zx spectrum 48k low -5volt rail

i have just received a 48k issue 3. initial tests show that all voltages are there but the -5V rail is only showing -4V. it does boot but i have turned off quick as to not incur any further possible damage to lower ram.i have not experienced this before, normally its either correct or not present at all.. i will start by replacing the electrolytic caps .TR4 and TR5 seem ok .. i dont suspect the coil as the -12 is correct and +5 seems good. is it worth changing the zener diode? maybe bad lower ram chip holding it down ?

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