Help Please Identifying This Transistor

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I am working on a Scott 312D Tuner.

I have found a bad transistor in the Audio Output board. The transistor is mark on the device and schematic as S1990 (Fairchild).

I have looked on the web for a replacement without luck. I cannot even find a datasheet.

I am OK at troubleshooting but lack the knowledge to make a competent choice on a replacement that might work.

Any recommendations is appreciated.

Here is the schematic. The part is identified as Q101.


  • Scott 312D Tuner Output Board.jpg
    Scott 312D Tuner Output Board.jpg
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No luck finding it for me either, pre-internet and obsolete. I think any small signal PNP transistor will do at a pinch, preferably low noise as its the input device to audio amp. You'll have to figure the pinout from the pcb.

The circuit has LC traps to filter out 19kHz and 38kHz stereo decoding pilot and carrier, its clearly fed direct from the stereo demultiplexer.
In the 1960's process noise was pretty common. Manufacturers could buy select low noise transistors under special numbers from ordinary die runs.
Today process noise is unusual. The only TO18 products I see in stock at newark today is 2n2907 & 2n3251 with a US pinout. There is a BC177 but download the datasheet, the pinout may be wrong.
If I had MPSA56 in stock I would not hesitate to try that. You have to bend the pins a little to fit a TO18 pinout. I can't exactly read this but I believe the resistors are 22 k and 12 k so the current should cause undue heating of a TO92 product. There were no heat sinks on it were there?
I'm assuming TO18 because of the age of the product, but TO5 is more problematic. There aren't any. At least TO18 has the same pinout, ebc.
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I could find no data for the part and looking at the circuit it is a PNP Si amplifier low noise min. 25V VCE in metal can TO-18 package. I think in the era, low noise PNP parts were hard to come by. The NPN 2N2925 (hFE ~470, NF 2.8dB) shows what Scott liked there.

It would be hard to find something like that and I'd put in a modern (plastic) part but watch the pinout differences.

I would suggest 2N5087 EBC, BC560C CBE, 2SA970 ECB would be fine replacements.

S1990 does cross to a NTE234 but that is silly high voltage 120V like the 2SA970.
ANY small signal PNP will work there.
Bias is fed from low value resistors, so Hfe, input impedance, etc. are not critical.
Signal level is relatively high so noise isn´t a problem either.

ELEKTOR magazine published "universal transistors" (and diodes) which could be used most everywhere ,the famous TUP TUN DUG DUS list.

Being European, they suggest same origin transistors, but seeing the parameter choice, you can easily expand the list with popular American and Japanese types:


that´s how interchangeable transistors are, go figure.
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