As vintage hifi gear has aged over the years, we see various components that have not been able to stand the test of time. Some more than others and some are just downright dangerous to the integrity of the rest of the components in certain pieces of gear. In this case...some transistors begin to collapse from age or stress, sometimes both. And when they go, they can potentially go BIG.
Here we see the 2SC1451, 2SA809, 2SC1940, 2SA798 (dual package), 2SC2291 (not pictured, dual package) and 2SA915. These are transistors that MUST be replaced, even if currently working. At best they will merely start to inject crackling, static bursts, etc. At worst, they will fail hard and take out other components downstream in the circuit. ALL manufacturers used them. Pioneer, Technics, Trio/Kenwood, Onkyo/Integra, etc.
Here is a Top-10 (now 19) list taken from Audiokarma showing the original, pinout, packge type and replacement. Search on Audiokarma for WAY more data and experiences than listed here. There are more transistors being added all the time and much of it is in gear from the 1970s. Although they were using better componentry by the 1980s, even gear being produced in the early to mid 1980s was STILL using some of these! By the late 80s, most of them had ceased being installed. Original - Pinout - Package - Replacement
2SA706 EBC TO202 KSA1220, 2SA1837, TTA004B*, MJE15033* Gain deteriorates with age
2SA720 ECB TO92 KSA1013
2SA725 BCE TO92 KSA992, BC327* Shot noise
2SA726 BCE TO92 KSA992, BC327* Shot noise
2SA798 (2x) BC560B, (2x) KSA992
2SA809 KSA1381 Poor heat dissipation
2SC458 ECB TO92 2SC2240, KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Leakage, noise, static
2SC631 ECB TO92 KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Prone to failure, noise
2SC632 ECB TO92 KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Prone to failure, noise
2SC871 ECB TO92 KSC1845, BC549*, BC550* Leakage, noise, static
2SC1000 KSC1815Y Leakage, noise, static
2SC1124 EBC TO202 KSC2690, 2SC4793, TTC004B*, MJE15032* Gain deteriorates with age
2SC1312 ECB TO92 2SC2240, KSC1845, BC337*
2SC1313 ECB TO92 2SC2240, KSC1845, BC337*
2SC1318 KSC2383, KSC2690A Thermal stress, thermal flex
2SC1344 ECB TO92 2SC2240, KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Static burst
2SC1345 ECB TO92 KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Poor mfg quality
2SC1400 ECB TO92 KSC1845, 2N3707, 2N3708, 2N3710, 2N3711 Static burst
2SC1451 KSC3503 Static burst, open circuit faults