1) I own a Quasar TV model# SP 3232A. I have a dark screen but good sound.What are the possible troubles. 2)Is a service manual available for this model

Yes.

Check all your channels. If you are getting sound on some channels but not others it is a good possibility that your SAP setting is turned on. Simply go into your audio menu and disable the SAP. Not sure about your model tv but some Tv sets had a switch on the back to disable the speakers and some have a setting in the menu to disable the speakers. If you have a digital cable box hooked to the tv it is possible the cable box volume is turned all the way down which will override your tv volume. Check these options and hopefully one of them works for you.

try adjust screen up, in you see horizontal line in bottom, the tv have the vertical ic with cold solder or bad.

Sounds like a failed vertical amplifier circuit. This is normally primarily contained in an integrated circuit mounted on a heatsink.
There are other possibilities along the line but the one above is the most likely.
Now all you need is a TV repair shop that is still in buisness.

Quasar TV SP3232. Picture disappears but comes back by striking the TV . The sound keeps coming. Perhaps something is loose

Solder any / all bad connections , esp. vertical output IC......

well it's not a wide screen tv show it won't really matter but if it is H/D you will find a red/green/blue connection in the back for picture and red/white for sound

you have an intermittent vertical ic or caps . it will need service sorry usually it's not to expensive werepair these all the time

well most of the modern tv today doesnt have vertical hold anymore. this symptom is caused by a trouble in the IC integrated circuit or the vertical oscillator circuit. let the service guy do the job if u dont know anything about this. good luck!