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Guest jimkarl

Help with vinatge transistor replacement

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Guest jimkarl

Trying to repair a vintage phonograph. Schematic attached. one channel was weal 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/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 Emmittor I get 0v, and open when moved to the Base. Swapping to Pos on the collector, I again get 0 to the emittor 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, I just want to understand my options.

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.

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. In the schematic, at test point  5 (or 13 for the other channel) 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 channel balance (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). What in the circuit causes that severe signal attenuation at that point and is that normal?

VM_3691_voice_of_Music.pdf

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Guest jimkarl

Update - I'm sure that Q7 transistor is blown. Diode checked/measured all the pin combinations on all 4 transistors. Only Q7 had 0V between Collector and Emitter in both directions. The others had 1v or just under from Cn to Ep and infinite in the other direction.

So I'm pretty sure I'm back to my original question - 2N2869 a valid alternative (since I can get one relatively cheaper) or any good sources for a low cost 30203?

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Guest liquibyte

Update - I'm sure that Q7 transistor is blown. Diode checked/measured all the pin combinations on all 4 transistors. Only Q7 had 0V between Collector and Emitter in both directions. The others had 1v or just under from Cn to Ep and infinite in the other direction.

So I'm pretty sure I'm back to my original question - 2N2869 a valid alternative (since I can get one relatively cheaper) or any good sources for a low cost 30203?

Have you tried a cross reference site?

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Guest jimkarl

Thanks I did check and found that the 2N2870 is also a valid replacement. Found this on ebay

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=390813514432&ssPageName=ADME:B:BOC:US:1123

Only thing is I'm not sure what the 7246 reference is that follows and have been unable to dig up a spec sheet on that manufacturer's version. I bought all 3 figuring I could return (at least 2 of the 3) if the first one doesn't work out. Interesting that that same seller had the same manuf/@n part Without the 7246 for $10 each, so there must be a difference in the two, I just don't know what....

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