08-03-2025, 05:37 PM
Hi MrFixr55!
Yep, I figured out that there were two AVC lines...finally. Every single resistor (except for 61) and capacitor (except for the mica caps) has been replaced. So, the AVC line filters should be good to go. The time constant on those filters is pretty small (~.1 second) so the AVC response should be fairly quick. Now, I did note that the schematic shows that on AVC line C the filter capacitor is connected after the resistor and this is the way it was wired initially. The schematic shows the filter capacitor on AVC line D is connected before the resistor. I thought this was weird, but I replaced the resistor and capacitor as they were initially wired. It's possible that this may be the issue. I probably should revisit that wiring to see how that AVC filter is physically wired. I think the capacitor should be after the resistor. Or is there something about the AVC D line filtering that is different?
I did check resistor 61 and it is well within tolerance. I did a general check on the negative bias on the AVC lines while tuned to a dead area on the band and it looks okay. But I did not check it while tuning the very strong stations (probably should have). Maybe for another day! I did clean up all of the tube pins and socket contacts so I think we're good there. I did note that there is one 47K resistor that is part of the 3rd IF xformer. Unfortunately, I did not check the value to make sure it was still okay.
I did a quick check with my ZTO, recently restored, and run into the same issue with the strong stations though it's not nearly as bad. So, I may do well checking the AVC bias on both lines. However, checking voltages in the chassis is a bit of a pain. The radio has to be connected to the internal antenna. When I extend the antenna connections, I notice a marked change just moving my hands around and keeping them short, there's not a lot of room to flip the chassis on its side.
I did some searching around the Phorum and noted that there is a section non Philco radios. I will try to post some info when I start the Zenith 12h650. I have my Sky Rover (made by Belmont) out of the case. So, I may post some info on that as well. I have not yet checked on Old Ron's Bar and Grill!
Thanks so much for your help and guidance!
Rich
Yep, I figured out that there were two AVC lines...finally. Every single resistor (except for 61) and capacitor (except for the mica caps) has been replaced. So, the AVC line filters should be good to go. The time constant on those filters is pretty small (~.1 second) so the AVC response should be fairly quick. Now, I did note that the schematic shows that on AVC line C the filter capacitor is connected after the resistor and this is the way it was wired initially. The schematic shows the filter capacitor on AVC line D is connected before the resistor. I thought this was weird, but I replaced the resistor and capacitor as they were initially wired. It's possible that this may be the issue. I probably should revisit that wiring to see how that AVC filter is physically wired. I think the capacitor should be after the resistor. Or is there something about the AVC D line filtering that is different?
I did check resistor 61 and it is well within tolerance. I did a general check on the negative bias on the AVC lines while tuned to a dead area on the band and it looks okay. But I did not check it while tuning the very strong stations (probably should have). Maybe for another day! I did clean up all of the tube pins and socket contacts so I think we're good there. I did note that there is one 47K resistor that is part of the 3rd IF xformer. Unfortunately, I did not check the value to make sure it was still okay.
I did a quick check with my ZTO, recently restored, and run into the same issue with the strong stations though it's not nearly as bad. So, I may do well checking the AVC bias on both lines. However, checking voltages in the chassis is a bit of a pain. The radio has to be connected to the internal antenna. When I extend the antenna connections, I notice a marked change just moving my hands around and keeping them short, there's not a lot of room to flip the chassis on its side.
I did some searching around the Phorum and noted that there is a section non Philco radios. I will try to post some info when I start the Zenith 12h650. I have my Sky Rover (made by Belmont) out of the case. So, I may post some info on that as well. I have not yet checked on Old Ron's Bar and Grill!
Thanks so much for your help and guidance!
Rich
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