BK 4040 Ext. Counter Repair

[watchmaker]

 BK Precision 4040 schematic 1.pdf (930.65 kB - downloaded 41 times.)My 4040 does great as a frequency generator.

However, there is an issue with the external counter function.  Square wave counts match my Siglent scope.  Triangle and sine waves are all over the map.

I pulled the schematic from BAMA, attached.

It looks to me like there are only two things differnt between the external input and the internal frequency counter.  One is U117B which is an 74L500.  The other is a 10K resistor, R210.

When I open it the first thing I will check is the value of that resistor.  If not it looks like I should replace the NAND gate. 

Do you see anything else?

Regards,

Dewey

[floobydust]

U117 is switching in either the internal or external signal to the freq. counter. So on EXT it will naturally pickup noise and not read the generator signal, aside from using the timebase.
I think EXT selects the analog front end (JFET Q108, Q109, U116 etc.) to U117A. I would check signal and DC voltages in that subcircuit if it is not working. Check C136 47uF has the right polarity, if the JFET pre-amp has failed it might go reverse.
J115 not sure where it goes but must be to EXT BNC.

[watchmaker]

U117 is switching in either the internal or external signal to the freq. counter. So on EXT it will naturally pickup noise and not read the generator signal, aside from using the timebase.
I think EXT selects the analog front end (JFET Q108, Q109, U116 etc.) to U117A. I would check signal and DC voltages in that subcircuit if it is not working. Check C136 47uF has the right polarity, if the JFET pre-amp has failed it might go reverse.
J115 not sure where it goes but must be to EXT BNC.

Thank you. Will follow up when I do the deed.

Regards,

Dewey

[watchmaker]

Decided to trace it starting from the source.  Turns out it was the cable at the BNC connector.  The pin is too short/small.  It does need calibration though.  Can be 10% off below 2500Hz on other than square wave.

But w/ the scope it is not worth sending out for calibration.  Will look and see if there is anything, but I did not see any trimmers for the IC.  So that may be up next after all.

Still need to look at all the pins on the LS00 and check the resistor.

I can always measure the sq wave from a generator and presume the freq is stable on other waveforms, but what fun is that.  In fact, not sure why this matters at all to me.

Just me being OCD.

Regards,

Dewey