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| Current source in Princeton Electrometer repair |
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| charles.ouweland:
I'm currently in the repair of a Princeton Applied Research Model 136 Electrometer. The input amplifier is not working. Unfortunately I could not find a schematic so I'm reverse engineering the whole thing. One of the problems is the noobtainium quality of some of the components, so it takes some thinking how the circuit was originally intended. But I'm coming a whole way. One of the things I don't understand is what appears to be a current source, the schematic of which is attached to this post. I have measured it and it delivers a consistent 0.16mA out of the collector of T11 over the whole range of -9V to +7V (the emitters of T9 and T10 are connected to nothing). That's great but can anyone explain how this circuit works? Question number two: how is the current set and/or is it possible to change the current? TIA |
| perieanuo:
that schematic is incomplete, those npn's wont open like that, so something is missing. or you have shorted transistor (the one you say it outputs cc) |
| charles.ouweland:
That's what I was thinking too, so I double checked. But then again, the bc-junction of T10 will conduct no problem and for T9: The 2N3565 specifies a Collector Cutoff Current (ICBO) of max 50nA under test conditions IE=0, VCB=25V and T=25ºC. That multiplied by T11's hFE of 200 gives 10µA, nowhere near 0.16mA. Remains a mystery to me. |
| charles.ouweland:
And here are the photos of the component side and solder side (mirrored) of the board, where you can clearly see that the top leg of both T9 and T10 are not connected to anything |
| fzabkar:
Measure the voltages. For a start, the base of T11 should be sitting at 1.8V below the 9V supply. |
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