I used the link and a few other of Andy's post for reference
www.worldwidedx.com
Ran a couple experiments on this chassis. After a tune up, I did the NPC with the resistor value being determined by using a 1k pot to find the value at maximum negative peaks before the positive peaks start to clamp. 250 ohms is what I found at somewhere around 190% positive peaks and the negative peaks were set at or near 95%. I ran out of AMC range at around 70% of VR5(AMC). Referring to Andy's post , I found that the stock values or R153 and R155 to be 3.3k and 1.5k respectively. At the minimum point of VR5's range , I measured 1.61v at the junction of R153/R155 and 1.82v at the maximum point of VR5. Using an online voltage divider, I found the source voltage to be 5.6v and the voltage divider to be at 1.75v. Using the same source voltage I found that if I use a 4.7k and a 2.4k values for the divider I get 1.893v. So right in the area where I need based on the increased modulation. After swapping R153(4.7k) and R155(2.4k) I ended up with 1.83v at the minimum and 2.02v at the maximum of VR5. At the minimum the AM envelope was at a little over 100% positive and 70% negative. The max point clamped the positives to around 220% and pinched the negatives, adjusting the AMC now gives me the ability to bring the positives to a rounded 210% and near 95-96% negatives. I will remove the NPC resistor and recheck with the 1k pot for the new value needed to bring the negatives to 98-100%
Uniden PC122, low AM modulation
Hi all, I received an unmodified PC122 that was still in the box, never used. SSB shows full 12 watts, but AM very little swing, almost unnoticable. I adjusted the AM carrier down to 2 watts and varied the AMC pot, but still almost no "swing". I found a couple mods online which might help...
Ran a couple experiments on this chassis. After a tune up, I did the NPC with the resistor value being determined by using a 1k pot to find the value at maximum negative peaks before the positive peaks start to clamp. 250 ohms is what I found at somewhere around 190% positive peaks and the negative peaks were set at or near 95%. I ran out of AMC range at around 70% of VR5(AMC). Referring to Andy's post , I found that the stock values or R153 and R155 to be 3.3k and 1.5k respectively. At the minimum point of VR5's range , I measured 1.61v at the junction of R153/R155 and 1.82v at the maximum point of VR5. Using an online voltage divider, I found the source voltage to be 5.6v and the voltage divider to be at 1.75v. Using the same source voltage I found that if I use a 4.7k and a 2.4k values for the divider I get 1.893v. So right in the area where I need based on the increased modulation. After swapping R153(4.7k) and R155(2.4k) I ended up with 1.83v at the minimum and 2.02v at the maximum of VR5. At the minimum the AM envelope was at a little over 100% positive and 70% negative. The max point clamped the positives to around 220% and pinched the negatives, adjusting the AMC now gives me the ability to bring the positives to a rounded 210% and near 95-96% negatives. I will remove the NPC resistor and recheck with the 1k pot for the new value needed to bring the negatives to 98-100%
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