Intresting, the CTC-16 does not have the VDR, but the 16XL & CTC-20 does.
https://www.ebay.com/itm/395361002948
This part, along with a change of the size of the 1w resistor, 150k vs 120k (82k), shown on SAMS.
If you have the CTC-16, not sure how to get it more stable. :/