Retired ME neophyte needs assistance

Discussion in 'General Electronics Chat' started by mesmith, Oct 28, 2005.

  1. mesmith

    Thread Starter New Member

    Oct 28, 2005
    4
    0
    I have a 1972 vintage 10 KW standby generator with a trickle battery charger while the unit's in standby. Online the generator provides a 4 amp charge via a reverse current diode from the generator to the battery. Offline an L1 and L2 powered step-down transformer protected via a 3AG, 1 amp fuse provides power through a rheostat and resistor in series with a selenium rectifier. The rectifier in series with a milliammeter is designed to provide a .02 amp charge to the battery. This charger is not functioning and I wish to test the milliammeter. If the meter is defective is is possible to test it by placing test probes across the terminals? Or should a terminal be disconnected and then place test probes in series with the meter? I have no idea as to the values or either the rheostat, the resistor or selenium rectifier. If the meter tests functional, then how do I test the selenium rectifier without removing it or taking the generator completely offline. All of the above is subjective based on an assumption that the step-down transformer and the X side of the transformers protective resistor is functional.

    Have as much fun with this as you desire. I'm not thin skinned but need direction.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    You might just shotgun a couple of parts before getting to serious troubleshooting. Just replace the fuse and get rid of that selenium rectifier - especially if it smells like a dead mouse. They paint up pretty, but smell horrible when they fail. Substitute a 1N4001 diode for it.

    It's easy to check your millimeter - just touch meter leads across it with the meter set to ohms. The millimeter will probably bounce the needle off a stop pin. Don't worry about accuracy - it will be fine if it responds at all.
     
  3. mesmith

    Thread Starter New Member

    Oct 28, 2005
    4
    0
    Thanks beenthere,

    I should have indicated the fuse had been replaced & is good now.

    I will check the meter as you suggested & reply back.

    The selenium rectifier looks as though it was new, no appreciable odor but the generator & control panel is in it's own building so I could have missed the burnout

    Now comes the good part, I quit attempting to stay current when transistors replaced vacuum tubes. That should date me. I was a hobbyist until then. Therefore I'm completely ignorant of diode installation especially since the SR has 4 terminals. I will search for a schematic for the recommended diode to replace the SR.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    With four terminals, the rectifier is probably a bridge type - there's four diodes in it. Your local Radio Shack should have bridge restifiers in stock. All you have to do is get one rated for more than one amp @ 50 volts. Once you have the + and - DC leads located, hooking it up will be no problem.
     
  5. mesmith

    Thread Starter New Member

    Oct 28, 2005
    4
    0
    Hello,

    You're correct as to the number of diodes so indicated by a schematic I've obtained from the manufacturer, who by the way did not tell me values of the particular components in this circuit. Other than a timer that starts & tests the unit at a predetermined 2 week interval & the battery trickle charge circuit, the generator operates flawlessly.

    I tested the MAM as you indicated with an ohmmeter. There was a momentary resistance indication & then my meter, which is a digital, fell back to (0.00). Is this to mean the MAM is operational? If so the only component left is the SR. I have an old RS as well as an Allied catalog, so I should be able to locate the Rectifier. I located a BR from NTE.

    I will deenergise the unit & install the BR. Once this is completed I will let you know the results. In the mean time, I wish to thank you for your time & knowledge. Without it I would still be searching.
     
  6. mesmith

    Thread Starter New Member

    Oct 28, 2005
    4
    0
    Hello Beenthere,

    I installed a new Bridge Rectifier, rather simple installation as you indicated, but the initial problem still remains. Not being sure how to check a step-down transformer, I made the following attempt after energizing the unit & receiving no meter response while adjusting the rheostat. I placed test probes across the secondary of the transformer & received a reading of 25 mv. The transformer's rating is 230 ac volts primary & 18 ac volts secondary. Should I be not reading 18 ac volts or is a load required? Since I'm not sure of the testing procedure for a transformer, I need your knowledge as to whether I should, as you put it, shotgun the transformer. This is one component other than the MAM & rheostat that could be faulty. Once again, there's a new fuse, a new BR, the MAM bounced an indication on my digital, the secondary transformer resistor tested 30 ohms which is stamped on the resistor.

    In my original request for assistance I stated the trickle charger should have a .02 amp delivery rate which should have been .2 amps. I took this into consideration when purchasing the BR which was an NTE 5314.
     
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