Question about HP 8640b Signal Generator Knob Set

2 Answers

Have a problem here. Generator, which is in good shape and cal, suddenly started to drop out RF output after warm up of about 10 minutes. The level meter DOES NOT drop out when this happens. After some initial success with tighening the output micro connector from the 003 option module which was way loose, it happen again. So, having a spare set of modules, I replaced the 003 module. Again, it worked for a while and then quit - same symptoms. Now I just was trying things and found that I can get the output to come back by just slightly rocking the band switch. Then I tried rocking the switch slightly to the side and leaving it there. Sure enough, the output stays on. I have a switch contact problem somewhere, but don't know where. I know that the band switch predominantly switches the ECL logic on the divider section, but am puzzeled as to why the output stays up on the meter (which is detected after the hybrid output module and right before the attenuator) indicating that output is there after the hybrid. Is there a output muting contact somewhere on the band switch that is killing the output during switching (and intermittantly when ?dirty?) ?? I do not see it on the block diagram. Thanks for your help on this.........Puzzled in Texas......Bill

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  • wcarns832 Aug 02, 2010

    Some good general advice here, but obviously not familiar with the HP8640B. This is a very complex instrument. Replacing all the switches/contacts and checking all the not applicable things suggested would be a nightmare and way unproductive. If you are not familiar with the 8640B, then you can not help me. I am a retired EE, MS in EE and Physics and a very capable repair engineer. I need to know where this is.

  • wcarns832 Aug 03, 2010

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  • Master
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Hi Bill,

My experience of HP gear is limited, so bear with me.

A previous generator that I used, original 8940, did have the "muting" function as you described. Per Agilent, it is a design feature to prevent an unloaded output (mismatch) unless specifically configured. I never was able to get that deep into mine, as it was complete with maintenance agreement (gotta love off lease equipment!), and was serviced by Agilent Tech. Services. I also seem to remember an output disable function if the external reference (10MHz) was lost or drifted too far.

Out of curiosity, is your generator also configured with the option for Automated Testing (ie assembly line)? I forget what the option was for it, but it was available with rackmount kit and moved the outputs to the rear (2 sets of outputs total) and each set was electrically selectable / enabled via GPIB.

Posted on Aug 03, 2010

  • wcarns832 Aug 03, 2010

    Thanks for the reply and the info. No, no automatic test function on the 8640B.......Don't I wish.

    I will continue to pursue the mute thing. Something is shutting off the output after the output hybrid and detector diode for leveling feedback. Not much after that.



    I am going to explore more this AM.



    Bill

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  • HP Master
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If it were me i would simply replace ALL switches/contacts on spec, I would imagine you have some sort of build up on the switch contacts, this may be acting as a resistor or maybe even a crude Diode... or as you suggest perhaps a contact that's not quite solid, it's making contact OK, but at a higher resistance than it should be, IE: It's acting like a series resistor? thus altering the current characteristics?
Also check your power supply for AC/RF ripple, check ALL electrolytic caps nearby. Also all RF filtering, and coupling/decoupling components.
Also check out ALL decoupling caps on the IC's you may find you have a leaky one?
It seems to me that something is damaging that 003 module, this is evidenced by the fact, that it was fine, then deteriorated,. I would imagine you have something that is slightly damaging, or causing to go out of spec, ECL is very susceptible to earth eddies, so ensure ALL earth connections are solid too. Spray ALL connections with a good contact cleaner and ensure ALL contacts are good and tight.
As for the meter not indicating as it should, there may be some sort of back feed to this meter, through a faulty decoupling decoupling component, this then would explain why it is doing what it does.
Hope this may help.

Posted on Aug 02, 2010

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