I had a faint crackling/frying pan noise in my ST120 power amp after I repaired it the first time. The second time I replaced the input 5 uf (microfarad) tantalum capacitor, with ceramic this time, it totally went away. Electrolytic capacitors get old in 20 years for high quality ones with great seals, 1 year for cheapos. Usually they just go low value and stop passing low frequencies, but sometimes the big ones in the power supply can short and explode. If your organ is transistorized and has no voltages over 24, you can take off your jewelry and start replacing, but if it has tubes or any voltage over 24, read aikenamps.com technical resources button technician safety button about not electrocuting yourself before touching anything metal. My first replacement tantalum cap with the crackle was bought off the bargain table in front of the supply house. I believe the analog these days where you buy outdated caps is E-bay. I suggest you buy any caps from a first line dealer like newark.com or mouser.com in the US. A WP25 iron with a chisel (not pointed) tip is fairly useful, as is scrap wire with flux on it to wick up the old solder. Wear safety glasses, solder can splash. Electrolytic caps look like aluminum cans with etched writing, or plastic or cardboard sleeves with a plus on one end or a minus on the other. Tantalum electrolytics look like peanut M&M's. Any value under 10 uf and 50 WV can be replaced by forever film caps or ceramic caps nowadays. Replacement electrolytics should be same to 40% higher in value, and same to 40% higher in working voltage (WVDC) Make sure you mark the PCB (printed circuit board) with the plus before you remove, as if you install them backwards they boil and explode. Timing electrolytics should be exactly the same value. You can detect timing caps, they usually have a tolerance of +- 20% or 10%, whereas the run of the mill hum bucking electrolytic is +80%-20%.
Contact cleaning cant hurt, except for the fire hazard. Don't unplug anything with the power on. Modern contact cleaners are gasoline with a fancy name in a spray can. No smoking, no open flame heaters lights or pilots within 30', no electricity turned on and off, use a fan. Some people here like expensive Caig Deoxit for tabs and keys, but I found lumber yard stuff fixed my problems.
A useful beginner book on solid state electronics and meter use for debugging is "Electronic Devices, Electron Flow Version" by Thomas L. Floyd, Prentice Hall, ISBN 0-13-363599-6. I got mine at Goodwill because the problems are obsolete. Your community college bookstore might have something just as good. Understanding capacitor and inductor filter and tuned circuits is interesting, wikipedia is very cheap. I started learning electronics with the Ford car manual and a dead car radio- the tube voltage warning was missing, and I learned about 440V in a 6v car radio the hard way.