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Looking for the serive manual of an HP 70951 optical spectrum analyzer

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_Wim_:

--- Quote from: nctnico on June 24, 2023, 01:34:05 pm ---Isn't there are similar machine from HP from the same era for which the service manual exists? It is likely the electronics are similar enough to get an idea of how ranges are controlled and how the amplifier works.

--- End quote ---

I do not think the was a predecessor to the 70950/51 series. Later models also do not have a service manual. I will try to retrace part of the circuit.   

_Wim_:
Today I did the following:
- Made a short board extender out of some IDC pin headers. At least now a big part of the circuit is accessible to probe
- I tried to use the transimpedance amplifier external input with a current source, could not get it to work (this was before I started the reverse engineering)
- I noticed the analog output is also broken (is stuck continuously around 8.9V). Currently I did not investigate further.
- Tested the voltage rails in circuit, +15 and -15V are fine
- In power meter mode, optical signals below -44dBm work fine, signals above to not work. I suspects this is similar in OSA mode, but it is a bit more complex as it depends on RBW and some other settings
- Started to reverse engineer the board (see attached drawing were I am so far)

Whenever K401 is in the "1" position (so using the "low signal" gain stage), the OSA works fine. When K401 is in the "0" position, no signal is seen. On U407 I see around 0.7V between + and - inputs, and the output of the opamp (AD797) is around 13.7 V . So I tried to replace U407, but got exactly the same result (was replaced by an AD829 I got on hand).

I do not understand the Q403 part of the circuit. There is another one of these on the board, and it measures differently in that position, so this part could be broken. I however do not understand how this could be a plain transistor, although the other one does seems to measure like one. If anyone recognizes Q403, please let me know.

nctnico:
Q403 could be a 2N7002 mosfet. It would be a good choice as a gain selector and a mosfet can be driven from a logic output directly (without a series resistor).

_Wim_:

--- Quote from: nctnico on June 25, 2023, 07:49:26 pm ---Q403 could be a 2N7002 mosfet. It would be a good choice as a gain selector and a mosfet can be driven from a logic output directly (without a series resistor).

--- End quote ---

I agree a FET would make more sense, but I measure diode junction voltages like typically seen for a transistor (from base to emitter and base to collector). On Q403 I also measure junction voltages in reverse (and only 0.17V) which made me think that it is possibly broken.

I did remove Q403 (I should have measured it out of circuit also, but did not yet do so) but this still did not fix U407 continuously hitting its rail.

_Wim_:
One more piece of information, K402 is most of the time in its "0" position (connector to GND) unless the reference level is set very low (below -50dbm if I remember correctly). At that point K401 also switches to its "0" position.

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