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Old 27th June 2011, 09:23 PM   #1
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Default Repair or Get something else....

I have a Kenwood KR-A47 receiver that was in use until about 3 months ago that I want to use in my garage. Everything worked just fine except the preset stations get lost when powered off. I did some research & found that I needed to replace a .047F 5.5V cap. That certainly isn't beyond my abilities, so I ordered one from digikey & did it. I powered it back up & started to test tune a station in, and I noticed smoke! After powering it off, I saw that a 2W resistor had badly overheated. Now, there is a positive indicator marking where the cap was, and I installed it accordingly. I really don't think that I misinstalled it, but I'm second guessing myself. I can replace the resistor easily enough, but I'm wondering if it is likely to fix the problem. Any thoughts?

Scott
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Old 27th June 2011, 09:33 PM   #2
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Scott,
Sounds like it has a short circuit, maybe a solder bridge on printed circuit board, stray solder blob or uncut lead on new cap too long. If you haven't already checked, thats where I'd start.

Mike
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Old 27th June 2011, 09:41 PM   #3
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OK, that sounds like a good idea. Is it likely that nothing else is fried? I'm torn between using this as a learning experience, getting in over my head, and throwing time & components after a lost cause. I should get a chance to look at it tonight or tomorrow night (I'm still at work....)
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Old 27th June 2011, 10:02 PM   #4
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Scott,
Hard to say without being there. If the electron Gods like you, only the resistor was damaged, if they don't, well... I'd just start with thorough visual inspection for shorts or other obvious problems like other components that overheated or burnt. If every thing looks good, you can try replacing the resistor and see what happens. But be aware, either that resistor was already bad, or something that was wrong took it out, most likely there's a short.

Mike
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Old 27th June 2011, 10:08 PM   #5
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It's always good to check the orientation of old parts before pulling them out. It might be a good idea to power up the unit without the cap installed and measure between the two pins; then you can be sure to install the new part correctly. I have seen errors on commercial boards. OK, I've even made errors designing commercial boards. A long time ago. With tape and pads.
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Old 27th June 2011, 10:28 PM   #6
gpapag is offline gpapag  Greece
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In old radios, I have experienced ceramic caps (from positive line to ground) to fail short circuit. This causes smoked series resistors to burned rectifiers or transformers.

I havenít come across an electrolytic or paper capacitor to fail short circuit.

Regards
George
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Old 28th June 2011, 11:16 AM   #7
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Well, parts are cheap. I'm going to order the resistor & another .047F 5.5V cap. I'll inspect everything very carefully for shorts, and I really like the idea of powering up the unit without the cap to ensure proper installation. Hopefully, I'll get the parts & try them out later this week. I really appreciate the input you guys have given me. I'll report back soon.
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Old 28th June 2011, 11:25 AM   #8
Mooly is offline Mooly  United Kingdom
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If you could post a circuit so we could see what that resistor feeds....
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Old 28th June 2011, 12:12 PM   #9
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I'm new here, and I'm not sure how best to post the schematic. I do have a PDF of the service manual (Including the schematic), and I have identified the resistor & cap by their number. The file is around 10MB. Do I need to put the file on another server somewhere, and link to it here?
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Old 28th June 2011, 12:56 PM   #10
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OK, try THIS link for the service manual. The capacitor is C125, and the resistor that overheated (Badly!) is R164. You can see the + notation on the schematic by the cap. I followed it, perhaps I erred? I'll measure the voltage/polarity there before I install a new cap.
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