HP-239A Oscillator Adjustment

[RichardM]

Hi All,

I have an HP-239A that works well and the THD measures low also. I thought I might try and calibrate the device as per the service manual section 5.11 (attached) but I am unable to get the frequency wwithin tolerance. At 10kHz I am always high and the best I can do is 10,069 Hz which is +- 10Hz as per specification. At 100kHz it is even worse and measures 92,296Hz. If I adjust C5 to match 100kHz my 10kHz becomes 10,916Hz.

Does anyone have experience in adjusting this device ? I note there is provision for an additional capacitor C45 (0-22pF) to adjust to 100kHz but that still won't solve my issue at 10kHz. I have no problems adjusting the gain per the manual.

Any suggestions welcome.

[wn1fju]

Perhaps one or more of the range capacitors or digit resistors in the frequency feedback network have drifted.  You could try to measure them in-circuit (with the power off) if suitable switch combinations can easily isolate them.

I just tried the adjustment on my unit.  Mine doesn't have the C45 padding capacitor installed.  By adjusting C5, I was able to get the two readings as 10,005 Hz and 99,890 Hz.  The values do change depending upon whether the aluminum cover plate is installed (about 5 Hz at 10K). 

[RichardM]

Thanks for the advice. C45 also isn't installed on my unit.