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.

 

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.

 

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.

 

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.

 

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.

 

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.