Sears Battery Charger 934.71830, Charge Output is Weak

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Capt Mack

New Member
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
 
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).
 
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.
 
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.
 
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?
 
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.
 
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.
 
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