I have a Infinity7541a amp. I have 7.5 volts dc to the amp. If I disconnect the power to the amp and read it its 12.2. Is something pulling it down in the amp

It should be 12 volts,check for a bad ground ,the clutch is just a electrical magnet ,it's either energized or d energized....if the 5.6 volts was AC,it should be DC volts !

Check the connections on the batteries. One or more may be loose or corroded. Also check the fuse for the batteries. It may have blown.

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.

Sounds like a loose or bad connection ether at the pto switch, the clutch or both. With the key switch in the on position and the clutch wire disconnected at the clutch, use an ohm meter set to dc volts and check and see if you have 12 volts at the clutch. Push and pull the switch several times and watch the meter. If power reads 12 volts each and every time you pull the switch, the switch will not be the problem. Clean the clutch terminals and wires using electrical cleaner and then try it again. At the last and most expensive cause would be a short in the clutch itself. Good luck. George

The computer's switch mode power supply produces DC (direct current) electricity.

I would first check the wiring connections to fan and get a multi meter , set on 12 volts dc and see if fan motor has 12 vdc input voltage.
Too cheap to buy a voltage tester at wal mart ot harbor frieght, you can use a 12 volt bulb and socket, if you already have one. Remove fan wires & hook up 12 volt light bulb & if it lights, you got the juice. Carefull,dont let live wires touch because you short out $26.00 big buddy fan transformer. work with fan transformer unplugged from wall and dont plug in until you sure no wires touch.
use electrical or duct tape. electrical tape better.

otherwise,just buy the multi meter, they only $8.00 to $10.00
read & understand instructions, before you try to use it. otherwise,you could burn out your new meter.

well worth owning &
learning to use multi meter. I use it thousands of times.

if fan motor has juice but wont run, check fan shaft to see if it turns freely or scrapes or is stuck. oil fan motor bearings ,change the brushes.
make sure that the fan blades are not touching the fan housing and can turn freely.
if it still dont work, buy new fan motor.
if no juice at fan motor, check mr heater switch with the power off,unplug transformer from the wall or remove batteries.,
and use the ohm meter setting of the multi- meter to test for continuity of the switch when the switch is in the on position.
power must be off when using the ohm setting of a multimeter.
if power is on & using the ohm setting of the multi meter, you will blow out the multimeter. The ohm setting uses a 1.5 volt dc battery inside the multimeter to test for a compleated circuit.
so, 12volts dc would blow it out on ohms setting.
TESTING FOR OUTPUT OF FAN TRANSFORMER OR BATERY PACK.
If the switch is good,reinstall it on mr.heater and test the output of the transformer or battery compartment using the 20 volt dc scale of the multi meter. the voltage scale you select ,must be higher than the voltage you expect to find ,in order to avoid damage to the multi meter.
observe polarity,positive to red and negative to black.

the dc voltmeter setting is totally polarity sensitive and a reversed hook up could blow out the meter.
multi meter means it can read ac volts, dc volts,ohms and dc amps. Read and understand the included instructions before using a multi meter
a good single battery in new condition reads 1.62volts dc
1.5 vdc is almost dead, a good used battery reads 1.58 to 1.6 vdc
below 1.58 vdc is weak battery and 1.48 vdc is usually totally dead, but may be enough for momentary usage under 1 minute.

buy a new power supply

You have a draw and a draw pulling more than a couple hundred milliamps.This will drain the battery in an objectionably short amount of time.. All the things you mentioned and more can cause this draw. The best diagnostic step towards isolating your draw is with meter that reads Volts DC or better yet a test light.(I think) ..disconnect B+ positive battery cable. Install meter or lamp in series with disconnected cable and battery post. You do have some smal draws like computer memories,clocks etc which will show a low amount of voltage hence a very dim test light. If you are drawing heavy ,obviously the meter will show a full blown 12 volts and the lamp bulb will be very bright. Remove fuses,unplug relays,dis-connect devices while watching the voltage or lamp.You should begin to isolate offending item once the circuit has been detected, Generally pulling all fuses individually will tell you at least which circuit is pulling the amps.

Here's a few things that come to mind. An auto charging system will operate normally from about 11 Volts to about 14.5 Volts, depending on what is operating (lights, blower,etc), and whether the vehicle is at idle or not. Most car amps will operate from about 10.5 to 16.5 Volts, and outside that range, they will probably shut down.
Another consideration is your 20 volt power supply. You didn't mention if it is a DC power supply, but it needs to be. The other factor is its output, meaning its amperage rating. I'm figuring it will need to put about 5 amps, at the least, for the Sony to put out some watts. I think the line fuses of the Sony are rated @ 25 amps each.
I don't like this type of situation, because I want to say 'try it and see what happens" and I want to say 'don't do it, something may blow".
If you decide to try the 20Volt power supply, maybe if you put smaller rated fuses (say 10 amp) in the Sony amp, they will act as a fail safe. Good luck!

Check the wall outlet for power, using a lamp or such. If you can get a multi-meter, set the meter to Volts-DC between 12 and 20. Plug the power supply into the wall outlet and test the other end for voltage. The power supply should tell you its output so the reading should be more than a couple of volts. If it is less or ZERO the power supply is dead. If it has power, then open the router and check for a fuse. If the fuse is dead, replace. If there is no fuse or the fuse is OK then the router may be dead. Best of luck.