P.E. LCF Meter Modifications

This project caught my attention as a replacement for a rather old and large Marconi LCR bridge, having purchased the pcb and obtained all the parts plus programming the PIC from the downloaded file at the P.E. web site the meter worked reasonably well, however some inaccuracies with capacitance measurements were found. After some research it was traced to the type of oscillator and cmos IC. I designed some modifications and sent full details to P.E however they did not publish, so I am publishing them here for anyone who may have had similar problems. This project is now completed, the machining of the LCD and main front panels was made very easy by the use of a Proxxon PF230 Mill/Drill.

I liked the idea of a LCF meter, especially the L part as this means more workshop space now that the Marconi Bridge can go. The L parts of the LCF meter works fine but the C part has a problem in the shape of spurious high frequency oscillations. The added 68pf referred to in the text file on your download site stopped the high frequency oscillations but it still started up in high frequency mode after changing Cx (the unknown capacitor), this required the meter to be turned off and on again to enable the checking of another Cx.

From the notes about problems I purchased the HEF version of the 4011, better, but still needs the 68pf. After checking low and high values of Cx I noticed value errors, using the correction factor helped but the errors were plus on low values and minus on high values of
Cx. It seems that the 68pf was changing the frequency from the straight line at low values of Cx.

I looked at the use of an TL072 op-amp however it would not work at the high frequency end. Using a 74HC132 for the capacitance
oscillator produced results which were near to actual values for low and high values of Cx. The note about problems stated that the HCT device could not be used as a replacement because it would stop the L part of the meter working properly.

To allow for the different frequency produced by the HC or HCT devices I adjusted the frequency to the values expected
from the 4011. Thus no modifications are required for the software.

Keeping with the 1n capacitor for C and using a larger R makes the meter
work fine at low and high Cx values.
To enable the modifications to be easily implemented I used a long wire wrap
pin socket for IC3 soldered thru a prototype board with the 74HC132 along side.
The whole assembly then plugs into the original socket for IC3.

Before plugging in the add-on modification unplug IC3 4011 and plug it into the add-on board. Remove R3, R4 and C7, this will make IC3 pin 5 free to be used by the add-on board (see schematic). Remove the wires from SW2 pins 2 and 6 and reconnect them to the add-on board.

If the additional 68p capacitor has been fitted then remove it.

Schematic and photo attached.

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