Kuba / Carmen Am Tuner


  #1  
Old 01-27-03, 10:07 AM
fireweedfour
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Kuba / Carmen Am Tuner

I have a old AM FM SW Turntable floor model stereo that has been passed down through the family. The AM tuner doesn't work at all but the FM does just fine. They are two seperate tuners and I was hoping that I could figure out what would be the best way to trouble shoot and replace the supposedly defective parts.

This is a Western German product.

Kuba Carmen - Stereo Radiogram

Nur Fur Wechselstrom 220V -50 HZ

Leistungsaufnahme ca.55W

Sicherung" 0,3 A trage

Vor Feuchfighkeit Schutzen!

Export Chasis 664

Any and all help is appreciated.

Rob

 
  #2  
Old 01-29-03, 01:25 PM
Teratum
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AM receiver is not hard to diagnose, given you have already known that the rest of the radio works.

1) check Vcc power bus in AM tuner section if it receives proper voltage. If you don't have schematic, notice the rating voltage of the electrolytic caps on the Vcc bus. Your tuner Vcc voltage should be close to the value but lower says 10-15%. Example cap rating 16V , your Vcc should be 12V.

2) Notice the 3 IF freq tuning cans, each should have diff color on top on the screws, like yellow, white and black. I don't remember the actual order. Look for a detector diode in the last IF amplify state and measure the if you have any AC signal at the anode (+) pin of the diode. If you do, you may have only wiring problem between this AM detector and your band selector sw.

3) If you don't go back one If state and measure the dc voltage of the IF transistor one at a time. If you have a scope will make it easier to see if you have signal at their collector.

4) Keep going back one state at a time if you don't get signal and look for abnormal voltages at transistor pins that can identify if you have a short between B-C or B-E. Typically, you should have a diode drop between B-E and your B and C should not have the same voltage which means it shorts.

5) If you still can not find problem, you will then need the check the local oscillator if it oscillates properly. Identify the oscillator transistor which should sit near some adjustable coil or the gang capacitor that uses for tuning the stations, measure if its DC voltages are OK. If you have a scope, you can check if there is any local oscillation.

6) Now the hardest part, your front end of the AM. Check transistors the same way then check the ferrite core antenna. Pay attention to the crack and those little wires coming out of the ferrite core. If the core is cracked, or one of the wires is broken, you will not receive anything.

If you radio is an antique, it's worth to keep it to its original working condition.

Good Luck.
 
  #3  
Old 01-29-03, 04:17 PM
fireweedfour
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Kuba Carmen

Teratum;

Well, I did ask didn't I?

OK this may be a little beyond my experience but I'm definetly willing to take a look. I don't have a scope but I do have a meter.

Please forgive my ignorance in any of my questions as I haven't attempted this type of repair before.

I will be attempting to do this troubleshooting with the chasis semi removed from the stereo as I do not have a test bench or equipment.

I have noticed that the tube that is on top of the AM tuner is glowing. I understand that this doesn't necessarily mean that it is working correctly but it may be an indicator.

Thanks for your help. I will check as best I can what you have suggested. I believe that it is a antique as far as stereos go. It is at least as old as me !

Rob
 
  #4  
Old 01-29-03, 11:52 PM
Teratum
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Did you say tube? Can you confirm is your radio tube or transistor?

If your set is still using Vacuum tubes, what I explained to you earlier might not work because it applies to transistor AM radio. however the theory of operation for AM radio is still the same no matter tube or transistor. But to fix it, you may need someone who are much older than me to give advice of how to look for the test points that I mentioned earlier, e.g. detector, IF amps, oscillator etc. so you can measure with the voltmeter.

Tube growing or not growing can be used as indicator that it is working properly or not. Any tube that its filament does not grow, it means the filament is broken so it will not be able to emit electrons to its anode or plate. You should replace it and see if it fix the problem.

There is another type of grow is the growing of the plate or anode that indicates excessive electrons hitting the plate. It indicates something wrong witht he circuit around that tube.
(Filament is at the bottom of the tube, the plate is on the top part of the tube. This is how you differentiate them. You can visually observe and use them as you problem indicator)

One word of caution about vacuum tube. Tubes operate at much higher voltage than transistor, so be extra careful not to touch anything. It's not only the heat that can burn skins but also the electric shocks.
 
  #5  
Old 01-30-03, 03:06 PM
fireweedfour
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Teratum;

Yes this unit is tube.

I understand that the unit is higher voltage than transitor. I also understand that the tubes can hold a charge even after the stereo has been turned off.

Thanks for your help, I will keep trying...

Rob
 
  #6  
Old 01-30-03, 08:25 PM
Gary Tait
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In the case of signal tubes, as in your tuner they don't hold a charge, it's cathode ray tubes that do. Don't turn off your fear
yet though, as the power supply capacitors can still hold a
possibly lethal, if not painful charge.
 
 

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