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Vintage electronics

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ussfox

New Member
Hello, I'm trying to fix an old 6 & 12v battery charger. I'm pretty sure the bridge rectifier is bad. It's an old mallory one, probably about 2" square plates with a 3/8 bolt through the middle. Can I replace it with a newer type? What rating? I have a 19v secondary xfmr voltage applied to the rectifier. Primary is fused at 2A. Max charge current is 20A at 6V.
 

tcmtech

Banned
Most Helpful Member
Any standard standard diodes with higher amp ratings will owrk. Just pick somenting withat high enoughvoltage and preferabley a 2 or 3 times higher amp rating than what the battery charger puts out!

Check out the internet for electronics components suppliers.
Or you probibly can get a 25 amp bridge at radio shack. They used to carry them.
The made a good replacement on many smaller battery chargers.
 

ussfox

New Member
Ok,sounds good. BTW, are you familiar with the type of diodes I'm talking about? How much do they have to be isolated to test properly? Right now, I have the 2 inputs unsoldered and one output lead isolated. These are two seperate components consisting of two diodes each.
 

Rolf

Member
Voltage drop ........

Any standard standard diodes with higher amp ratings will owrk. Just pick somenting withat high enoughvoltage and preferabley a 2 or 3 times higher amp rating than what the battery charger puts out!

Check out the internet for electronics components suppliers.
Or you probibly can get a 25 amp bridge at radio shack. They used to carry them.
The made a good replacement on many smaller battery chargers.
Just remember that a modern silicon diode will produce a greater voltage output
than the selenium rectifier, so some additional series resistance might have to be added.
 

mneary

New Member
And remember that we've finally learned that what we bury eventually comes back in our drinking water. Dispose of the selenium rectifiers properly, please.
 

ussfox

New Member
Well, after further troubleshooting and tinkering, The charger started to work.I now have 29 vdc output. This transformer has a tapped secondary for 6 or 12v. The reostat that actually shorts the windings has two small contacts.I think from moving them back and forth, I was able to make better contact. At one point during troubleshooting I was measuring output voltage. Had 29vdc but it started to slowly drop, eventually went to zero. What would cause that?
 

Sceadwian

Banned
Bad contacts sounds plausible to me. If you weren't loading the output the slowly dropping voltage was just the filter cap discharging.
 

ussfox

New Member
Started charging a 12v car battery today. My hydrometer broke so I'm not sure how low the battery was. 15v 4A was what I was putting out with the charger up all the way. I noticed only one cell was bubbling. Battery is only a year old. Does this sound like it could be ok? Eventually the voltage dropped off to 13v and I stopped the charge. Any thoughts?
 

mneary

New Member
One cell bubbling means it was charged and the others were not. Perhaps the charges equalized, perhaps not. You really need to test with a hydrometer.
 
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