Repairing a NEScaf filter

Some years ago I built a NEScaf switched capacitive audio filter for my QRP station. This little gem is produced and sold by the New England QRP Club and has to be one of the most useful pieces of gear any QRP operator could want.

About six months ago I must have applied power to the filter incorrectly because I heard a popping sound and it stopped working. I remembered instantly that there is no reverse polarity protection. I´d have to repair the filter when I got a chance.

When next at my local electronics store, I bought a new 78L09 9v regulator and an LM386 as I figured one of these would probably need replacing. The heart of the kit, the MP100 IC chip, is not available locally so I could not pick one of those up as a spare while I was there.

When I got back to the shack, I opened the NEScafś enclosure, unsoldered all the connecting leads and removed the PCB.

The cleaned-up enclosure once the PCB had been removed.

I then removed the dead 89L09. It was then that I noticed that the replacement I had bought would not fit as it was the TO-220 type and not the smaller, neater TO-92 type. So it was back to the shop I drove, only to discover that the TO-220 was the only one they stocked. There would be nothing for it but to make a plan and mount the regulator onto the back bulkhead of the enclosure, with leads soldered onto the three legs going to the original holes in the PCB. The end result worked fine business.

The new voltage regulator mounted onto the enclosure.

Once that was done, I re-soldered new connecting wires to the PCB (boy, there are a lot of them!) and dropped the PCB back into the enclosure.

Looks like a mess with all the loose wires waiting to be connected to the right places.

Then it was a matter of working out what wires went where and soldering them into place. The instructions I found to be rather vague. For instance, when wiring up the dual gang 50K Ohm potentiometer, the instructions say to take care that the wiper connections go to the centre pins of the pots, and while looking at the front of the pot, connect the right tab of each pot to the correct circuit board location labelled W (for wiper). I eventually worked out that the word right  should have been the word correct. I also took ages to work out the correct way to wire up the DPDT switch so that the filter could be hooked up to my Hendricks PFR-3 QRP rig and switched in or out from the front paned of the NEScaf enclosure. I eventually figured it out, completed the soldering and then tidied it all up by binding the wires together nicely with short lengths of dental floss so that it all looks neat and tidy.

The wiring all neat and tidy thanks to dental floss.

My NEScaf filter is now fully operational again, just in time for the upcoming COCQ QRP Day Contest on 3 September.


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