NC-101X Issues

Obtained a nice NC-101X and for the past couple weeks, I've been restoring it, including the power supply, S-meter circuit, and recently temperature compensation to stabilize drift but I have two nagging issues remaning. Feedback most welcome.

My temperature compensation caps arrived on Saturday and after some trial and error, found that the Collins 10 pF/N2200 coefficient [uncased silver mica] stabilizes the '101X to within half a dial division on 40m after a 5-minute warm-up period. Adding the cap is much easier in the '101X than the HRO. All that's needed is to attach the cap across the rotor and stator of the variable HF Osc tuning cap section, located on the main tuning mechanism. Within the first five minutes, the 101X drifts quite a bit, then the shift slows significantly. After five minutes, the '101X's power supply is already generating heat and the N2200 cap takes over. What started with 10+ dial divisions of drift is now less than a single division.

I also Zener-regulated the HF Osc plate and screen supplies to +200V but the result is less dramatic than it was in the HRO although the two circuits are nearly identical. The 101X's frequency is still too sensitive to changes in RF Gain on the high bands. This is odd since the mod worked superbly in the E-76 HRO and apart from the 101X using a 6K7 instead of a 57, the circuit is the same.

The HF Osc is coupled to the first mixer through a 0.01 uF cap. I then drew out a Thevanin equivalent circuit and concluded that the terminating Z at the grid of the mixer is 130K-ohm [(100K || 50K) + 100K]. Running the math, the coupling cap only needs to be about 1 pF and still retain sufficient injection on 160m. I had hoped that slightly unloading the HF Osc would do the trick, but it didn't. Still, to prove the math, I installed a 5pF in place of the 0.01 uF coupling cap, and the injection is the same. Why National chose such high coupling values is a mystery to me. On the E-76 HRO, it's 0.1 uF -- again, way more than is needed. With the HF Osc. Zener-regulated and lightly coupled to the mixer, something else is at work here. There are no other connections to this free-running oscillator. I'm going to put a frequency counter on the HF Osc output, vary RF Gain and see what happens.

Second issue is that on 20m, CW signals sound raspy. The degree of raspiness changes depending on the tubes. I rotated though several within the set and it has gone from really bad to still noticeable on 20m. 160, 80, and 40m are all fine. I have a stash of 6J7 and 6K7 tubes en route. Power supply caps were changed but I'm also going to add local filtering and decoupling to the B+ and B- bus and note any changes. Still why 20m, and a bit of raspiness on 10m? I pulled the catacomb modules for those bands, inspected, tightened hardware, then re-installed back into the catacomb.

Anyway, the good news is that the 101X is now frequency stable. I'll keep at this until the last two bugs are worked out!

Paul, W9AC