Two threads going with same topic.
https://pinside.com/pinball/forum/topic/first-restoration-gottlieb-volcano#post-5988944

R10 is a 680 ohm, 1%. 1% not needed but tighter tolerance won't hurt anything. Appears to be 1/4W resistor but will only be dissipating about (4*4)/680 = 0.023 Watts. To me - the color code says the part is actually 690 ohms which is a common value for 1% values. Blue, white, black, black, brown. But.. I suppose the white could be gray. Either way - 680, 690, AC, DC, that one just doesn't matter.

C2 is a non-polarized audio capacitor. Considering where it is used - not a great choice but will work.

Need to fix solder joints at E3 and E4. This very well could be the source of the 9.9V versus 8V.
What part number is installed at CR7? Check part numbers on it as it may have been substituted.
Good solder joints at both anode and cathode of CR7?
If the diode is the right part - you should be measuring 7.79V to 8.61V.

HV is bad - still need voltage measured at E2. Voltage at E2 is what sets the output voltage and gives us a starting point on where to start looking.

Fuse F7 - is that your GI fuse? If so, that doesn't go to the power supply. You may have a shorted lamp or lamp socket somewhere. Did you bump a wire to a lamp socket within the light box?

High voltages are now within tolerances.

But -

8V supply still needs to be fixed - there is definitely something wrong with it. There are *only two* parts to that supply. One or both has an issue.

See ---
https://pinside.com/pinball/forum/topic/first-restoration-gottlieb-volcano#post-5988944
Look at post #14, photo #2.

CR7 -- in that photo, that should be 1N4738A or equivalent.
Yet when I look at the photo - I can make out the ending "0A".
If that is a 1N4740A then that explains why he sees 9.92V.
To make it right - replace CR7 with a 1N4738A.

You can reuse the 1N3445 but in many cases, this diode is pretty badly burned and is hard to find.
If you replace the resistor then you can use the more common 1N4738A.

This display offset voltage is a very low current. How low of current? I never measured. I am truly curious and plan to measure this in my System 80A machine... someday.

Gottlieb used the same basic displays and voltage regulation with the System 1.
The system 1 used 1N4738A/100 ohm resistor pair for the voltage regulation and it still appeared to be more than required.
The System 80 machines used the same displays, the 80A games used 7 digit displays but the current increase was negligible.
I think the original designers had no idea as to how much current would be required so they bumped it up to the max.

Minimum current required must meet the minimum knee current which is about 1mA. That part is easy.
Maximum current is determined by the resistor. If resistor value is too high, the resistor becomes the dominant factor in setting the voltage. I normally use 470 to 680 ohms. This resistor value is low enough the supply the minimum current for the zener and high enough to keep the resistor/zener cool. There is a max value to the resistor but I haven't hit it yet. The max value for the resistor is where the load current through the resistor becomes the driving factor in determining the voltage drop.

This is indicative of a bad connection. Probably a bad solder joint, often at the legs of the power transistor on the hot plate.

I would look to others before the RD power. Those have several posts where the wrong parts were chosen/used on the boards resulting in burned parts.
Gulf most closely 'followed' my design, I would look into those.