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  • Author Author: sverk
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Repairing a HP5308A counter: Where find help/forums/discussions?

sverk

Hi, this is probaly not the right forum, I'll be grateful for hints on where to go, in element 14 or some other group/forum:

As surplus from my old job I have an old HP5308A counter/timer that does'nt work and I'd like to fix it.
I have manual and schematic for the HP5308A part of the instrument, and
testing around it seems the problem is that the 10 MHz clock signal is dead.

Unfortunately, the clock is generated in the top section, HP5300B, the measuring unit,
and for that I have not been able to find an manual or schematic.

Any help appreciated,
Sverker

  • in reply to sverk

    Hi Sverker,

     

    That's great that it is confirmed to work with the external clock! You have a few options, one could be to probe around the internal oscillator to see if there actually is no oscillation at the buffered output (using datasheets to see the pinouts of the logic gates) or if the INT/EXT switch is faulty.

     

    If the oscillator itself is faulty then you could try to replace some of the logic or the crystal, in case one of these is faulty; it should be a cheap repair.

     

    The TCXO option entails finding where the oscillator signal travels, isolating the source and then using a TCXO (see here http://uk.farnell.com/c/crystals-oscillators/oscillators/temperature-compensated-tcxo-oscillators  ) instead. You may need to solder it all on a little prototyping board perhaps, and also possibly level-convert from the 3.3V output of the TCXO to the supply voltage that the rest of the circuit is using (likely 5V) unless the upstream circuit can handle the 3.3V logic output. Also, make sure the frequency is correct, sometimes a different frequency (e.g. 5MHz) is used internally,  even though the external reference needs to be 10MHz.

  • in reply to shabaz

    Hi shabaz,

    Your advice was just right !
    When I took the two halves of the instrument apart, sure there is an OScillator INT/EXT switch, and an OSC-BNC connector on the back panel.

    With a simple function generator (a UDB1210S) at 10 MHz plugged in, the instrument came alive and is working all right.  Great!
    You mention using a TCXO to fix this permanently, how could I do that?  I have no knowlege about TCXOs, are there any examples of installing it available?

    The internal osc is crystal discrete comp type on the board.  This cute 75 MHz timer/counter is of 1965 vintage, built mostly with SN74S-series chips.

    Further advice appreciated,

    Thanks a lot,

    Sverker

  • Hi Sverker,

     

    I've no idea about this particular device, but many of the old counters just had a discrete or logic gate based crystal based clock source in their base model, unless one paid for something better like an ovenized crystal oscillator as an option.

     

    If it isn't functioning, I'd be tempted to overhaul that part of the design completely, perhaps replace it with a TCXO (cheap) or ovenized (a lot more expensive, might make it not worthwhile).

    If you can, you could test if the rest of it works by feeding an external 10MHz clock if there is an input for that.