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#1
09-17-2010, 06:24 PM
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KLH 21 with issues...
A guy at work said a radio his folks owned all their life was having issues, so he let me have a crack at it. I've already replaced all the power supply capacitors, but it still fails to tune in any stations. The audio amp works perfectly, so something is up with the tuner. Are there any common faults with these? I can hear white noise through the speaker, but rotating the tuning knob dos nothing. Ideas?
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Evolution... 
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#2
09-17-2010, 09:49 PM
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The oscillator transistor in the front end of the tuner is most likely dead (open). I've seen at least 3 of these with this failure in the last 4 years here at my workplace. Its a germanium device and I believe there is an NTE/ECG equivalent if you're willing to look around awhile.
However, the tricky part is getting the sealed tuner apart and getting it into a position to where you can check the RF amplifiers while the set is running. See if you can get your hands on a schematic. It will help alot in your troubleshooting.
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Jordan
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#3
09-17-2010, 10:04 PM
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That was my first guess. To which transistor are you referring? I got the schematic from Phil's site, you can find it here: http://www.antiqueradio.org/art/KLHT...1Schematic.jpg
I obtained a new RF PNP transistor from my brother, in case one of them does end up being bad. And I already have that metal cover off, a 250-watt solder gun helps a lot with jobs like that. 
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#4
09-19-2010, 12:34 AM
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Looks like Q520 and 521 are your RF amplifiers. Make sure they have proper supply voltage. A failure in the RF section might severely hinder the sensitivity but you'd still be able to tune a station or pump one in via signal generator. Q522 is the oscillator. If its got 12 volts on the emitter, scope it out for 10.7 mhz. An FM signal generator is extremely helpful here.
If the front end is working, then check for 10.7mhz at the entrance to the IF strip, and see where it disappears. However, based on my experience with these sets the oscillator transistor seems to have the highest failure rate.
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Jordan
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#5
09-19-2010, 01:41 PM
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Measure the DC voltages on Q522. There should not be more than about 0.22 volts difference between the emitter and base. If so, the transistor is bad.
If in doubt, measure the base while moving the dial from end to end. If no change, the oscillator is not running. The collector will always measure 0 volts in that circuit since it;s grounded through the oscillator coil. If the RF or mixer transistor were bad, you would likely hear some strong, nearby station.
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