johnson thunderbolt

[wa5bti]

i have a johnson thunderbolt that has a bad bottom panel meter the one that reads grid current& voltage. i found the top meter on ebay the one that reads watts& high voltage my question is,does the two meters have the same movement,if so i can change the scale from the bad to the good meter.wa5bti

[W2INR]

If you are sure they meet the same specs yes. This should be easily confirmed in the manual parts list.



G

[WQ9E]

The plate current meter (also marked with watts input) reads 750 mils full scale without the use of an external shunt.  The other meter uses external shunts (and a multiplier for HV measurement).  You would need to open the ebay meter to determine what internal shunt is used and this will let you determine whether or not the meter has sufficient sensitivity to replace the multi-meter.  At the very least you will need to do some meter surgery to remove its internal shunt and perhaps also change the values of the existing shunt and multiplier resistors for the other meter circuit.  Also be sure and determine what caused the death of the existing meter, if one of the shunt resistors is open then this would cause excessive current through the meter coil damaging the movement.

Unfortunately it won't be as simple as just switching scales in this case.