I just bought a used valveking 100 half stack and the 1st channel does not work, second channel works but seems to have a loose input jack as connection is intermittent and affected by wiggling the input cable... what do i check first for fixing the 1st channel?

I think you may have a bad input jack, or it has come loose from the circuit board.. If you need to take it to a repair shop, ask about their bench charge. It may be cheaper to just replace the amp.

Looking at the schematics my 1st guess would be a defective capacitor C114 20uf 50volts. If you are not an experienced electronic tech I would not recommend you work on this yourself. This amp operates with dangerously high voltages that remain even when unit is unplugged. If you touch the wrong thing it CAN kill you. You can probably have this repaired at professionally shop for around a $100.
Below is a link to download the schematics. Take them with you to the repair shop:

http://ultimate-guitar-valveking.wikispaces.com/Warnings

This could be a transformer failure. Take it to the repairs shop.

Does your M foot switch work on the Fender? If you know how to use a multimeter you could check continuity by putting meter leads on plug of M foot switch(1 on tip, 1 on sleeve ) You should see a change in the multi meter when you depress switch. If so switch is good. Another easy test is with cap off a spare 1/4 plug -plug it in to foot switch jack.put a alligator or some such metal clip or wire and join(called short or shorting) the two inputs . If connecting and unconnecting results in switching then the foot switch is suspect. One fairly common thing is that foot switch and input jack are often sautered straight to a circuit board to save $ on production cost. Used to be all point to point where a wire was attached to 1/4" input and then wire went to circuit board or tubes. The wire has give, but not so with direct wiring to board. So the nut holding the 1/4" amp input gets loose(doesn't have to be loose for the following to happen) and a pull on the chord causes a break in the soldier and thus a break in the sound chain. Sometimes it will work intermittingly by putting a little pressure left, right, up or down on the plugged in plug. If so, it's the input jacks connection to the amp. If not it still could be that because putting pressure on plug may not be sufficient to affect a temporary reconnection. Since switch(the new one you made)works on Fender, but not Marshall I suspect the jack or foot switch circuitry past the input jack. If using some of the above the above tests leads you that direction,remove housing(unplug a/c power 1st ) inspect jack,wiggle it slightly see or feel a break then resodier from back side. You should be able to use the soldier already present without adding extra.

The missing nut is not likely to kill the channel. You just bought this so exercise your warranty... unless it is used in which case you bought someone else's problem. Somebody probably blew out the channel by zapping it with high voltage that destroyed the input circuit of the channel. Often there are bypass capcitors right at the input which short out. If this is boiught used, take to a repair shop and if new, exercise your warranty. I suspect they removed the nut from the connector to mark the dead channel.

The answer all depends on the impedance of your speaker(s). A single speaker can be plugged into either jack. Set the switch to the impedance of your speaker. If you use two speakers, set the switch to one half of the impedance of the lowest impedance speaker. Very few speakers are 16 ohm types. Most are 8 ohms and some are 4 ohms. You could plug an 8 ohm into each of the jacks and set the switch to 4 ohms for example.

Here you go... the parts list is at the front.

http://elektrotanya.com/peavey_valveking_100_212.pdf/download.html

Scroll down to "Get Manual" at the site and click on it to download the manual.

Buy ONE new 12AX7 and try it in location V1 and if it doesn't help, try it in V2. This tube has TWO heaters in it and if one is intermittent that could cause your problem. The vibration MIGHT be "fixing" a briken heater after some time.
There are MANY components that could cause this, so you might have to take it in for repair if it isn't a simple tube problem. Since it works on the clean channel, V3 should be OK.

I suspect you MAY have a severe ground loop. The green light indicates channel crossover if nothing plugged in.

MAKE SURE ALL connected equipment is powered from the SAME receptacle. If you service amps from receptacles at one end of the room and this board from the other end, you are ASKING for trouble.

Read up on "common mode" as it pertains to balanced lines. Equipment can be at risk if grounds "bounce" as they often do.

When green light with no inout shows up, slide all faders down first. Then unplug input lines one at a time... turn off the amps and then unplug the output lines one at a time. See if one of these inout or output lines was causing the green light.

You should be using BALANCED lines to your amps... either XLR or TRS types at both ends.

it could be your guitar pickups. try using another guitar or getting your pickups waxed at your local music store. also if your playing in a small room kind of loud, or if your using single coil pickups (like fender statocaster pickups) you will definitely get a feedback noise. if that does not work let me know.