at my wits end - Videokarma.org TV - Video

radio nut · #1 01-03-2012, 10:45 AM

My Capehart by Farnsworth 21n2 is been an interesting project. It seems to be one problem after another. After fully recapping it I had a distortion problem and found two bad mica caps in the treble adjustment. Then I would lose the high freq. fm stations and found an original solder joint that was bad in the osc. circuit. I still have a problem with the tuning eye.It is a dual sided eye tube(6af6). both sides are balanced on AM but not on FM. The diagram says to pull the last IF tube and adjust a variable resistor to set eye balance for the FM. I tried that and while it does make a difference it will not balance the two sides. The difference between AM and FM on that tube is where it gets its power from. The FM signal is routed differently and I have checked ALL resistors and caps around that tube and am lost. I got the diagram from nostalgiaair. Would an expert please look at that diagram and point me in the right direction.

Reece · #2 01-03-2012, 12:26 PM

I'm no expert on this radio but looking at the schematic: when you adjust the eye-balancing rheostat and get it the best you can is it at either extreme of its range or somewhere in the middle?

radio nut · #3 01-03-2012, 12:55 PM

Its at one extreme.........which one I do not remember. Even then as I am looking at the set from the front the right side stays closed. The left side is changed on the adjustment. On AM though both sides are equal and very active. When playing FM the right side stays closed and then overlaps when the staion is tuned in correctly. I replaced any out of tolerance resistors in the power supply chassis and the radio chassis already and all tubes are known good. The only thing that I can think of would be a resistor or cap thats bad that I am not seeing.

radio nut · #4 01-03-2012, 05:30 PM

I really need to do voltage checks since the diagram lists them on a seperate page but my auto -ranging digital voltmeter gets confused if there is AC and DC on the same line.

Reece · #5 01-04-2012, 07:07 AM

Be a good idea to use an analog meter on this. Yes, there may be a bad component somewhere (are there any caps or resistors hiding inside IF cans? The schematic would show.) Where I was going with the setting of the rheostat: if you're at maximum resistance and could use more, you could put a resistor in series with the 'stat. If you're at minimum resistance with the best "eye" setting, out of luck on this tack. Could be the eye tube is not within spec but it's what you have to work with.

radio nut · #6 01-04-2012, 04:16 PM

I bought an analog meter today and will start checking voltages hopefully tomorrow. Since the eyetube is balanced on AM stations I am feeling confident that the tube is within specs. I think since the right side of the tube pretty much stays closed on FM that it is being fed too much power and that the voltage check should lead me in the right direction fairly quick. When trying to balance the eye the right side is already closed and maybe that is the real issue. I HOPE!!!!

Einar72 · #7 01-05-2012, 09:44 AM

You might want to start by looking at the input to the 6SN7 driving the tube when on FM. Note the 2.2 Meg resistor #17 taking voltage off the discriminator output. Also the 2.2 Meg coming off the disc. tube going to the AGC line and with a .05 cap to ground. The small 'lytic in the disc. circuit is almost certainly needing replacement as well.

Where i"m going with this is you may have insufficient or unbalanced output from the discriminator to give the right input to the eye-tube circuit. Measure the voltage on that first resistor I mentioned, and you might find the discriminator is way out of whack and producing a large voltage which will knock the eye tube off. Think of the eye-tube being a center-tune meter - the expected voltage should be zero if tuned in properly, IMHO.

radio nut · #8 01-05-2012, 01:53 PM

Ok thanks, I will end up checking this out this weekend...... no time til then . Oh, the small lytic in the disc. circuit has been replaced as well as the .05 cap. unless I missed one ,the resistors in this set are within tolerance. If not I replaced them. If the disc. is way off adjustment is there any way to calibrate it without a scope. Long story short I do not have one. I can borrow one but Its from a friend who lives outside of my town.

radio nut · #9 01-08-2012, 10:53 AM

Well, I have through this set MANY times and well I missed one resistor behind some wires and sure enough its was one of the 2.2 meg resistors that was pointed out. The eye is working correctly now. voltage checks that were suggested did point that direction also. Thanks for the help and I will post pictures soon!