I wish I could give you a 3-sentence instruction on how to fix it, but that's simply not possible. There are a few things that you need to know. The first is that you most likely can not test transistors in-circuit. What this means is that you'd need to desolder and remove the transistor. And then depending upon the type of transistor (there are probably millions), you'd have to find out the pin-outs to determine which pin is the Base, Collector, and Emitter pins. Once you've determined this, you can use the diode mode of your meter (the red mode to the left of the red-10A mode). You can just google how to check transistor to find out how to do it, I'm not going to type in a whole essay here on how to do that when it's all over the internet. If it's bad, you'd need to replace it. If it's good, reinstall it and move on. Alternately, in most service manuals, you'll possibly see circuit diagrams showing transistors with their leads identified along with voltage specs. What you'd need to do is using your meter in DC volts mode, test the test points, especially those near any transistors, to see how close they are to spec. If they are off, these are hints that need to be interpreted. If you have a chart listing the transistors (Q101, Q201, etc.), along with the voltage specs (should-be/currently-is), perhaps someone can help you. If you are thinking whoa! that is a big pain in the ass. My response is, you bet your ass it is. That's why it's better to have just 1 toasted transistor than having a bunch of blown components by randomly and blindly trying to sabotage everything. And that was the point of my previous post. After all, it was clearly your own fault,
but the damage has already done so...who to blame, well.. me, myself & i.
In other words, it wasn't a matter of fault, rather it was that there is absolutely no point to probe around unless it was methodical and a specific result was being searched for (voltage or resistance check, etc.).
BTW, voltage checks as specified in the service manuals are conducted "live." It's better for you to do it with batteries instead of AC. That's because your meter is not autoranging so you should select 20V range (next range is 200V). Unfortunately, on many boomboxes, it is entirely possible to find over 20V with no load, which could damage your meter unless you switch to 200V which might not provide the accuracy and resolution. Your meter probe tips should be insulated except for the very tip when doing live checks. This reduces the chances of the entire 1" metal tip being a grounding rod. You can use a small heat shrink tubing or electrical tape. Lastly, be especially careful when check voltage around the IC's due to the close lead spacing.
Ok, about transistors: Some service manuals will tell you the B/C/E assignment of the pins, but not all. If yours doesn't, then you'd need to look at the transistor in question, and search online for the datasheet. This sheet will provide you the transistor characteristics and lead (pin) assignments to help you identify the Base, Collector and Emitter. If you don't know which is which, then the test is meaningless.
Honestly, this may simply be something you'd have to pay someone to fix for you. Circuit diagnostics is not a novice skill. Unlike changing belts which just requires some amount of mechanical aptitude, electronics is not. If you don't know the basics, the chances of getting it fixed is slim. (Sorry for being party pooper). If you come to that conclusion, then the only question is whether the boombox is worth the cost of the repair (to you).
