3335A Output Level Fault

[veedub565]

So my recently acquired HP 3335A seems to have a problem. With output level set to 0dBm I get the following output levels

50MHz = -24dBm
25MHz = -30dBm
10MHz = -38dBm
1MHz = -58dBm

If I change the output level from 0dBm to -10dBm, all of those levels drop by exactly 10dB. So I'm templated to say the attenuator is working. If the attenuator was stuck then the output would be down by the same amount regardless of frequency.

Any idea's before I start delving into this one ?

**EDIT**
Just been and tried it again... working perfectly now, weird 

[bsdphk]

The 3336 uses a special dual thermal converter to level the output, and traditional step attenuator to increase the possible range.

If the 3335 is roughly similar design, then that dual thermal converter is your first suspect.

Check HPJ or Principles of Operation to find out how it is supposed to work ?

[veedub565]

I had left it plugged in (but not switched on) so maybe it warmed up a bit in between being miles out, and working perfectly.

I've got the service manual, I haven't gone through the principles of operation yet, just wondered if it sounded like a common problem, or others had similar issues.

It is an old instrument, so maybe just getting a bit temperamental. It's probably very nearly as old as I am, and I'm getting rather cranky and temperamental these days too

[veedub565]

Just tried it again and it's back to how it was. It's definitely got issues 

[BigBoss]

Seems to me a serial capacitor fault. Because the Output Level Decrement is inverse proportional to the Frequency.
A unseen crack of a ceramic cap may be a cause.

[veedub565]

I'll have a look around the levelling circuits then

Interestingly I tried it again today and initially it was faulty... and then started working properly again. Knocking it up and down the attenuator range a bit seemed to have an effect. I may write a little GPIB program to hammer it up and down the attenuator range, just in case there's some crud on the contacts to clear off.