blower fan works fine. condenser fan and compressor both start. than they both shut off after 5 sec. repeat. 5 sec run, 5 sec off. start capacitor is melted. has a good amount of black oil crud leaking since it is mounted sideways. looks like melted wax. so it is run cap or start cap that may be suspect?

What you describe certainly indicates the start capacitor is bad. Is it a round black cylinder shape? Make sure the power is turned off. Look at the end where the terminals are hooked up and you'll see the black goo came out of a round hole. If it weren't for that hole, the capacitor would explode.
If you are going to replace that capacitor you may want to replace the start relay also. Although I have seen times when just the capacitor went, most times it's the fault of the relay. The start capacitor can only stay in the circuit for about 7 seconds and then it will blow. It is made differently from the run capacitor which can stay in the circuit indefinitely. So if the relay points have a tendency to stick or are fuzed together, it'll take out the capacitor. I can't quote prices because they range wildly depending on area of the country and from supplier to supplier. In my area, the relay would retail for about $15.00 and the capacitor for about $8.00. Make sure you replace it with the same MFD and voltage rating. It should be stamped right on the capacitor. Take a look at the fan and compressor run capacitor. If it is swollen or leaking, you'll want to replace it too. If you have a capacitor checker or know how to check capacitors, give it a check. Otherwise, just for a margin of safety, since you're already in there, replace that too.

SC is small, black. mallory 72-88mfd. the RC is large. 2x4x9 and mounted vertically thru the bottom of control box. i have seen online info on RC where it might be referred to as dual cap or something like that. also, much more expensive. some places want 100 or more for them. the schematic shows many parts but each description says they may be optional. i used to design refrig pop machines so had access to lots of parts but don't work there now. does the 72-88mfd sound right for a house central air unit? seems low.

Joe, 72-88 sounds just about right for a start capacitor. The Run Cap would be dual in that it runs both the compressor and the fan. Some of them are fused on the inside and when the fuse goes, the whole capacitor has to be replaced. If lets say the compressor side went but the fan side is ok, the whole run capacitor would have to be replaced.

i also tore down unit sheetmetal to vacuum off condenser fins. the sides of enclosure are slatted, and very hard to vac easily. matted down crud. gulp. my fault. i have done the cleaning process many times over the years. am pretty good at it. hosed out fins too. now, to dig up a cap today.

took off old cap. is is marked 330v. local supplier only as 220v in stock. have to special order 330v version.

got new cap. condenser fan starts. compressor does not. after 10 sec or so, the compressor tries to start again and start cap blows a lot of smoke. did not change start relay. unit does not trip breaker. unit seemed to try and start before with old bad cap. comp/fan seemed to start but shutoff after 5sec? than repeat.