NEScaf builder feedback

It has been reported that the relatively high input impedance of the NEScaf circuit can create audio level-matching issues with the RockMite. Specifically, the sidetone level appears quite a bit louder through the filter compared to the level without it. A simple fix is to lower the input impedance of the NEScaf by placing a low-value resistor (10-47 ohms) from the audio in terminal to ground. This will allow the rig feeding the NEScaf to see a more reasonable load.

(Thanks to Charlie, KE2SP for this tip).

Here's a copy of Chuck's post to the RockMite Yahoo group:

"Chuck,
The nescaf has a high input impedance, hence the great variation in sidetone level between when using it versus using the RM since the RM is set up to work into low inpedance headphones. If you put a low value resistor (say 22 ohms)across the input jack of the nescaf then all should be OK. the RM should then only need about 4.7 K in series with C8 and yield about the same sidetone level between using the RM plus nescaf versus using the RM alone.
Charlie ke2sp"

Has anyone an idea why the LED comes on when the usual stereo headphones are connected to the audio out jack, regardless of the on/off position of the NESCAF PWR switch? This is with a power supply connected and on of course. I confess I connected the SW1 to the power jack through the gnd instead of the Vcc wire to the circuit board.

Is there an adjustment I should do to R9? I have left it alone so far.

Thanks,

Bob Pendergast
N5JYW

I would assume (untested) that with the headphones connected, and Vcc always available to the lm386, that
you have found another way for ground to get back to
the battery, probably through the chassis

R9 is used in conjunction with R10 to set the center
frequency of the filter. Set the panel mounted pot to
its detent position, and adjust the board pot to the
offset center of the rig of choice.

/dave

Thanks for the advice, Dave. I also had some wiring problems of other sorts, but figured it all out-rationally and empirically--and the unit is working fine.

N5JYW

I completed the construction of my NEScaf filter, mounted into a Altoids Tin. Very nice audio and nice filtering. It worked fine for about 5 minutes and then it started ringing and then a very loud buzz or HASH sound. I rechecked all of my voltages on IC1, IC2, and IC3. Voltages appear okay. Does any one have recommendations where I might start checking?
73 K2CG
Chuck Gehring
I removed IC1 MF10CCN and checked the audio, everything sounds fine audio amp working. I replaced the MF10CCN and the "Crashing" hash sound returned. Should I order another MF10CCN? or continue to troubleshoot?

Hi Chuck.

I'd check a couple of basic things first. Be certain that none of your connections to the bandwidth and freqency pots have come loose. Check the power supply voltage. the MF10 can start oscillating if the power supply voltage is too low. Like with a weak battery? Also, check the output of the 555 clock chip. If you have a freq counter or scope, make sure it is oscillating properly (at 10x the center frequency desired) And of course check the soldering, and for any shorts to the case, etc.

GL, Bruce N1RX

I plugged a stereo ear phones into the mono output jack. Would this have caused a short and damaged my NEScaf?

73 K2CG
Chuck

Plugging stereo phones in a mono jack will not cause a short. Plugging mono phones into a stereo jack can cause a short. The mono plug has no 'ring' connection, so that portion of the plug shaft is tied to common, or ground.

Since the output of the audio amp is capacitor-coupled, this should not damage the NEScaf either way. But, it is best to use the same 'flavor' jack as plug. For stereo phones, use a stereo jack, and tie the tip and ring connections together.

In your previous post, you mentioned that the amp checked out OK with the MF10 removed. It doesn't sound like you damaged the amp at all.

-Bruce N1RX