Proton D1200 Question

[1965GTO]

I need to change the voltage output on connections I and J from +/-22v to +/-18v. I am having to change out the old Mitsubishi M5219L to a new OnSemi NE5532ANG. Suggested working voltage for the OnSemi device is max 20v. So in the power circuit I see there are a couple of Zener Diodes, ZD901 and ZD902, rated at 22v. Is it as simple as changing the Zener Diodes to 18v ones? I am assuming there needs to be a change in R905 and R906 value as well. Any help in providing values and solving my problem is deeply appreciated. Attached is a Jpeg of the Power Supply Schematic.

[mjf]

hello.
volt drop across the base of bc546(bc556) is around 0,6v...........so you should measure 22,6v at the base.
volt drop across r903/r904(39k) is 97v-22,6v = 74,4v
and current through r903.......74,4v:39k=1,9ma.
current through the 22v z-diodes should be around 4.......5ma (bc639/bc640 ignored)
and the bc546b(bc556b) have a current gain(hfe) of 250 typically..........so only a tiny current of 4ma:250=0,016ma flows into the base of this transistors.
if you change the 22v zd to a 18v zd........the factor is 22:18=1.22.
so r905(r906) should be around 12k:1,22=9,8k.........so 9,1k or 10k should fit.......this is only estimated,not exactly calculated!
and if you like you can increase r907(r908) to 15k x1,22=18k.
it is late night here and i am listening to "the proms/bbc".........hope i am not totally wrong.
greetings

[mjf]

........and perhaps r919(r920) increased to 2,4k...........2,7k.
annother idea is to connect z diodes (3,9v......4,7v/1,3w) in series with the outputs i,j..........
greets