[Eric]

I have a Stromberg Carlson 430m I've recapped, and replaced all resistors that were out of spec. finaly got a replacement for the 6AF6 eye tube which was bad. Here is the schematic   
http://www.nostalgiaair.org/PagesByModel...021539.pdf

The radio plays fine but the eye doesn't operate when tuned to a strong station, I retested and it shows the open and closed position just fine on the tester. 
The voltage test I think has a misprint here is what I have
     printed                actual
 nc   1   90                  0 not used
 H   2   ----               -------
 G   3   ----               121
 G   4  220               147
 P   5  110               237
 x   6   52                   -----
 H  7  6.5                  6.3
 K    ----                   100 
1&6 are not used but the print shows 90v on pin 1 and 52v on 6

What I'm I missing?
Eric

[Mondial]

Yes, the voltages on the print are all messed up. The highest voltage should be on the plate pin 5, then control electrodes pin 4 and 3 are lower and finally the cathode pin 8 should be the lowest. Your measured voltages seem about right, although the control voltages on pins 4 and 3 are a bit high if you are measuring them with no station being received. 

The control voltage is developed across the screen grid resistor of the 6K7 IF tube. As a station is tuned in, the AVC voltage on the 6K7 control grid goes more negative, causing the screen current to be reduced. As the current falls the voltage at the screen rises, and since this voltage is connected to the 6AF6 control electrode, the eye closes.

Check the screen voltage on the 6K7 as you tune in a strong signal. If there is little or no variation in voltage, then that's why the eye is not operating properly.

[Eric]

Yes, the voltages on the print are all messed up. The highest voltage should be on the plate pin 5, then control electrodes pin 4 and 3 are lower and finally the cathode pin 8 should be the lowest. Your measured voltages seem about right, although the control voltages on pins 4 and 3 are a bit high if you are measuring them with no station being received. 

The control voltage is developed across the screen grid resistor of the 6K7 IF tube. As a station is tuned in, the AVC voltage on the 6K7 control grid goes more negative, causing the screen current to be reduced. As the current falls the voltage at the screen rises, and since this voltage is connected to the 6AF6 control electrode, the eye closes.

Check the screen voltage on the 6K7 as you tune in a strong signal. If there is little or no variation in voltage, then that's why the eye is not operating properly.

Here are the numbers on the 6K7
  P  3   253v
G2  4   129v
G3  5   3.7v
K    8   3.7v
did find an unsolderd connection in the eye tube plug pin5 re soldered but it made no difference
I get no variation in the eye or voltages when tuning from strong to no station in BC
On SW I do get some voltage and eye movement from 129v to about 160v 
R21,25 says should be .47meg I take this to be 470K am I correct?

[morzh]

Your phono switch does something that might disable the tube though then why it is only at BC......

[Eric]

Mondial,
came across this
http://www.antiqueradios.com/forums/view...=1&t=91953

Pretty much dicsribes what is going on?

[Mondial]

Eric, if the eye works on SW, then the 6K7 and eye tube circuit are working properly. The 6K7 screen voltage should rise as you tune in a station, which you confirmed when on SW. The whole system is dependent on developing enough AVC voltage to control the screen current of the 6K7. 

It appears that you are not generating enough AVC voltage on BC to control the 6K7. This could be due to a lack of gain somewhere in the BC antenna circuit, such as might be caused by an open coil winding, bandswitch contact, etc. Even though you are receiving signals on BC, there is not enough signal to activate the AVC, and thereby control the 6K7 and the eye tube.

Check the voltage on R1 to chassis ground, the 27 K resistor in the detector ( demod) circuit. There should be several volts of negative voltage there when receiving a strong signal on both BC and SW. 

[Eric]

Eric, if the eye works on SW, then the 6K7 and eye tube circuit are working properly. The 6K7 screen voltage should rise as you tune in a station, which you confirmed when on SW. The whole system is dependent on developing enough AVC voltage to control the screen current of the 6K7. 

It appears that you are not generating enough AVC voltage on BC to control the 6K7. This could be due to a lack of gain somewhere in the BC antenna circuit, such as might be caused by an open coil winding, bandswitch contact, etc. Even though you are receiving signals on BC, there is not enough signal to activate the AVC, and thereby control the 6K7 and the eye tube.

Check the voltage on R1 to chassis ground, the 27 K resistor in the detector ( demod) circuit. There should be several volts of negative voltage there when receiving a strong signal on both BC and SW. 

It reads 36k so I changed it I get about -14v doesn't make a difference where I tune

[Mondial]

What voltage do you get there on SW? There should be close to zero volts with no signal and several volts negative when tuned to a station, but is should be similar in BC and SW.

[Eric]

What voltage do you get there on SW? There should be close to zero volts with no signal and several volts negative when tuned to a station, but is should be similar in BC and SW.

    station   no signal
BC -.8          -.59
sw  -.75       -.59

eye shows no change except at the high end of the sw scale 18m.  1/2 the eye closes even though there is no station.

the two coils read 9.5 ohms

[Eric]

I think the problem is now alignment, I have gone through it once and I see an improvement in the eye opening and closing this has a lot of adj not sure I have done it right the first time so I will as time permits keep working on it.

eric 

[Mondial]

Seems like you are making progress. If the radio is out of alignment it will result in low sensitivity and an unresponsive eye. You can peak the adjustments for maximum negative AVC voltage at the 27K resistor, which will correspond to greatest deflection of the eye tube

[Eric]

Seems like you are making progress. If the radio is out of alignment it will result in low sensitivity and an unresponsive eye. You can peak the adjustments for maximum negative AVC voltage at the 27K resistor, which will correspond to greatest deflection of the eye tube

Thanks that is a great idea I now finished that and get pretty decent eye movement the only one that didn't seem to make a difference on the volt meter or in the tone of the signal gen was the wave trap step?

eric

[Mondial]

Glad to hear you got it aligned and working properly.

Don't worry about the wave trap adjustment. It was only used to prevent interference from high power communications transmitters, which no longer use those low frequencies. Most radios don't even have the trap at all.