I am working on a SANSUI 3300 and have been going to Audiokarma forums for help on substitution of transistors for that unit. I found that several people gave advice for a particular power transistor 2sc792 replacement as MJ21194G and others a different transistor. When I looked up the specs on the datasheet they seem to be over kill. Is this something that is ok or should I opt to look for something more close in specs? What is the theory for replacements? Is it ok to alter the specs and if so to what degree? All Transistor website was given as a place to go but that site always results in a list that also shows replacements that are many and seem way different than the original. How does one know which to pick?
Hopefully someone with far more knowledge chimes in as well. This is what I look at in a replacement. Case, try to pick one with the same physical characteristics so you don’t have to make huge modifications to the equipment being worked on. Voltage, try to find something with the same or higher voltage specs for all junctions. Gain,or Hfe a number that is usually difficult to match perfectly, but you want to keep as close to the original as possible. Power dissipation, the same or higher. Same with max collector current. In the real world you’ll find that sometimes you just have to use something much bigger just because there are no other options. Or you’ll have to use a transistor with a different lead configuration and orient it on the board differently. Ive been “playing” at this for 5+ years and most people have forgotten more than I’ll ever know. Keep good notes. When you find a good substitute for an obsolete part order as many replacements as is practical so you’ll have them for years to come, as they are constantly discontinuing them
On transistors it’s important to replace with specs that are as close to spot on as possible.
But more generally speaking, opting for a higher capacity transistor is sometimes allowed as this only allows the transistor to run at voltages less than it’s maximum design capacity which helps them run cooler at any particular volume removed link And that’s not necessarily a bad thing.
Modern transistors are likely to have higher ratings and your selection of current products is very limited. I suggest you go to Mouser or DigiKey, drill down to the BJT category, select the TO-204AA and TO-3 case and also ‘stocked’ and/or ‘normally stocked’ to see your options. (DigiKey has ‘marketplace’ items so you might find a closer match at a much higher price.) Export the results as a CSV so you can use a spreadsheet program to sort on specific fields. Keep in mind that you are likely going to have to adjust the bias and may need to replace other transistors to get the new ones to work. A dim-bulb tester is your friend when trying new outputs as the ‘magic smoke’ is easy to release.
Another other option is try to find matched-pair exact replacement on eBay but, unless you know the seller to be truthful, you’re likely to get fakes.
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