Error Code - 7704 - DC Contactor Error

This error occurs when there is a failure at the inverter's DC contactor. This can be caused by many things, and it varies for different models of inverter. 

500HEUS & CPUS

When a 7704 error occurs in a 500HEUS the first place to look is on the side of the DC contactor. There is a connection on the side of it that indicates to the control board (MMTCON) when the contactor is either open or closed. It's essentially a feedback loop, if any part of it is loose then the inverter will open the DC contactor and trigger the fault. 

Advanced Troubleshooting - 500HEUS

In some cases the fault may be intermittent or difficult to determine the source of the failure. The DC contactor needs 230V to close its coil, this can be done by jumping the signal to close from the A1 (MMTCONT) board X5 connector to the appropriate signaling wire. 

Use a small wire to "Jumper" these connections together to CLOSE the DC contactor as soon as control power is applied. This can be done with the F1 breaker once the jumper is in place. Close the DC contactor by applying the 230V signal multiple times until a failure happens. Notice which DC contactor is failing, and look for possible causes.

If the DC contactor is failing because of the auxiliary contacts on either side, then they may need to be adjusted. One on the left side of the DC contactor can be adjusted with the spring/plunger. The contact on the right side may need to be creatively extended as it is not adjusted-able. If after adjustments have been made and the contactor is still not operating as intended, setup voltage measurement probes at the terminal blocks above the DC contactors - X450 and X460 input #1 on both terminal blocks. The tolerances for closing the coil are +/- 10% of 230V. Anything outside of that range could be causing the failures. Potential failure points Further inspect the Aux contacts to ensure the issue is not the aux contact on the right side being worn down and not making a good connection. Extend/modify as needed to ensure it closes each time successfully If voltage is dropping > 10% at either connection point there may be a resistance issue - either in the terminal blocks or the conductors themselves that will need to be fixed.  The MMTCONT may be bad if the DC contactor closes multiple times with no issues. If 230V is good, DC contactor closes successfully 20-30 times, then the MMTCONT is the failure point and it will need to be replaced. If Voltage is good, Aux contact is working as intended, and the DC contactor has failures when you attempt to close it then we need to replace the DC contactor. It should be obvious which one is having the failures.

Right Side DC contact Left Side DC contact

CP10 / CPXT

For the other models of inverter, CP10 or CPxxx and the CPXT series, the failure is either related to the DCcontactor itself and its undervoltage release coil, or something within the feedback loop circuit attached to the DC contactor, this can even be temperature sensors in some cases. Check the inverter for any loose cables within the DC contactor circuit, ensure all of the feedback/power wires are secure at the top, and then as a last resort replace the DC contactor.