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G0HZU_JMR · 5th Mar 2021, 7:05 pm

I think 868 ohms is a bit on the high side as that would reduce the current to about 5mA. This would probably give a slightly lower noise figure (maybe 0.5dB lower?) but the signal handling of the amplifier would be much worse.

I just spotted that you have bought some 2SC5010 parts. I'm not sure if you realise but the 2SC5010 is a modern low noise RF part in a tiny plastic minimold package. It won't be a drop in equivalent in terms of the package size. I used it as a model in Genesys because it was the nearest fit in terms of the RF performance at 2GHz.

I think the original part could an older NEC 2SC2150 in SOT-173X package as mentioned back in post #20. This will have slightly worse noise figure than the 2SC5010 and probably lower gain by about 1dB at 2GHz. The 2SC2150 is used elsewhere in the TR4172 so this was my best guess as to what the actual device is.

I'd expect the 2SC5010 to work in this circuit but it won't fit the artwork neatly. It would be best to fit the emitter leg as close to an RF ground via as possible, There is also a slight risk it might be unstable. It's also only rated to 6V Vce so it would be a bit marginal in this circuit.

Hopefully your RF transistor is OK and it is just the bias board that is faulty.

5th Mar 2021, 7:05 pm	#62
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: Spectrum Analyser TR4172 I think 868 ohms is a bit on the high side as that would reduce the current to about 5mA. This would probably give a slightly lower noise figure (maybe 0.5dB lower?) but the signal handling of the amplifier would be much worse. I just spotted that you have bought some 2SC5010 parts. I'm not sure if you realise but the 2SC5010 is a modern low noise RF part in a tiny plastic minimold package. It won't be a drop in equivalent in terms of the package size. I used it as a model in Genesys because it was the nearest fit in terms of the RF performance at 2GHz. I think the original part could an older NEC 2SC2150 in SOT-173X package as mentioned back in post #20. This will have slightly worse noise figure than the 2SC5010 and probably lower gain by about 1dB at 2GHz. The 2SC2150 is used elsewhere in the TR4172 so this was my best guess as to what the actual device is. I'd expect the 2SC5010 to work in this circuit but it won't fit the artwork neatly. It would be best to fit the emitter leg as close to an RF ground via as possible, There is also a slight risk it might be unstable. It's also only rated to 6V Vce so it would be a bit marginal in this circuit. Hopefully your RF transistor is OK and it is just the bias board that is faulty. __________________ Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 5th Mar 2021 at 7:19 pm.