Author Topic: Looking for the serive manual of an HP 70951 optical spectrum analyzer  (Read 6118 times)

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Offline _Wim_Topic starter

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I know it is a long shot, but does someone here has a service manual for the HP 70950/51 optical spectrum analyzer? Together with the 70004 display module, this combination is also called HP 71450/51. I already have the "installation & verification manual", "quick start guide", "user guide", "reference manual" , "programming manual" and "quick reference guide".

According to the reference guide (see attached screenshot) a service manual has existed. I already looked on the usual places and googled quite a bit, but so far no luck. If the manual is somewhere for sale, that would also be an option for me.

My unit has the following error I am trying to fix: "Cal Failed (2C) ERR 7055". If any-one knows more detail about the "2C" hex code, that would also already be very helpful!

Thanks for looking,

Wim
 

Offline zrq

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Although very unlikely with the service manual, we have a similar HP OSA at work, so I hope I can answer some questions.
Does it scan normally? These grating based OSAs have moving parts, so inherently have limited lifetime. The oldest Yokogawas we have at work makes screaming noise when scanning and from time to time aborts with monochromator errors. Good luck.
 

Offline _Wim_Topic starter

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I have been doing various tests this evening, and the monochromator part seems fine I think (my unit has the monochromator out option, so I can use it as an optical "preselector", and this works fine for the optical signals I could test with).

There is at least an issue with the transimpedance amplifier, as the signal comes and goes depending on the sensitivity setting, so probably some gains do not work any longer (could be a faulty COTO relay, will try to check tomorrow).

Attached some pictures on the transimpedance section. If not the COTO relays, the DG441 cmos switches would be my next guess...
 

Offline _Wim_Topic starter

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Forgot to mention, I also replaced 2 opamps on the X-Y control board (U401 and U402) because they were running quite hot (>100°C). After the replacement, U401 dropped to about 85°C, and U402 to 60°C. I did however not have any TL072 on hand, so I replaced them with AD712 as a temporary solution, so this might also explain the temperature difference. Still, this board remains suspect also, but I first want to concentrate on the transimpedance section, because debugging is much easier when a signal is present.
 

Offline _Wim_Topic starter

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This morning I checked both the COTO relays via their driver (74ACT244), and they were fine. The two analog switches (U400 and U402) however both had their port 4 damaged (other ports are working fine). As I do not have any ADG441 cmos-switches in stock, I will have to wait on the parts to continue the repair.
« Last Edit: June 18, 2023, 07:02:18 am by _Wim_ »
 

Offline _Wim_Topic starter

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The HP journal 44 no. 6 has some very interesting articles about these optical analyzers:

* Optical spectrum analyzers with high dynamic range and excellent input sensitivity
* A double-pass monochromator for wavelength selection in an optical spectrum analyzer
* A high-resolution direct-drive diffraction grating rotation system
* A two-axis micro positioner for optical fiber alignment
 

Offline _Wim_Topic starter

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Offline _Wim_Topic starter

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This morning I checked both the COTO relays via their driver (74ACT244), and they were fine. The two analog switches (U400 and U402) however both had their port 4 damaged (other ports are working fine). As I do not have any ADG441 cmos-switches in stock, I will have to wait on the parts to continue the repair.

The ADG441 parts arrived, but unfortunately this did not fix the issue. Probably some more parts are damaged in this area. As the HP70951 allows you to connect and external signal to the transimpedance amplifier, this probably damaged this board, so I will check what is in series with those damaged ADG441 ports.
 

Online nctnico

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Opamps running hot may also be caused by a missing power supply rail. I see lots of tantalum capacitors. I'd measure them all to see none have formed a short circuit.
The reed relais on the board may also have become sticky.
« Last Edit: June 24, 2023, 11:14:10 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline _Wim_Topic starter

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Opamps running hot may also be caused by a missing power supply rail. I see lots of tantalum capacitors. I'd measure them all to see none have formed a short circuit.
The reed relais on the board may also have become sticky.

Thanks for the tips. The relais were tested before and are ok, and I have verified that the rails are not shorted to ground with a multimeter. However, testing all the rails when the board is powered is harder, due to the way it is installed in the module. I did check all rails of the 70004 main frame, and these were fine (and no overload light blinking).

I also verified that whenever channel 4 is not selected on the DG441, I get an actual signal on the display, so this means whatever is broken on the amplifier board, must be wired to channel 4 only.

Hopefully tomorrow I have some time in the lab to further test what is wrong.
 

Online nctnico

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I'd be carefull with saying those reed relays are OK. I had an HP function generator and after a while (with the internal temperature rising) the relays started to act up causing all kinds of weird behaviour. It took me quite a while to figure out the cause was in the relays because they tested just fine.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline _Wim_Topic starter

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I'd be carefull with saying those reed relays are OK. I had an HP function generator and after a while (with the internal temperature rising) the relays started to act up causing all kinds of weird behaviour. It took me quite a while to figure out the cause was in the relays because they tested just fine.

I agree that there could also be and inconsistency issue with them, but when I tested them (cycling each approx. 5 times) they worked flawlessly, and the issue the 70951 has is very consistent (does not change when just started up to hours later).

I get only a signal on high sensitivity settings, and that signal can be partly blanked-out when "auto-ranging" is "on" (so when it tries to switch the gain in the middle of the sweep). This auto-ranging is presumably done with the solid state switches, although I still need to verify if that is the case.
 

Offline _Wim_Topic starter

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On lower sensitivity settings I get something that looks like this (so essentially equal to the blanked out section above, but then for the entire spectrum). So it really seems the unit is unhappy with "strong" signals, indicated it might have been severely overloaded in the past
 

Offline zrq

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I'd be a bit surprised if it was an overload that caused the problem. The photodiode will likely die before the electronics.
 

Online nctnico

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Can you burn out an optical attenuator? I suppose there are some mechanically operated optical attenuators somewhere in the signal path. And with the device working for low level signals, it could be these are switched in / out of the path. I'd check whether these attenuators still work.
« Last Edit: June 24, 2023, 01:22:50 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline _Wim_Topic starter

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I'd be a bit surprised if it was an overload that caused the problem. The photodiode will likely die before the electronics.

One of the read relays selects between the internal photodiode (including initial amplification) and the external photodiode input. I suspect some previous owner overloaded the external photo diode input (max 10V and max -10mA).The internal photodiode would not be damaged is this case, as the reed relay provides sufficient isolation. This would also explain why the very sensitive amplification was not damaged (external signal is only injected after this part) but some parts of the following sections are damaged. Off course, this is just a theory, and a schematic of this amplifier section would be very handy!

Worst case I could brute force and replace all active components in the transimpedance section, but for now I still want to find out what was exactly damaged.
 

Offline _Wim_Topic starter

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Can you burn out an optical attenuator? I suppose there are some mechanically operated optical attenuators somewhere in the signal path. And with the device working for low level signals, it could be these are switched in / out of the path. I'd check whether these attenuators still work.

As far as I can see (and also browsing through the manual) I do not think this OSA has an optical attenuator fitted.
 

Online nctnico

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I just checked the specs and the maximum input level is +20dBm which is far more than I expected. So yes, the problem could be in transimpedance section. I'd try to retrace the schematic and make more measurements before going for a brute force replacement of random parts. IF you can't find the service manual. 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.
« Last Edit: June 24, 2023, 01:36:38 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline zrq

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I have never seen an OSA with optical attenuator.
Is it possible to manually inject a current and see how the TIA respond in different ranges? (maybe you have already tried this)
 
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Offline _Wim_Topic starter

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I have never seen an OSA with optical attenuator.

They do exist, my other OSA (Anritsu MS9710C) has one built in, it allows to increase the range from +10dBm to +23dBm.

Is it possible to manually inject a current and see how the TIA respond in different ranges? (maybe you have already tried this)

That is indeed my next plan (currently not in the lab). I plan to use the keithley 220 in program mode continuously stepping between 2 currents. This way is should be easier to trace where the actual signal is. Ideally however I would have a board extender, as currently probing is not easily possible (apart from the IC towards the top, it means pulling the board out, making the connections with micro grabbers, reinstalling the board (and hoping the grabbers to not disconnect), measuring, repeat all over again...
 

Offline _Wim_Topic starter

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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.

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.   

 

Offline _Wim_Topic starter

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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.
 

Online nctnico

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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).
« Last Edit: June 25, 2023, 07:55:53 pm by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 
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Offline _Wim_Topic starter

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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 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.
 

Offline _Wim_Topic starter

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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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