I didn't know about the official Fluke battery mod, but it looks like they have done something similar. They have a couple of resistors to boost the voltage from 1.25V to 1.35V to match the mercury battery, but it is not really necessary.
If the NiCd batteries are dead, I am not sure how you can test any of the units. The NiCd batteries are the voltage regulators for the unit, and so if the battery packs are dead, the supplies will be all wrong or at least noisy. I never try and use the 540B while it is plugged into the mains myself.
The key thing is that the sensor must be OK - if it is blown, the unit will be worthless. It is probably OK to swap the A54 modules, but I have no experience in that.
Here is my guide to checking the sensor.
Get two multimeters, including one with a decent mV range that can read to 0.1mV.
Find the ohms range on the other multimeter that outputs the most current. In some multimeters, the Diode range has a higher current then the low ohm range. It must be less then 5mA but if it can do 0.5 mA to 2mA, that should be fine.
The first 3 steps to check the sensor are done with the 540B OFF.
1. Put the second ohm meter across the shunt connector on the A54. On my meter, it reads 87.8 ohms. I do not know the tolerance, but 80 to 100 ohms is probably OK. If it is open of above 100 ohms, the sensor may be burnt out and the whole A54 module will be doomed.
2. Unplug the HIGH FREQUENCY CONVERTER plug on the bottom left of the 540B and put the sensitive multimeter on a fixed mV Range between the lower pin on the socket (black lead) and the right upper pin on the socket.
You should get about 0.4mV/mA of current from the multimeter connected to the shunt input.
You can press the Polarity switch on the A54 and it should stay the same. If the multimeter on the shunt is disconnected, the mV multimeter should settle on 0.0 mV
3. Put the HIGH FREQUENCY CONVERTER plug back in.
4. Switch the 540B ON and on the A54, to 0.5V and DC TRANSFER. the resistance between the DC input pins should be slightly higher then the shunt ohm reading. If it stays open circuit then either relay 701 (the transfer enable relay) is not on, or the protection circuit (relay 801) has tripped. If the protection circuit is faulty - like a leaky transistor, pressing the PROTECTION DISABLE switch does nothing.
Now to check the galvanometer. If you have dud batteries, the supplies may not be adequate, even if the meter says they are OK. If you are running off mains, there may be a heap of 50/60Hz ripple on the supplies with bad batteries.
5. Switch the 540A ON and the SENSITIVITY to HIGH. The GALV toggle should be in the center OPEN position. The A54 settings do not matter - it can be off. You should be able to zero the galvanometer with the inner know on the SENSITIVITY control. It should not be drifting. If you cannot, then there is a problem with the galvanometer circuit or the supply to the galvanometer circuit.
6. Set the SENSITIVITY to LOW and the three REFERENCE ADJUST knobs all fully clockwise. Switch the GALV toggle to LOCK. The galvanometer should stay at 0.
7. turn the COURSE REFERENCE ADJUST one click to the left. The galvanometer should move left to about -10. If this works, then the galvanometer circuit is probably OK.
8. If a unit passes all the above tests, it is probably serviceable, so it is just a matter of looking for the least corrosion from the batteries, and do the sniff test on the A54 to make sure there is no smell of burnt resistors. If you can pull out the A54 and remove the white plastic cover on the range selector, all the better. It does include a lot of custom wire wound resistors, and I would hate to have to replace them.
If there is the sign of battery leakage or corrosion, some of the wires the the battery module and to the lower board my be corroded at the ends, and you will have to strip the wires back to an uncorroded spot and resolder. But this is all work that is achievable.
The two things you want to avoid are a dud sensor, and a dud galvanometer circuit (it just looks very difficult to debug).
Good Luck.
Richard.
If this tests works, then at least the sensor is probably good.