Sears Battery Charger 934.71830, Charge Output is Weak

[Capt Mack]

My Sears 6/12V battery charger 934-71830 has a low charge output as indicated on the charge meter. I have the manual. There is a choice between a transformer replacement and a rectifier assembly replacement. My question is, which do I replace? Or how do I troubleshoot the schematic

 

[Nigel Goodwin]

It looks like an antique, and only seems to have a transformer and a rectifier - low charge output could only be caused by the rectifier, which is probably a selenium type? (which are very prone to failure).

 

[Diver300]

As Nigel says, there is probably no smoothing. The voltage will often read low when there is no battery connected.

If the voltage shows up as good when you connect to a charged battery, then it's probably fine.

If you want to troubleshoot, disconnect the transformer from the rectifier. The transformer should produce around 10 - 12 V ac with no load. Between each of the AC input of the rectifier and one of the + or - connections, there should be a diode, so you should get continuity with your meter connected one way, and not with it connected the other way.

 

[crutschow]

Sears Partsdirect (amazingly still in business) shows it as a 30A charger with the schematic below:
Likely it is one of the rectifiers that is bad.
A replacement should be a 30A rectifier with at least a 40V rating.

 

[Capt Mack]

I'm headed to the hangar to try out all this superior advice I am receiving . Thank for responding.
Steve

 

[Capt Mack]

I am definitely Joe Btfsplk. No reading coincides with what I am above told. Transformer ac voltage output: Through the switch positions: 6V=15.7 / 12V slow=22.3 / 12V fast=27.0 /boost=30.6. Transformer voltage to the center wire to ammeter=7.6 . Diodes equally confusing, Using 2000K setting on ohmeter #1 reads 0 and reverse wires 21 Ohms / #2 reads 8 and reverse wires 10.
Maybe it's time to use this charger for a boat anchor?

 

[danadak]

Picture of rectifiers ?

Did you read AC voltage on meter selection ?

Can you lift one leg of each rectifier and repeat forward and reverse
measurements ?

Meter Check of a Diode | Diodes and Rectifiers | Electronics Textbook

Read about Meter Check of a Diode (Diodes and Rectifiers ) in our free Electronics Textbook

www.allaboutcircuits.com

Use diode check selection on meter, see half way down page.

All screw terminals for connections tight.


Regards, Dana.

 

[Nigel Goodwin]

Like I said, it's almost certainly a duff rectifier, as they were prone to failure - the transformer will either work or not, but selenium rectifiers are pretty unreliable.

 

[crutschow]

selenium rectifiers are pretty unreliable.

And they give off a noxious spelling gas when they fail.

 

[Nigel Goodwin]

And they give off a noxious spelling gas when they fail.

They can do