Thank you for sharing the pictures.
I assigned some labels to a few components that may help you test and perhaps fix your unit. Being able to change a light bulb is a step in the right direction (we all started there at some point in our lives as far as learning electrical goes) and perhaps you can find a friend or relative that may be able to help you with measuring a couple of resistance values. From what I am seeing the coil test portion of the original schematic appears to be similar to your unit but cannot say for sure. I am thinking that the shunt resistor R7 is the large resistor connected to the two terminals on the function switch as I have labelled it but cannot say for sure which is why there are the question marks. From what I can tell the colour bands are blue, grey or white and black which would indicate a value of 68 or 69 ohms of resistance. If you have a multimeter you could check the value of it – make sure the power supply is disconnected and set the selector switch to the off position so the meter is not part of the circuit. You should get a reading of 64 to 74 ohms across the two resistor leads. If it is less than the lower reading then it could be the reason the meter is low since too much current would be allowed to flow through it and not enough will go through the meter coil. Looks like the capacitor C1 should have a value of 0.15 uF 400 volts DC. You could also check the value of it the same way as testing a condenser if you put a piece of paper in between the contacts on the point set and then measuring across the leads of the condenser. I would suggest using a separate capacitor tester to do this and again have the power supply disconnected and set the selector switch to the off position. If the capacitor is partially shorted it could also make the meter read low as well as not accurately perform a coil test – given the age of the unit that is a possibility. There could also be a problem with resistance caused by tarnish or grease at one of the selector switch contacts going to the meter when the selector switch is set in the coil test position. These are the best suggestions I can offer to you – I hope they help.
