We take a look at a Marantz CD4000 with a very dim display. Dim as in WAY too dim to read. If you jam your face up against the screen and switch the lights off you get something so it is ‘working’ but it is impossible to use. So what is to be done? These displays are based on the VFD technology was very popular in audio and video hardware back in the day. It has fallen out of favour in recent years with lower power and cheap LCD/LED display alternatives that are much more flexible. These replacement displays have the advantage of being non-specific, they can be defined entirely in software. The VFDs are fixed for a specific purpose and that is generally very limited, i.e. can be used for 1 thing only. For example here is a VCR device:
You can see that is will work only for a single role.
The Marantz CD4000 is a highly integrated device. There is mostly empty space within the case but that is typical for the era. Not a bad thing not good either. Sometimes it makes repair harder.
Referring to the service manual found online here is a circuit of the VFD:
F11/F12 and F21/F22 are the heater connections. They provide the bias that drives the display. Typically these are 3.3V dc.
Over time the cathodes get caked and degrade. There is a potential to increase the voltage across the terminals on a permanent basis and that can be possible by modifying the circuit.
There is some evidence it is possible to hot-wire the VFD and burn off some of the degradation on a temporary basis. Using a 9V battery to do this means that the volage is 3 times the normal rating but that would be for only a few seconds. The intention is to do this for a very limited amount of time.
Seems like when this is done it does have a positive impact and restore the brightness a little bit.
Here is a video of me doing this