Battery charger repair

[hanick]

I have a Schumacher 1275A-PE 2/12/75amp charger that I just spent $30 on a circuit board that is still broke (feel like an idiot) and I am wondering if you would help me. http://www.centurytool.net/229900091...2299000911.htm
Plugged in lights up quiet hum on 2amp 8.6v output, 12amp 10.8 v outputand loud hum on engine start 13.6v output. Take off rectifier leads after 10min transformer very warm. Transformer output 30v ac on leads to rectifier, 15v on either one to the center. Diodes tested seperately to the plate, infinity one way reads 440-475 the other. arectifier plate reads 13.2v. Could the SCR be bad and how do you test it? Circuit breaker is fine. Meter always reads 100%. On light is on. Full charge light is on when I hit engine start.

 

[RonD]

I looked up that model but couldn't find a schematic, it does seems to have an auto sensing circuit so won't overcharge a battery.

So to test output voltage you need to hook it up to a 12v battery.
First check battery voltage , write it down.
Then hook up charger to battery and turn it on, lowest setting, and check battery voltage, write it down.
Do the same with the other settings.
Obviously the voltage at the battery should go up if charger is working.

Generally speaking you need 1volt above battery voltage to maintain a charge, so if battery voltage is 12.6v then 13.6v would only maintain it's charge, above 13.7v would be charging a battery with 12.6v, you can usually run up to about 15v before battery damage will occur, but most electric chargers, are not 100amp alternators, lol, so just above 14v would be their maximum output for recharging.

The amp meter on the charger being high could be a problem in the circuit, amp meter is the measurement of current flowing into the battery, so amp meter should show low if battery is unhooked(no current flowing) and high only if battery is very low on charge, battery shows 12.0v or lower.
So Full Charge Light and high amp meter doesn't make sense.

These units are $70 new, so I wouldn't spend any more on it without a firm knowledge of the failed circuit.

 

[4x4junkie]

Sounds like something's loading down the transformer if it's humming and getting warm real quick...
Can you test it with the transformer hooked up to the rectifier, but with the circuits downstream of the rectifier disconnected? If so, this may help you determine if the rectifier is bad (blown rectifiers are pretty common on stuff like that, and occasionally not always obvious testing it with a meter either). If the transformer is still humming and getting hot with the rest of things disconnected, then the rectifier itself is likely bad (shorted/leaking current).

 

[naford]

I looked on line and you can buy the diodes and SCR mounted on the heat sink for 22 dollars. If you have a schematic or if you can draw one it would be helpful. What is bothering me is that it looks like they are using the heat sink as one of the leads for the diodes. This means that all diodes have one lead in common which means it's a half wave rectifier with 2 parallel diodes per side. Without looking at this to hard. You should test the diodes with voltage on them. Measure the voltage across each diode using both ac and dc voltage settings on the meter. It shouldn't matter what amperage setting you use on the battery charger but change that to and re measure the diodes. I don't think the SCR is bad. My reasoning is that the SCR controls the output voltage and the output voltage changes as you change the output settings change. From the output voltages if you measure them hooked to a battery I would think the lowest setting would give you a reading of 12.5 to 13 volts. The next setting would be higher and the last higher still. So I'm guessing you have a bad diode because of the voltage drop. If somebody could measure their battery charger and confirm that the voltages will be higher as you change the output settings it would be helpful.