Hewlett-Packard 1741A Calibration Frequency Drift

[El_Berto]

Greetings, first post.....

I have a 1741A and a couple frequency counters.  Just futzing around while reading the manuals I decide to plug the 1741A calibration posts to the counter. The documented frequency of the 1741A calibration posts is 1400 Hz. The counters tell me another story, both agreeing to something  like 1611.x Hz. Each counter using their internal clocks, as opposed to a shared external time base. On their internal tests, the counters are spot-on. 

There is a volt peak-to-peak adjustment in the 1741A but not a frequency adjustment, even then a +-14% offset just does not make sense. With a slow DMM the output is measured at 0.495v, which seems sort of normal since the signal  is a 50% duty cycle square wave. I am thinking that there is something going on inside the scope, i.e. some component(s) on it's way out.

Has anyone seen this in a 174x or similar scope? Is this frequency used internally for measurement results and if so what? Any suggestions would be welcome.

Thanks Dave. Cheers, Bert

[factory]

Did you see my reply on the HPAK group, instructions state the frequency is "approx" 1400Hz. It's used externally for checking/setting the probe compensation.

The calibrator circuit is on the A3 board, I would look at R100 (301k) & C47 (910pF), it's possible the Mica cap has lost a little value, due to silver migration problems that Mica caps are known for, or the resistor has gone high in value.
The output of U4B is used for other things in the scope, this is common with the analog storage scopes from HP.

David