any luck with this monitor yet ?
this looks to be a cheaper version of the Wei-Ya 429 chassis, these are both very close to the Wei-Ya 825HR & Wei-Ya 826H schematics
These are a very versatile chassis with only changing a few components
tube size selection is done via C909 0.01uf & C912 0.0047uf .. these are very wide & you may find the adjustments at the max
you can change C912 down to 0.0022uf to make dialling in a 29" easier.. & the 0.01 uf is best suited to a 27" tube.. about center of the H. Width potentiometer
So you now want to know what to use to suit any tube ?
C909 0.01uf & C912 0.0022uf will give best for 27" & 29" tube
C909 0.0082uf & C912 0.0022uf will give best for 25" & 27" tube
C909 0.0068uf & C912 0.0015uf will give best for 24" & 26" tube
C909 0.0068uf & C912 0.0022uf will give best for 23" & 25" tube
in short, you need about : 0.0072uf suit 23" , 0.0082uf suit 25" , 0.0104uf suit 27" & 0.0122uf suit 29"
Horizontal adjustment range is done via C908 0.33uf & C908A 0.1uf..
you then may need to change or trim C908 from it's 0.33uf.. so 0.47uf will give smaller picture & 0.22uf a larger picture, you want to aim for about center of the H. Width potentiometer
also trimming C912 either up or down from the values i gave above will a tighter tolerance on tube size..
P.S. i am yet to try these down to a 20" size tube.. but thinking C909 to 0.0047uf & C908 to 0.47uf will suit.. not sure.. but will up-date..