01-19-2011, 12:23 PM
Update - I have the second IF transformer (Z3) working now. I found that 120pf capacitors resonated at 455KHz with the slugs in the middle of the their adjustment range!
I don't have a frequency counter, but I do have several digital radios. I adjust my signal generator around 455KHz until the second harmonic shows up at 910KHz on my digital radio (Grundig G5).
I pre-aligned the IF can prior to installing, taking time to mark which 120pf capacitor goes to what side of the IF transformer. The clip that holds the IF can in place prevents installing the IF can with the 120pf caps soldered in place. I used super glue to stabilize the solder pins of the IF can. Super glue was unfazed by repeated heating from soldering the pins. I tested the radio to make sure my work was ok before proceeding with repairs to Z1 and Z2. I found a noticeable improvement in sensitivity!
Thank you 7estatdef for your suggestions. I tried a NOS 7A8 but it had no effect on the tracking/drop in sensitivity around 900 to 1200KHz. Once I have the mica caps replaced in Z1 and Z2, I will go through all the alignment steps again.
I assume the coupling capacitor shown in Z1 is not a critical value. If the Z1 cap is leaky, it could be affecting the bias on pin 6 of the 7A8 converter. Anyone know what a typical voltage should be on pin 6? For those interested here is a link to the 50-922 schematic
I don't have a frequency counter, but I do have several digital radios. I adjust my signal generator around 455KHz until the second harmonic shows up at 910KHz on my digital radio (Grundig G5).
I pre-aligned the IF can prior to installing, taking time to mark which 120pf capacitor goes to what side of the IF transformer. The clip that holds the IF can in place prevents installing the IF can with the 120pf caps soldered in place. I used super glue to stabilize the solder pins of the IF can. Super glue was unfazed by repeated heating from soldering the pins. I tested the radio to make sure my work was ok before proceeding with repairs to Z1 and Z2. I found a noticeable improvement in sensitivity!
Thank you 7estatdef for your suggestions. I tried a NOS 7A8 but it had no effect on the tracking/drop in sensitivity around 900 to 1200KHz. Once I have the mica caps replaced in Z1 and Z2, I will go through all the alignment steps again.
I assume the coupling capacitor shown in Z1 is not a critical value. If the Z1 cap is leaky, it could be affecting the bias on pin 6 of the 7A8 converter. Anyone know what a typical voltage should be on pin 6? For those interested here is a link to the 50-922 schematic
