Repair or Get something else....

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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
 
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....)
 
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
 
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.:cool:
 
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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 haven’t come across an electrolytic or paper capacitor to fail short circuit.

Regards
George
 
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.
 
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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Very difficult to see on that manual.

R164 feeds a 14 volt regulator (Point A) and from here a 5.5 volt regulator is fed from this rail (its output at point B). The 5.5v rail does feed the back up cap but via a 47 ohm resistor so I think we can discount any problem there.

It looks like you either have a short on the input to or output from the 14 volt reg.
 

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.047 caps are not usually polarized. You can check for short cap with dvm . On stuff over 20 years old usually all electrolytic caps are low value or failing, These are the polarized ones, minus marking on one lead, cupped plate on diagram. Mark pcb with sharpie before changing. If you like it, get all parts on one order. C133 &C134, 3300 @ 50 are pretty likely tired or ready to go, unless super quality (8000 hour caps). If you don't like doing stuff over in 5 years, buy 2000 hour up caps. Farnell.com has the hours in the selector chart. Mouser makes you download the datasheet. Panasonic makes 8000 hour caps, also 1000 hour ones, depends on the part number.
As far as Conrad Hoffman's opinion that every cap should be measured before changing, I have 3 broken meters with C function that lasted less than a year each. How much c******* made **** should I buy? Everybody else buys a new printer cartridge every 6 weeks, but I'm making do with a 10 year old typewriter ribbon on a Remington because consumer products are such **** these days.
As far as answering questions, I could watch soap operas, or go out in the sun and dig some more holes or cut more trees. This is more entertaining.
 
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Wow, that's pretty detailed! I should be able to get some time to look at it tonight. Hopefully, I can find an obvious short.

Do you guys ever get tired of questions from amateurs like me?

We luv it - and the pay is Greeeeeeat!!!! :rolleyes:

Actually - we love giving something back annnnnd it helps keeps us sharp. I always tried to encourage the "new" techs to ask any question about anything that was business related because sometimes those "newbie" questions could lead to a) a better understanding on their part b) a better understanding on my part c) an accidental discovery of something interesting. :D:D:D
 
Well if it was a .047 F, it was electrolytic and the minus has to be put in the minus hole. If backwards, they blow the relief seal and leak out, right before they short.
Could also be the 3300's were your problem all along, or another. Anything over 10 microfarad is probably electrolytic, and dried up after twenty years. 1,2,5,8 mf (uf these days) can also be electrolytic with water and seals, but nowadays people put film caps in in which is long life.
This year I've recapped a 1961 tube amp, a 1998 disco mixer (which had design problems also), a 1961 tube preamp. All sound great. Last year I recapped a 1968 organ (70 caps) and a late seventy'ish Readers Digest all band transistor radio (20 caps). (gets WNAS-FM from 15 miles away). Right now I'm recapping a 1964 organ and repairing a 1994 PV 1.3K power amp and recapping it as I go. Replaced caps are all electrolytics.
 
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