Hi Jim

just had a look at my 'so-far-un-restored-840A'.
I has just one bulb, which is described as a pilot light, or as an indication
that the receiver is 'on'.
The is bulb is in series with the valve heaters, so if blown, the receiver would
not work. It may be that it has the wrong type has been fitted, such that it's
so dim, it can't be seen. It's situated top right of the tuning scale. There is
no other scale illumination.

My 840A is number six on the list of receivers to 'do', so it'll be awhile
before it has any voltage applied to it !

Good luck
Pete