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G0HZU_JMR · 27th Apr 2021, 10:26 pm

The technology of the components is certainly quite varied in that probe. I'm going to guess that Q130 (PNP BJT) in your schematic is biased at something like 13mA. I'd therefore guess that the impedance at the emitter will be 0.026/Ic = 0.026/0.013 which is about 2 ohms. In reality it will probably be a bit higher than this but it may mean that the ideal value for the R190 resistor is in the order of 47R as indicated on the schematic. This will sum with the 2R emitter impedance to get close to an ideal broadband 50R here.

I suppose you could use a pair of 0603 SMD resistors in parallel here of values 150R || 68R to get close to 47R for R190.

27th Apr 2021, 10:26 pm	#25
G0HZU_JMR Dekatron Join Date: Sep 2010 Location: Cheltenham, Gloucestershire, UK. Posts: 3,077	Re: Tektronix P6201 FET Probe repair The technology of the components is certainly quite varied in that probe. I'm going to guess that Q130 (PNP BJT) in your schematic is biased at something like 13mA. I'd therefore guess that the impedance at the emitter will be 0.026/Ic = 0.026/0.013 which is about 2 ohms. In reality it will probably be a bit higher than this but it may mean that the ideal value for the R190 resistor is in the order of 47R as indicated on the schematic. This will sum with the 2R emitter impedance to get close to an ideal broadband 50R here. I suppose you could use a pair of 0603 SMD resistors in parallel here of values 150R \|\| 68R to get close to 47R for R190. __________________ Regards, Jeremy G0HZU