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

[_Wim_]

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).

I think you are correct. I now start to doubt myself about the measured junction voltages, I will hopefully have some more time tomorrow evening. I will short the pads for low gain, this should then at least show a signal, albeit too low.

[nctnico]

Removing Q403 would be a good idea. If it is broken, it could inject enough DC to overdrive the opamp to get it stuck to a rail.

[_Wim_]

Removing Q403 would be a good idea. If it is broken, it could inject enough DC to overdrive the opamp to get it stuck to a rail.

That was indeed what was happening. However U407 was also broken. After I replaced U407 and got the same result (railed to the positive rail) I wrongly assumed nothing was wrong with U407 and reinstalled the original part  Afterwards I removed Q403, this did not fix it also.

However both Q403 and U407 were broken! By removing Q403  and shorting Q403 to the lowest gain setting I now have a signal on lower sensitivity settings also. Off course some gain switching is not correct, I have some 2N7002 on order, as I did not have any suitable FET in stock.

But certainly good progress! Will off course also reinstall an AD797 for U407.

[_Wim_]

A short update:
- Q403 has now been replaced by a 2N7002, this seems to work correctly, although I do not see much difference between the temporary short and with the 2N7002 in place (which is normal I guess, as it only adapts the gain slightly)
- broken analog output: is now fixed, this was also caused by a damaged AD797BR opamp (U105)
- non working transimpedance input: this was a user error, I did not configure the OSA correctly to use the external transimpedance input
- replaced U401 and U402 again by the original TL072, temperature went again above 100°C, which I guess is than normal (absolute max is 150°C according to the datasheet)

TODO's:
- verify all functionality
- find a keypad (not urgent as currently I am using the rubber buttons and a screenshot of the keypad)

[_Wim_]

Some further updates that might be useful for others in the future:

- KE5FX 7470 plotter emulated works mostly correct to take screenshots (using an agilent 82357 GPIB usb adaptor)

Some information is the plotter screenshot is not correct or missing:
- Start,center and stop wavelengths are missing
- RBW is missing
- Video averaging number is correct, but is also shown when video averaging is off
- screenshot reference level is at top of display, while on the 70951 this is at the 9th graticule.

I have also done wavelength and power calibration using a 1550nm single mode laser. Wavelength was off by 0.6nm, power was off by 1.5dbm, but for power I am not really sure about the accuracy of my calibration. For wavelength I verified the result using an Acetylene reference cell (installed in the Anritsu MS9710C), and the result was bang on.

[_Wim_]

TODO's:
- find a keypad (not urgent as currently I am using the rubber buttons and a screenshot of the keypad)

In the spirit of DIY, I deceived to make a 3D printed one myself. Not as nice as an original, but it works fine and is much easier to use than the rubber buttons. Attached the 3D design files should any-one be in a similar situation in the future.

[_Wim_]

I have made some improvements to the 3D panel file to ensure the panel is more rigid, and to allow some additional travel for the buttons. The updated files are added in the post above.

[_Wim_]

Another update:
- I tested all functionality as it seems everything is working now 
- As suggested in this blog (https://medium.com/@HP70K/70004a-display-screen-capture-43f7954d32b), I bought an Epiphan VGA2USB LR to allow me to easily take screenshots with all data (as the KE5FX 7470 plotter is missing some info). Attached an example when I was characterizing 2 Thorlabs  F240FC-980 collimators
- I routed the external input and outputs to the front for easily accessibility (see shelf picture)
- The OSA is now installed on my shelf with all the other optical test gear

This will be a very useful additional characterizing unknown optical parts bought on Ebay or when reverse engineering industrial optical analyzers.

If somebody would still come across the service manual, still interested to have a read through it. Also very interested in any software applications that work together with this OSA or functional that can be installed on the OSA via "measurement personalities".

[smaultre]

Hello _wim_

Just received HP 71450B  -002 option White Light source module (separately).

I have OSA Agilent 86146B (not tested) and would like to test it with light source.

Additionally i have an Anritsu MT9810A optical test set with one power sensor module.

I would like to check the power of mine White Light source module.
And check Agilent 86146B with them.

Can you provide the spectrum of your built-in light source? I would like to compare with my.

[_Wim_]

Hello _wim_

Just received HP 71450B  -002 option White Light source module (separately).

I have OSA Agilent 86146B (not tested) and would like to test it with light source.

Additionally i have an Anritsu MT9810A optical test set with one power sensor module.

I would like to check the power of mine White Light source module.
And check Agilent 86146B with them.

Can you provide the spectrum of your built-in light source? I would like to compare with my.

Here you go. Please also post yours, always good to have a reference.

[_Wim_]

It does seem mine does not meet its spec any longer. According to the datasheet it should produce -50dbm/nm, but is only achieves -55dbm/nm...

[smaultre]

I have now only on dB measurments.

[_Wim_]

Hi Smaultre, when measuring light with an optical power meter, you always measure the "broadband" power. The only thing the wavelength setting does is adjust the correction factor of the photo diode to convert to power for that specific wavelength, but it is not possible to measure the power of a specific wavelength from a broadband source. 

When I connect the HP 70951 light source to a VIS-power meter, I get -20.4dBm, when I connect it to a NIR power meter, I get -8.2dBm (both set to a wavelength of 850nm). This means it also makes it hard to compare results, as the sensitivity spectrum of the photo diode is different for different power meters. As you measure around -9dBm, I expect you are also using a NIR power meter (InGaAs sensor or Ge), but this does certainly not mean your source has a slightly lower output.