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xrk971 Pocket Class A Headamp GB

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#121
dwpeterson said:
Thanks for the quick reply. I've got decent ventilation for my range. Pb isn't a problem when my wife is away...
Click to expand...

I am using a 6in square pan that looks like this on a $10 electric hotplate. Works very well - takes 2 minutes, you can see that the middle of the board liquifies first and it spreads out. Within 1 minute the rest of the board is shiny silver solder. I remove the pan and place it on wet paper towels to quickly quench heat and limit exposure parts in case I overheated.

14286916


Hot plate looks basically like this:

4dce3995-b00a-48b0-b615-94cd364226b3_1.4b6066465716fbf1422b8deb4c3ca250.jpeg
 
#122
REFLOWING

Good setup. I was going to get a that of type pan instead of a fry pan with 2-3" sides. I was also thinking about putting the PCB into an Altoids tin to make it easier to remove from the pan and put into a pan with water to cool. There are probably many ways to do this and cool this without encountering painful rework. I've also considered using a gas chromatograph (20K$ oven at my place of employment) to do this, but this oven doesn't permit the operator to view the part and make the decision. I'll do the experiment to see if this chromatographic oven will match the profile recommended by Kester (EP256). The Kester thermal profile looks like ambient (25C) to 150C in 90 seconds (87 degrees C/minute; then 150 to 180 degrees at 20 degrees/minute; then 180 to 230 at 75 degrees/min; the 230 to 150 at ~-70 degrees/minute and presumably down to ambient.
 
#123
Don't overthink it. I tried it using a simple square aluminum cup like container from a medicine packaging. It worked ok. The square skillet worked much better - more even. The only problem I had was one of the four parallel resistors got twisted because it got pulled to one side since that side melted first and surface tension pulled it. So keep paste neat at four parallel resistors and don't use excessive paste there. It was easy to rework with an iron. I have had luck making 6 boards this way. No damage to parts. Once I quench the pan on wet paper towels you can pick up the board by hand on the edges in one minute.
 
#124
How to drill those holes.

I'm drilling those holes in the tin. It's tricky to do right using a drill as the bit grabs and can move the hole center. I have ended up with 3 bad tins because one of the holes keeps drifting during drilling. I use center punch and pilot hole bit. Then successively larger drill bits. Still not perfect. I did a search for CMOY hole methods. Looks like center punch then pilot hole then either Uni-bit or Dremel grinder, or tapered hand reamer. Normal drill bits don't work. Don't forget to put a wooden block behind the metal being drilled.
 
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#127
#137
I just saw this on Amazon

https://www.amazon.com/gp/aw/d/B00NPZDRD4/

These are machined brass with set screw:

https://www.amazon.com/gp/aw/d/B009DAHPLU/

Machined stainless steel:

https://www.amazon.com/gp/aw/d/B00AO73MFO/

Maybe a class A amp deserves a nice machined brass or stainless knob as opposed to a aluminum plated plastic knob? 🙂

Just be careful of max diameter to fit in between the jacks - especially if your headphone jack is thicker than normal. I just found out the nice machined knobs above are 18mm dia - probably too large.
 
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