Electronics > Repair

HP8566B unexpected INSTR CHECK 1

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Bill Woodbridge:
Does anyone know about / have experience of the behaviour of the INSTR CHECK I indicator while the 8566B is running please?

My 8566B powers-up perfectly – the two INSTR CHECK lights go off immediately - and every front panel function I’ve tried appears to work as expected (although there are plenty of more advanced features I haven’t tried of course).  There are no error or diagnostic messages in the display (once the oven has finished warming up).

But I’ve recently started playing with saving and recalling the instrument state.  I’m trying some FM slope demodulation at 97 MHz, so among other things I changed to zero span, fiddled with the resolution and video bandwidths, moved to linear and adjusted the reference level so I can then see baseband audio on the screen, like an oscilloscope.  Again, all of this appears to work perfectly.

But then when I saved these altered settings to memory ‘5’, as soon as I pressed the  ‘5’ key the INSTR CHECK 1 light came on.  Even so, the store and recall of the settings seemed successful, at least on the surface.  If I then reverted to the power-on default state with one of the FULL SPAN keys, the light went out again.  I could then recall ‘5’ which brought it back correctly to my demodulation settings, but again the light came on.

I’m writing all this in the past tense, because I then returned to the instrument a couple of days later for more experimentation.  I hadn’t left it in standby but the battery had preserved the demod settings correctly on ‘5’, which still brought up the INSTR CHECK 1 light as soon as it was recalled.  But I then set the same settings up again from scratch and managed to store them on ‘4’ without the light coming on.  I then stored the new version on ‘5’ (ie overwrote the old ‘5’) and the light stayed off.  (I know I should have tried saving the old '5' to another location first and see if the light followed it, but unfortunately I didn't).  And after about another 10 minutes of intensively bouncing various setups between all the memory locations 1-6 with SAVE and RECALL, I couldn’t reproduce the light coming on.

I’m asking because I’ve only seen references to the INSTR CHECK lights in the context of the power-up self tests, not suddenly coming on ‘in flight’ for any reason.  The service manual indicates that INSTR CHECK 1 is mainly about problems with the digital storage, which seems an unlikely place for instrument settings to be stored.

If anyone else has seen this sort of behaviour, I’d be very interested to know please.

py-bb:
I've got most of a HP 5420A I'm repairing for a client (missing interconnect cables and very obsolete.... not sure what to do with it)

If this would help (comparing between the two or whatever) please let me know.

I also "know computers" and have a very very strong familiarity with a few HP things a bit later on (3562A, 163x, 1650, 16500, several protocol analysers) - as in their "computer side".

Standing by.


By computer side I mean if you split it internally into power supply, computer/control and "analogue bit" - I know the first two well.

Bill Woodbridge:
Many thanks for the offer, it’s very kind.  But I can’t actually think of a relevant comparison to suggest, as I’m not sure there’s much in common between the 5420A and the 8566B apart from the fact they’re both split into two interconnected units.  The 5420A is an all-digital signal analyser to 25kHz, whereas the 8566 is an analogue (until you get to the display side of things) spectrum analyser to 22GHz.

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