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The Parallax 7155 converter should not require battery support for any 12 volt load amperage that is less than the maximum amperage capacity of the converter. A standard RV incandescent 12 volt lamp draws about 1.5 amperes for each bulb. With the battery disconnected and other 12 volt loads off, turn on lamps (not fixtures) and calculate the number of bulbs which can be turned on. If you run out of lamps, turn on vent fans etc. The idea here is to try to get to 55 amperes of DC load if possible without going over 55 amperes. At a sustained DC load of about 25-30 amperes or higher, the fans should cycle on continuously after about 5 -10 minutes. Lamp brightness should be normal and the measured voltage from the converter should be above 13 volts if under 55 amps of DC load. If the voltage drops with only a few lamps on, a converter issue would be likely.
Diagnostic tests an RV technician should make would be to:
1. Disconnect the battery system.
2. With the converter system powered on, measure the DC amperage with a DC ammeter capable of reading at least 60 amperes DC from the blue lead at the DC fuse panel terminal VCC.
3. Measure the DC voltage at terminal VCC and DC ground.
4. Record both readings.
At any DC load amperage less than 55 amps, the voltage should be above 13.0-13.2 volts DC. This method should take out all guesswork and confirm a DC converter section issue or not.
Look to see if the charge light is on when you connect the power. If not, try moving the power jack around slowly in a circle. If you get power with the cord/connector at certain angles, then the motherboard will need to be removed in most cases and the power connector on the motherboard will need to be resoldered to the board, and then the unit will have to be reassembled.
If moving the connector doesn't ever put the charge light on, then there is either a picofuse blown or more likely some SMT MOSFETS are blown. If you have the skills, identify and replace the MOSFETS and you should be ok. If a picofuse is blown, replace it with the same type and rating fuse. DO NOT JUMPER the fuse...sometimes they just go, but most often they go for a reason. If the new fuse blows, start looking to the MOSFETs and electrolytic capacitors in the DC/DC converter section.