Thorne --
1. With the unit turned off, temporarily connect two 47K resistors (wattage uncritical, but low wattage works best. should be close in value) in series (one lead of one resistor to one lead of other resistor). Now, connect the remaining free lead of one resistor to Test Point #3, and the other resistor's free lead to ground.
2. Using the schematic, determine where test point #4 is inside the set. It will be in the same general area as Test Point #3. Once Test Point #4 is found, for now, just note where it is.
3. Turn the unit on, and let your 500C fully warm up -- 20 minutes minimum. Set for FM Mono. Carefully tune in a notably weak station for precisely maximum signal strength. Once achieved, DO NOT touch the tuning knob again. Leave the unit running like this throughout the entire adjustment process. Do not have any florescent lamps or even nearby LED lamps on, as these all emit gross amounts of RFI noise.
4. Adjust in this order Z1, Z2, Z3 top and bottom and then Z4 (only one slug) for maximum signal strength. If the signal becomes very strong at all, wad up the antenna to reduce the signal strength reading. You want all the "IF" stages operating at maximum amplification, which only happens on weak signals. If the signal becomes too strong, then the adjustments won't have nearly as much impact on the signal strength reading. This is why it is important to start with, and maintain using an appropriately weak signal throughout the adjustment process. When the signal is appropriately weak, then each adjustment should produce a very definite and defined peak in the signal strength reading. If any one adjustment does not produce much change, or the peak produced is very broad in nature, that is typically indicating a problem in that IF transformer.
5. Now connect your DC voltmeter red lead to Test Point 3 and black lead to chassis ground as you did once before. Reading will be negative. Adjust the BOTTOM of Z5 for maximum reading on your voltmeter. NOTE: With the voltmeter so connected, you could use it to adjust the slugs in Z1 - Z4 as well (always adjusting for maximum reading), but I have often found it is easier to adjust for a peak with an analog meter like the signal strength meter where possible. Your DVM reading will also indicate a peak as well, but the reading will not be absolutely stable, making it tougher to discern.
6. With Z1 - Z4 done and the bottom of Z5 done, now disconnect the meter leads, and carefully connect the red meter lead to Test Point #4 found earlier, and the black meter lead to the junction of the two 47K resistors installed earlier. Adjust the TOP of Z5 for a zero (or nearly zero) indication.
Your done. Now turn the set off, disconnect your meter leads, and remove the two 47K resistors temporarily connected earlier.
Let us know how it goes!
Dave