I am rebuilding this radio that was my grandparents and have a question on capacitor #9. Its a .25MFD with the coil wrapped around the one end.
Pretty sure its one of those 455KC traps and was wondering if I left it out would it make any difference in the performance? Its ref# 9 in the schematic but I can't link to the Nostalgia site since they show the wrong page for it. Its actually in Riders 12-48.
Hi Jim
Most likely you will be fine with a new 0.22 uF cap, without the coil. However, should you have any issues, it is easy enough to wind some 20 or 22 gauge wire around the new cap, replicating the direction and number of turns of wire on the original.
Thanks Ron. Maybe I'll just restuff it and keep the original winding on the shell.
That's a good idea. I've done that before. I've also wound new wire over the new cap, without using the old shell, and it worked fine when installed.
Comes up once a month or so. Used to be marine and other beacons around IF frequency. Shielding no longer needed, but does no harm.
I've decided not to restuff the old capacitor since all the others have been replaced with newer ones. It would just look out of place. I'll just replace the .25UF with a .22 without the coil and call it a day. Have to resleeve the speaker and field coil leads first.
"Comes up once a month or so. Used to be marine and other beacons around IF frequency. Shielding no longer needed, but does no harm."
That answer also comes up every time
and is not accurate. The wire coil added to the cap was an effort to 'retune' it closer to the IF frequency for improved bypassing characteristics - usually between B-minus and chassis. The goal was to prevent undesirable feedback caused by floating 455kc on the B-minus line and the series-resonant circuit accomplished that more efficiently than the cap alone.
The old paper cap had a significant amount of self-inductance due to its construction. The new caps don't have this problem and don't need the remedy.
-Bill
I stand corrected, Xray, I have been hearing the obsolete beacon interference line for many yearsalong with the "special 3 legger cap" Agreed, no need for a winding or wingding with a modern cap.
Got everything back together and want to check the alignment. Where is the AVC line in this radio? Off the bottom side of of the 2.2M resistor #27 or the 4.7M resistor #34?
The AVC line starts at the bottom of resistor (27) and runs to the secondary of the first IF transformer, and to the secondary of the untuned interstage RF coil (10). A separate AVC line goes from resistor (31) to the RF tube's suppressor grid, and to the RF tube's control grid through antenna coil (1) and loop antenna (3).
The bottom of resistor (34) goes to B- and to one side of the
AC (
not AVC) line - don't put your fingers on that one!
Thanks Ron. Which one would you use for an alignment though, #27 or #31?
I would use (27). (31) feeds delayed AGC to the RF amplifier (only).
I'll try that Ron, thanks.
Well, the alignment is not going well. I am getting stations, although some not in the correct spot and the lower stations have motorboating in them. I cannot get an IF signal thru this thing at all and there is no AVC action that I can see. No IF signal even injecting at the 7B7 grid. All paper caps and OOT resistors have been replaced. Tube pins and sockets have been cleaned. The cracked rubber wiring was replaced and routed as close as possible to the original layout. So what am I missing here?
Playing around with this radio some more I have finally got some AVC action although still no audible IF tone thru to the speaker. Reading about -6 volts using the high output of my signal generator connected to the control grid of the 7A8. Decreasing the output does not really change the AVC voltage much, maybe around 1 volt at best.