Blue sealed volume pot. Drill hole at tip of arrow.Don't drill deep! Just deep enough to penetrate plastic.
Photos of what this amp looks like with the top cover on.http://hificollector.blogspot.com/search/label/Yaqin
About seven months into the Yaqin MC-10L EL34 ownership club, the volume control started spouting static, the likes of which I haven't heard since the initial power-up of my Sansui 5000x. Anyway, I realized that the only ways to resolve that would be to A) Find a replacement volume pot, or B) Spray some DeoxIT tuner cleaner inside the control. Problem was, the volume pot was sealed. Solution? Drill a tiny hole, just big enough for the DeoxIT spray tube to fit into the pot.
It worked! And now I have that hole accessible for the next time the static starts... it's inevitable.. it will start again. Always does with these things as you know.
Kinda dissappointed that Yaqin didn't put a higher-quality volume pot on this model, but hey - the sound is still sweet, and that's the only problem I've had after a year of ownership.
By the way - nice to know those large, green domes have something under them! These transformers are big, heavy and mean serious business. Check out these photos with the cover - http://hificollector.blogspot.com/search/label/Yaqin