I measured the trimpot and it is set at around 2.18K. I just noticed that the original R52 that I removed was indeed 10K 5%. Odd. I really don't understand what was happening when I replaced it with 1K, 3.92K and another 10K (!). But after 3 days trying to fix it, now that it is working correctly, I think I'll stop messing with it and actually spend some time playing it!
Best regards.