Realton 'Variophon Standard' Mains Assembly
At least the 'Variophon Standard' bears lethal electrical risks and must not be operated without modifications or safety precautions as described next:
As can be seen in the pictures above, the mains voltage is routet to the mains fuses and switch by the same low insulation/diameter flat ribbon cable. Even if the flat ribbon cable may be rated 300V-RMS (in the US) this is not sufficient for 230V mains voltage interconnections; even worse is the parallel routing of low-voltage signals within the same cable - seems the Realton Company had never heard of the appropriate safety-regulations.
There is a high risk of shorts and mains voltage access to e.g. the metal control-panel in case of overvoltage or insulation failure. There are dedicated mains-cables availabe (even in 1980) which prevent those issues and comply to the regulations.
What can be done? When no modification to the instrument shall be made, a separate sufficient mains insulating-transformer must be used. It is connected between the mains and the Variophon's mains-connector and prevents current flow from e.g. the Variophon's metal control-panel to the user in case of failure.
If the Instrument can be modified, e.g. because the heavy and bulky external insulation-transformer shall not be used, the mains routing has to be separated from the low-voltage cabling. This can be easily done by disconnecting the flat ribbon-cabling from the mains connector/fuses/switch (including removal of mains traces of the printed circuit board) and re-wiring the connections using dedicated mains-cables, as shown in the pictures below. In addition, a mains filter (incorporated in the IEC-Jack) was added. A new fuse was mounted near the mains filter directly to the lower housing.