Vr1 and vr2 are usually blue in later boxes black and in the centre there is a slot for adjustment. They should have 2 legs in line and the middle 1 slightly off set a little so not quite in line. They are different values one of them is 100 ohms and should have two sets of numbers on it, one of the set should read 100.
The other one will have 1m this means 1 ohm, So its 100 times less resistance.
I bought 2 of each off Mallinson Electrical for less than £2.
Cheers Arry