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potentiometer replacement

Discussion in 'Troubleshooting and Repair' started by leonel, Jan 19, 2013.

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  1. leonel

    leonel

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    Jan 19, 2013
    I built a SCR throtlte power supply for my model railroad quite a while ago and I am now experiencing a problem with it. I think I narrowed it down to a potentiometer shorting out. My problem is not being able to find or replace with an equivelant or near equivelant pot. Radio Shack doesn't carry what I need and I can't seem to find a hobby supply specializing in electronics. I am in need of a 250 Ohm linear taper pot (the original is:Mallory VW-250). I am hoping to find a replacement or a different pot which will work. Any advice is appreciated.

    Thank You; leonel
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    It would be rare for a potentiometer to short out.

    Does this pot need a particular wattage rating? (Ah, I see it is a 5W wirewound pot)

    I's check digikey (on the basis of an unfounded guess that you're in the US).

    This, or this might be a good option (I have no idea what the difference is)
     
  3. leonel

    leonel

    28
    0
    Jan 19, 2013
    hi Steve;

    Enclosed you will find the article of the power pack I built for my model railroad. It explains everything you will need to know.

    When I hooked it up to my Christmas display and set everything up as I wanted the train to respond to the settings; it would not start unless I tapped the Power Pack between the LOAD and POWER pots. This resulted in the train starting at the speed I set and it had no momentum at all. This happend a few times. Now when I try to run a train the overload indicator light(L1) comes on and this in turn causes loss of control and the voltage is grounded out. I felt this problem started with Potentiometer D3.(The article explains this action in the paragraph describing the "loss of regulation" feature.

    I hope I exlpained my problem thorouhly and not in a confusing way. The article of this power pack was in a 1977 Model Railroad magazine. If need be I will also take pictures of what I built and send them to you also.

    Thanks a bunch;

    leonel P.S. did the pictures of the article come through to you?
     
    Last edited: Jan 20, 2013
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, I don't see the article. Are you trying to post a URL, or something else?
     
  5. leonel

    leonel

    28
    0
    Jan 19, 2013
    again
    Hi Steve;

    I took pictures with my digital camera and now I will take them over holding the camera the normal way.

    talk to you in a while;

    Leo
     

    Attached Files:

  6. leonel

    leonel

    28
    0
    Jan 19, 2013
    Here they are Steve. The best I could at the present time.

    I hope this worked!

    Leo

    (PS) Many Many Many deserved thanks
     

    Attached Files:

  7. leonel

    leonel

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    Jan 19, 2013
    Yes Dave your right. BUT what about all the challenges that makes life interesting!

    Leo
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I'm going to have to read all this at my leisure.

    The "right way up" copy of the first page is hard to read due to camera shake, but the previous version you posted is easier to read. I should be able to get what I need from the first copy :)
     
  9. leonel

    leonel

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    Jan 19, 2013
    I would think you would like to know what operational problem I am having. Let me know when you would want it. Leo
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yeah, that would be useful :)

    I thought you explained it above, but I was clearly remembering something else...

    Give me all the information you can.
     
  11. leonel

    leonel

    28
    0
    Jan 19, 2013
    attachments

    Hi Steve;

    Here is all the info I have. I tried ALL settings to find out what the voltage meter and ampmeter readings and how they correspond to the different settings.

    I started out by setting the SPEED, LOAD and POWER settings before I turned on any power. I then turned on the AC power followed with the DC voltage. The resalt was the Control Disable light comming on and the Voltage was 1 3/4 volt.. The light remains on untill thePower pot is rotated to th 2 Oclock position where it turns off.(during this travel the light displays some dimming back and forth) The 'V' varies from the 1 3/4 volts to almost 8 volts up to the maximum Power setting. The Ampmeter exibits no movement at all during of these settings.

    When I touched the track lead wires to a locomotive(I used two different loco's for comparison) the 'V' dropped to '0'. Ampmeter displayed no movement.

    I turned the Service Brake On and Off. the Voltage lowered and increased according to the Momemtum Rate settings. When the Emergency Brake was applied the 'V' dropped to zero within the one second time mentioned. The voltage recovered at the preset momentum rate when the brake was disengaged.

    The Reverse Switch worked properly.

    The Control Disable Switch was turned to the off possition and after a short while the voltage started to drop very slowly as predicted. I did not wait for the voltage to drop to nothing to assume it was working properly.

    What ever setting I turned the LOAD pot to; the momentum reacted accordingly (faster or slower).

    When I turned the DC Voltage switch off the voltage dropped to zero immediately. Not sure if this is right---should a compacitor just drain off or should it be grounded out???

    I hope this is the information that is needed. I hope I didn't delve into parts of this problem unnecessarily. If I did, please excuse my eagerness.

    As Always;

    Thank You,

    Leo
     
    Last edited: Jan 26, 2013
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, thanks. I'll take a look at it with those things in mind.
     
  13. leonel

    leonel

    28
    0
    Jan 19, 2013
    fixed loose solder joint

    Hi Steve;

    I had nothing to do today and I decided to take another look at my power pack. I was looking for components that looked overheated or discolored ect. What I found was a loose solder joint on resister #12 and resoldered it. Well, I turned it on and to my surprise the power pack works!!!
    I tried it at all the settings I forwarded to you and everything works fine.

    So if you have had the time to study this problem; I appreciate the time and labor you put into trying to resolve this trouble for me.

    Again I appreciate what you did or was going to do very much.

    Thank You again;

    leonel (Leo) :D:D
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    Excellent fix. I'd put your circuit aside to look at in detail this weekend (I am sorry I didn't get a chance to look at it earlier).

    I may look at it anyway as it seems interesting. It will be a lot more fun now that I don't have to try to figure out your fault at the same time.

    I think this says a lot for the Mk:1 eyeball.
     
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