Marshall VS65R no -15V

[Psabin8951]

I have a Marshall valvestate VS65R combo here it was making lots of loud buzzing even with volume at zero. Caps have now been replaced and I now get clean sound from power amp but no sound through pre-amp. Caps around ZD1 and ZD2 have been replaced with matching parts and as board have clearly got hot there, ZD1 & 2 replaced with 5W 15V zeners. I'm seeing +15V across ZD2 but only -2.86V across ZD1
Any thoughts what will be keeping this low instead of -15V?

[J M Fahey]

Caps have now been replaced  .... Caps around ZD1 and ZD2 have been replaced  ..... I'm seeing +15V across ZD2 but only -2.86V across ZD1
Any thoughts what will be keeping this low instead of -15V?

Either that Zener or its matching electrolytic were inserted the wrong way.  Recheck that carefully.
That cap goes with *positive* to ground.
Please post results.