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Reversed polarity on am/fm/sw radio

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jjpaul

New Member
I picked up a radio in a thrift shop with an attached wallwart. I plugged it in at the shop and no power. I looked at the wallwart and it was 7 volts center positive but I don't remember what the current was. I looked around and found the one that belonged to the radio...6v 500mA neg center, plugged it in and no power. I bought it anyway for a $1.50 hoping maybe it was repairable. At home I had it plugged in and noticed the power supply was warm and unplugged it. I also tried batteries and no power but when I removed the batteries they were pretty warm. No burning odor. Had a limited peak at the boards and didn't notice anything but I didn't remove everything for a close examination either.

I don't know if the radio was working when it arrived at the thrift shop or not but if it was could the reversed polarity have ended it's life and if so how would I go about troubleshooting it and possibly repairing it? Thanks for any advice!
 

Les Jones

Well-Known Member
Most Helpful Member
Some products have reverse polarity protection. If it was the series diode type it would have caused no damage. If it was the shunt diode type then is should have had a fuse before the diode. This cannot be the case with your radio as is drawing current. If you are really lucky this diode will have failed short circuit and can easily be replaced. It is more likely that there was no reverse polarity protection in which case devices in the audio outpur stages ore the most likely to be destroyed. The audio output stages could be discrete transistors or an integrated circuit. If it uses transistors then check for collector emitter shorts on both output transistors and the transistors that drive the output transistors. Do this for both channels if it is stereo output. If it its an IC audio output check for a short between it's power supply pins. You may have to cut tracks on the PCB to be sure that a short is not elsewhere on the circuit.

Les.
 

jjpaul

New Member
Hi thanks for your reply.
I should have also added that there is a LED display for time, alarm functions which does not display anything.
 

dr pepper

Well-Known Member
Most Helpful Member
Reverse diode near the connector first call.
If not that then anything could have blown, likely candidates though are power supply decoupling electrolytics & the audio amp Ic or o/p transistors.
 

cowboybob

Well-Known Member
Most Helpful Member
Take some well lighted and focused pics of the radio's circuit board area where the connections for the battery and/or external power wires are attached.

Include additional components near to and around those connections.

Without that (or those) all we can do is guess...
 

Aleph(0)

Member
jjpaul So FWIW I agree with other posters advising inspection of on-board psu filter caps and active AF power amp semis. (it's an easy repair if finals are discretes or generic IC{s} also even unmarked discretes are vry easy to ID or adequately substitute based on just looking at bias circuit:)).

One other possibility I've seen is shunt diode protection w/o fuse:facepalm:! If that's case it's possible excessive forward current (during inverse polarity from external PSU connection) damaged the diode causing high reverse leakage so plz look for and check shunt diode if present):)!
 
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