s,& were sold as a "loudness control".
    They came complete with the external components.

    They were pretty much a short lived fad,& were aimed at DIY HIFI buffs.
    The DIY market collapsed,& most were eventually sold off as "New Old Stock" at a dollar or so each.

    Whether Astor just used these,or designed their own,is anybody's guess.
     
  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Metalmann likes this.
  6. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
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    I'm sorry I haven't replied earlier,I forgot to tick the instant email alert.Duh! What if I added a 25k ohm resistor to each channel on a normal 250k ohm dual gang pot, do you think that would work OK?
     
  7. tom_s

    Member

    Jun 27, 2014
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    solved a lot of 'crackly' faulty pots in the old days with a very small squirt of crc/wd40.

    personally, that's what i'd be doing.
     
  8. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I tried doing away with the RC circuit it worked in a fashion but I had no balance? Ive found a suitable Alps pot on ebay with a loudness pin,Ill see if it works with a 25k and 5,6k ohm resistor in series? By the way don't ever use CRC or WD40 on pots because it doesn't last,you'll end up having to pull it apart every few months to do it again. De-Oxit is the best,when its sprayed into the pot it fizzes and reduces the oxide back to metal again.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    I don't see how the RC network would have anything to do with balance. :confused: All the RC network does is change the frequency response.
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    dont forget to use a log taper pot, linear pots will give you a real strange volume control.
     
  11. Lestraveled

    Well-Known Member

    May 19, 2014
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    I don't see how a pot can fail and cause the symptoms you describe without causing other significant symptoms, like intermittent volume level and scratchiness. I suggest you measure the pot with two leads disconnected.
     
  12. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I was using the original volume pot at the time? Ive pulled this radio apart and rebuilt it a few times (trying to get it right) with all out of spec parts replaced and exactly as the schematic says. If I measure the volume pot out of circuit from the 2 x 25k ohm loudness taps to the 3rd solder tag looking at the pot front on,I get 26.5k ohms on the front and 29.1k on the back channel? To the 2nd or middle solder tag I measure the same 26.5k & 29.1,that's with the volume turned off(fully anticlockwise)Fully on (clockwise) I get 214k front and 249k back.Then to the first tag 214k front and 249k back with the volume turned fully both ways? There are some 15 ohm tweeters in this circuit that are turned on and off by a switch that isn't working so Ive soldered on a link to have them on permanently,Is there a way that I can upload the service manual here so you can have a look at it? It is 5MB?
     
    Last edited: Aug 13, 2014
  13. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    OK,Its taken a fair while but I have sourced an Alps pot with the same specs and it made no difference at all, ( I really thought the volume pot was the problem) so now I'm completely clueless on what to do next? It isn't the balance pot causing this problem either because Ive tried a new one,Ive stripped down and cleaned the bass and treble pots,checked and replaced anything out of spec and duplicated the negative wiring to earth by soldering another wire alongside the original wires just in case one was broken inside? To refresh your memory when the balance pot is turned fully anti-clockwise, that channel works as normal.When the balance is turned fully the opposite way both channels are on and the volume pot does nothing? The treble and bass seem to be working OK. Is there some way I can upload the service manual here so you can see the full schematic etc..? It is a 5MB file too large to go through the normal upload. Any ideas I could try would be muchly appreciated.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Why is a 5MB file too large to upload?
     
  15. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    A message came up saying it was to large when I tried to upload it.
     
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