My BL-40 audio processor just crapped out. Looks like the problem is in the audio output amplifier section. The output transistors draw excessive current, pull down the regulated + and - 18v to about 16 volts, causing the power transformer to overheat and stink (because a previous owner had substituted the 1/4 amp fuse with a 3 amp one), and the low-ohm resistors in series with the collector and emitter of the output transistors to overheat. I found a replacement transformer, but the old one seems to work OK; I grabbed the replacement just in case. A couple of electrolytic capacitors were swollen and oozing, so I replaced all electrolytics in the power supply, the ones in the +/- 18 v lines to the output amplifier, and the main coupling capacitor to the output transformer, as well the overheated resistors in the output stage, but that didn't solve the problem. Temporarily disconnected the +/- 18 v lines between the power supply and the output amplifier, and the voltage jumped back up to normal, and both the RMS and peak limiter seem to work normally. I suspect one or more of the transistors in the final audio section bit the dust. So it should be a simple fix, a 3-stage transistorised audio amplifier, using three small-signal transistors followed by a complementary par of PNP/NPN power transistors? Not so fast; evidently many of the transistors used in this 1974-76 era unit are no longer available. I can find the NPN output transistor, but the matching PNP one is apparently unobtanium. Same with the small-signal ones used in the driver stage; two are readily available, but not the third one. I haven't yet tried to test the individual transistors to maybe pinpoint the bad one(s), nor the electrolytic capacitors I didn't replace (a PITA because that requires removing them from the circuit board and re-soldering back in good ones (without lifting a circuit board trace), but the problem remains finding replacements for bad semiconductors no longer available. Haven't found much help in on-line substitution guides so far. That's one problem with solid state equipment. There must have been a jillion semiconductor types manufactured over the decades, and types are being discontinued all the time, often a couple of years or less after they first came out. A manual covering semiconductors in the manner of the RCA Tube Manual would be like a New York City phone book (back in the days when those were published). On-line searches bring up all kinds of irrelevant crap and occasionally something that might look useful. And don't even bring up the subject of SMT. Anyway, I am hoping to get this thing back into operation before summer static season is over. The output transistors are types U05 and U55. The U55 is the one apparently made of unobtanium. The small-signal transistors are 2N5087 (still available and cheap) and 5117 (no longer available). A 2N5088 was suggested on one of the guitar amp/audiophool sites, but the characteristics between the 5087 and 88 don't seem to match up. This is one of the reasons I have stuck mostly with hollow-state. It's easy to find substitutions for tubes and parts for equipment made 50-85 years ago, or often, fabricate one's own. This thing is "only" 47 years old and many parts are unavailable. I am much less knowledgeable about how to re-design transistorised circuitry to make it work with currently available semiconductors, than I am modifying tube stuff to keeping it on the air. Oh, yes. The rest of the unit is comprised primarily of little round metal-case plug-in op-amps. Like the audio transistors, some of the op-amp types are still available, and some are not, or at least not in the round metal case wire pin format. Hopefully, none of those will fail any time soon. At least I do have a spare T4C optic attenuator, a proprietary item but I have seen replacements on line (not cheap). No way will an expensive modern-day transceiver last as many years as a 75A-4. Probably some $5 proprietary IC or other component will soon be discontinued and turn it to a costly door-stop.