help me with this variable wire wound power resistor

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jul 18, 2012.

  1. sdowney717
    Joined: Nov 2010
    Posts: 955
    Likes: 53, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I emailed Vic at Raritan on this old convertor-charger to see if they can tell me what the specs are on the component.

    It is the older “R” series which predated both the “B” and the “F” series.

    It is the lowest part on the circuit board. Riveted on the ends and soldered into the circuit.
    [​IMG]

    I measured 14 million ohms across the terminals which must be too high dont you think?
    Perhaps corrosion has affected the winding under the adjuster.

    Also any ideas on where to go for a replacement?
     
  2. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    You have to measure it from the sliding point to both sides and see what you are able to get for a reading. Wire wounds are normally linear. Thus if you measure halfway the one side per example , 70 Ohm and the other side 14 million Ohms, you can assume the resistor is 140 Ohm. Tou can also keep the one probe of your meter on the one end and ritz the other probe over the wirewound resistor. It should tell you something.
     
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    A nice picture sdowney717, I haven't seen that for decades.
    You are probably right that the resistance wire corroded away under the slider.
    Judged by the size this is a 2 or 3 watts type. Measure as BertKu suggested and replace the part with a wire wound pot from Bourns or AB, connected with short wires. You won't find the exact replacement for this museum piece I'm afraid.
     
  4. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I thought so too! But I resoldered the connection and it is working again. It reads 330 ohms. Raritan responded back that they think it is a 400 ohm for the 12 volt charger.

    I very closely examined the exposed metal winding with a glass and it looks intact. It is possible that the resistor is still failing intermittently.

    I cleaned up everything and tested on the car and it is putting out now at 13.6v which is good. It must be connected to a battery to get feedback to drive the proper voltage. Unconnected reads 10 volts, run a DC blower motor and no battery reads 11.2 volts. hook up battery reads 13.6 volts.


    Now this here.
    I was trying to read out the component numbers in case needs future repairs.
    I got everything except a few are too difficult to fully resolve.

    Such as the 3 Westinghouse stud diodes feeding the battery bank, located on lower plate.
    They look like 368 'something' SCR 7120 BT with diode symbol --|>-- ??

    Their are 2 westinghouse stud diodes from the transformer located on upper plate which I verified as
    368C 7147 --|<-- with diode symbol reversed, so perhaps not the same parts.

    Their is also two round transistors. One is a 2N525 which I can find ,
    other has a large metallic heat sink on it which says Thermalloy
    22118?? I assume the number is still on the transistor case if it was
    pried off?

    Also the single silver metal diode, numbers obliterated. immediate left of
    thermalloy transistor.

    The gold zener diode next to red wire reads 1N3829 or 1N3828?
    The 2 stud diodes on the right sense if the ignition is on and are 1N1343
    There is a GE C38U phase control SCR to the extreme left on its own large heat sink plate.
    I am surprised the 100MFD capacitor is still working since this is made in 1970.

    [​IMG]

    This is a very well built charger. To get outer case removed disconnect front lid wires. Remove 4 side screws and the entire outer cover lifts off.
     
  5. Lt. Kludge
    Joined: Jul 2010
    Posts: 18
    Likes: 1, Points: 0, Legacy Rep: 23
    Location: Massachusetts, USA

    Lt. Kludge Junior Member

    Your Resistor is made by Ohmite. The value can be determined as mentioned above or probably legible on the under side. The metal posts only go in about 1/4 inch and can frequently be bent back out of the body enough to replace the part. It is available from Allied electronics.

    Stud mounted rectifiers can be a problem these days. There are some datasheet search websites that can help determine the specs for some of these parts from the number on them. But replacement may take a little more creativity in mounting/heat-sinking a TO-218 in place of a DO-5.
     

  6. sdowney717
    Joined: Nov 2010
    Posts: 955
    Likes: 53, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Raritan sent me the pdf manual for the unit with a lot of info.

    here is the schematic and part numbers. Much better than trying to decipher corroded lettering.

    Do you think those Westinghouse stud diodes are no longer available?
    found as a R3120 here
    https://avnetexpress.avnet.com/stor...=&rohs=&storeId=500201&term=R3120&topSellers=

    [​IMG]

    [​IMG]

    for some reason the web site is washing out the schematic
    so I attached the file and the manual.
     

    Attached Files:

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