Tektronix DA4084 volt level problem

[kirby64]

Hi guys i got a tektronix DA4084 distortion analyzer this is the military version of the aa5001. I went thru the entire calibration procedure with no problems but for some reason when i measure anything above 13khz frequency the volt level reading accuracy starts dropping more than the + or - 4% stipulated on the specs once i make it to 20khz it only measures about half the value it supposed to read, once i go above 30khz all the way to 50 i have no reading any ideas where to start looking thanks

[alanep]

Hello Kirby64,

A couple of things:

1. What signal generator are you using for the freq. tests? I see in your previous posts that you were working on a HP8903, are you using that?

2. The DA4084 has a 30KHz filter. Is that activated? Try cycling through the filter buttons & see if it changes.

Beyond the above, it gets a bit harder to diagnose without more information.....

Just a bit of speculation:

There have been past instances with the AA5001 input attenuator reed relays misbehaving. I've also seen corrosion near where the wires enter the glass of input protection incandescent lamps. In both cases, one leg of the balanced input won't be internally connected through to its associated opamp circuitry. This will cause lots of problems including poor frequency response.

The filters are internally selected via a 4052 CMOS switch, perhaps a filter is stuck on either because the 4052 input selects are incorrect or the 4052 is faulty.

Because the AA5001/DA4084 module is plugged into a power frame, signal tracing/fault finding (with it powered) is a real pain without an extender. The alternative is to solder & run individual wires from the points of interest on the PCB.


Regards

Alan