[Mondial]

Looking at the schematic, the screen grids of the RF, Det/Osc and IF are all connected together so the voltages on them should be identical. In itself, the voltage reading of 75 to 80 V is not too significant as long as all three are the same. This voltage is somewhat dependent on the grid bias of the tubes, so if you are not receiving a signal, the screen voltage may be a bit lower than 90 v. With a signal, an AVC voltage is generated which increases the negative grid bias in the tubes, which in turn will raise the screen voltage.

In any case, with a 75 to 80 v screen voltage you should still be receiving signals, so you may have to look elsewhere for the problem. You might check and see if the oscillator is operating by tuning a nearby AM radio to the receive freq plus the IF of 260 KHz and see if you can hear a carrier (for example, with 71B tuned to 1000 KHz, you should hear the osc at 1260 on a nearby radio).

If there are no miswiring errors then a check for open coils is probably in order.