I suspect you are getting too much signal from the bridge. Connect a 10k resistor across the test terminals, set the range to 10k and monitor the signal at the input of the amplifier as you swing the balance pot from one extreme to the other. You should see a dip in the amplitude around the null point. This would show if the bridge is OK. Next monitor the collector of TR2 and check the amplitude changes. If it does check the voltage at the collector of TR3. This should vary considerably rising to around 8V at the null point. If it doesn't vary it may be worth adding a sensitivity control.
I think one reason why the DM70 is in good condition is the lack of use after all these are not turned on for hours at a time.
Keith
I think one reason why the DM70 is in good condition is the lack of use after all these are not turned on for hours at a time.
Keith
