Amplifier build adventures


jerry@tr2.com
 

All,

When I ordered the QCX+, I also bought the matching amp & case.

The build was mostly uneventful - Hans' instructions for winding the toroids were excellent. Some of the resistors were just a little long for the hole spacing, but it worked out.

As usual, I measured each part before installing. Not so much because I thought I'd find bad ones ( didn't find any bad ones ), but modern resistors can be hard to read, with all the little bands, pale colors, and different formats. ALSO,
measuring the parts forced me to slow down a bit and think as I worked - to help guarantee not putting the wrong part in the hole.

When it came time to slide the card into the case, life got "interesting". It's quite a little Chinese puzzle - you install the screws at THIS end, don't install them on THAT end - that lets you slide the card on - but since you haven't soldered the final transistors yet, you have to do this with everything upside down.

I couldn't find my electrical tape. Nowadays, I don't use it much - I mostly use shrink tubing or self-fusing silicone tape. Where did I leave that roll? I found a few scraps stuck to the workbench, but they didn't work very well. Too old and gooey, and the silicone pad didn't want to stick to it.

I thought I had it done. Soldered the transistors. Whups, forgot to clean and inspect the board! Gotta take it apart again! Unsolder? Nah - you just unscrew the screws and pull. The soldered transistors slide nicely over AND you don't have to do it upside down.

I must have taken that thing apart & put it together 10 times. The silicone pads kept coming off & sliding around. I abandoned the scraps of electrical tape and tried some Kapton tape. Nope, not sticky enough. Finally in desperation I tried some 3M packing tape - and first, I scrubbed the electrical tape residue off the heat sink and silicone pads with alcohol and Q-tips. OK, that worked.

I didn't install the supplied DC power connector. This is a 20V item, and I didn't want a 20V supply laying around with a connector matching all the 12V stuff.

Instead, I used an AMASS XT60 connector. This is a really elegant, tiny & cheap connector rated at 60A continuous. Sparkfun claims that they will tolerate 65A. They have a round gold-plated pin and a round receptacle. When you plug them together, they inspire confidence. I soldered a couple of short silicone-insulated wires to the circuit board, brought them out through the "DC Power" hole, and soldered them up to the XT60.

- Jerry KF6VB


ke9uw
 

Get a new roll of tape for the next one. Just a 1/16 of overlap on the insulators seems to work well with fresh tape.


jerry@tr2.com
 

There's a special tape I was thinking of using. It's up on the shelf somewhere.
It's the 3M double-sided sticky tape that they use to stick cell phones together.
SUPER aggressive. I bought a roll to fix one cell phone, and now I have enough
to last me a century or two.

But the packing tape worked.

- Jerry KF6VB

On 2021-06-03 17:51, ke9uw wrote:
Get a new roll of tape for the next one. Just a 1/16 of overlap on the
insulators seems to work well with fresh tape.
Links:
------
[1] https://groups.io/g/QRPLabs/message/66721
[2] https://groups.io/mt/83290275/243852
[3] https://groups.io/g/QRPLabs/post
[4] https://groups.io/g/QRPLabs/editsub/243852
[5] https://groups.io/g/QRPLabs/leave/10312280/243852/1190336630/xyzzy


Greg Walters
 

On Thu, Jun 3, 2021 at 02:13 PM, jerry@... wrote:
Instead, I used an AMASS XT60 connector. This is a really elegant, tiny & cheap connector rated at 60A continuous. Sparkfun claims that they will tolerate 65A. They have a round gold-plated pin and a round receptacle. When you plug them together, they inspire confidence. I soldered a couple of short silicone-insulated wires to the circuit board, brought them out through the "DC Power" hole, and soldered them up to the XT60.
That is a great idea, Jerry. I fly RC planes and such and use those XT60 all over those batteries. They are good connectors and can't be plugged in backwards, nor shorted (given that they were soldered correctly). I have used these connectors on other projects as well. 

I still have my QCX+ to build, and may consider doing the XT60 there as well. 
 
--
73,
Greg
KY4GW