Trying to repair a vintage phonograph. Schematic attached (and BTW 256K is a completely unreasonable size for an attachment. You can;t get a single page pdf, a single page JPG, or even GIF in that size!!! Argh!).
Anyway - one channel (right) was weak and distorted, so I suspected the output transistors from the start but didn't have a schematic yet to debug. I'm an old guy relearning electronics from scratch again, so I figure the safest thing to do was get the schematics and walk through the audio path from the input on. Eventually I think I did confirm that one of the output transistors was bad (Q8 in the schematic). The schematic labels the four output transistors as 2n2869 but the actual transistors currently installed are 30203's so not sure if they had been changed at some point. Any, using a multimeter I did a diode check between the Collector and base and again C/emitter, and see a very low voltage .09 whereas I think it's supposed to be between .4 and.9 (tens times higher). With Neg on Collector and Pos on Emitter I get 0v, and open when moved to the Base. Swapping to Pos on the collector, I again get 0v to the emitter but .09 to the base. So my questions are:
1) Some please confirm this is a bad transistor?
2) Assuming it is, what would be the impact to the circuit if I put a 2N2869 in as a replacement, which is what is listed on the schematic. Any substantial volume increase/decrease on that channel as a result? Actually not looking for more power, just what the volume to be level across the channels? Of course the 30203 will be the closest match since it's the same as the other 3 currently install, I just want to understand my options and the impact if I go the other part number.
3) Are there any good source for online replacements - at low cost and used is ok. I obviously searched EBAy (and surprisingly only a couple hits there) and did a quick google, but both of these parts generally come up in the $20-$35 price range which honestly is a more than I think it is worth to fix up this unit. I've already put in about $15 for some caps and it's getting to be more an expensive venture than I had planned for what it is. I did find one source that had the 2N2869 for $9 (plus shipping) at even that will be at the edge of my $$ tolerance I think for the entire unit expenditure (original price plus parts)
4) Somewhat unrelated, as I was debugging the audio path, I was using a small battery powered amp to trace the audio through the circuit. Note that my input source was not actually the phonograph. I had disconnected that for logistics & testing reasons and hooked up a cassette player. Obviously a different gain/impedance input and perhaps that has something to do with my debugging problem I'm about to explain. In the schematic, at test point 5 (or 13 for the other channel) whihc is right after the volume control the audio is loud and clear. As loud as if I had connected directly to the input source itself (as long as the volume set to 100%). All good there. However, right after the next stage (channel balance at test point 6 or 14) the audio volume is severely diminished and I don't understand why. That is with the balance set at 50% (and it works appropriately swinging to the left or right channels). As I said I still learning but with that result I would have initially thought I had a dead preamp circuit from that point forward - yet the unit runs fines with lots of volume (except on the right channel with the bad transistor of course) so obviously it's getting all it's gain back in later stages, but I don't understand the circuit design and why the gain loss right at the balance stage. What in the circuit causes that severe signal attenuation at that point and is that normal? Is it related to the audio source I'm using and some kind of expected impedance or voltage mismatch? I'm not sure if the Parallel RC just prior to the balance is the reason (or even what purpose that circuit there serves?) While none of this is the cause of my real problem (bad transistor) I just wanted to understand the theory behind what I am seeing occur. Thanks
