Board for WA1GFZ MOSFET Audio Driver

[W1RKW]

John, Good find on the soldering iron. 65w should be perfect for this building job.

Your plan is similar to mine with tapping, do that first before building the board. Some careful attention to detail is needed here.

And concur on the filter cap arrangement.  I will consider my options based on the email conversations with you and Frank. One of my issues will be how I'm going to shoehorn the board and heatsink into my modulator chassis.

One step at a time I guess.
 

[W1RKW]

Received all my parts.  Ordered them Saturday and they were in my mailbox on Monday, what a surprise.  Heatsink is on the way.  Should have that by end of this week and then I'll start drilling and blasting. 

[w9jsw]

I have 2 boards so will use one for the drilling and tapping and decided to go populate the other one. All parts fit just right as ordered from the BOM at Digikey. Agree with Frank that I should have made all of the R pads larger. Everything is fine but larger would have been better.

Here is a pic of all of the parts placed except for diodes and fets.

John

[KC2ZFA]

just placed the order, everything from digikey...who cleaned out all the RES 50K OHM 10W 5% AXIAL ?
on backorder, estimated available 5/10...

John or anyone else who's building this, could you document the process of adding the heat sink ? I also
bought the one from eBay mentioned above.

Peter

[W1RKW]

Peter,
Allied Electronics has plenty of the 50kohm resistors.

[KC2ZFA]

yeah, but I got careless with the digikey online software that sucked
in the BOM...I can wait, should be done grading a mountain of exams
around that time.

[W1RKW]

I had the same issue and unfortunately, I will end up with an extra set of resistors.  If I recall correctly, Digikey said the lead time was mid to end of May. I figured I didn't want to wait.

As far as prepping the heatsink, my plan is the following:

I'm going to tape the PC board to the heatsink.  I will carefully align it so I can minimize drilling into the heatsink fins.

Center punch the holes carefully to ensure they are well centered.

Remove the PC board then drill pilot holes. I'm going to use a drill press to make sure the holes are perpendicular to the heatsink surface.

Gradually increase drill hole size using the drill press to the desired size then  carefully tap the holes.

[w9jsw]

I am going to try something new. I had a guy suggest to use an end mill to cut the holes. Lots less flex. I will use one to pilot the hole, then make it the right size with the correct numbered drill. I am also planning to just do two diagonally opposite holes, tap them, then screw the board down before marking the rest of the holes. That should get the rest of the mounting holes spot on. The fet holes are less critical but will do them the same way.

Tapping aluminum. I use WD-40 for a lube, and I go slow backing out the tap a lot to keep the threads clean. I use a hand tap.

You can get end mills from Enco or Grainger.

On the resistors, you can use 20J50K-ND as a substitute. They have 268 in stock at this time.

Mount those big resistors off the board a bit. They run warm.

John

[W1RKW]

Received my heatsink.

Width was not as advertised.  The board over hangs the edges.  Waiting for a response from the seller.  Hopefully it's just a mix up and will be quickly rectified.  Keep you posted. 

[KC2ZFA]

tell him I’m not happy either

just saw the same issue...

[W1RKW]

Peter,
Seller said we can return it.  Unfortunately, he does not have any with the advertised dimensions. I might just keep it for another project, sell it and search for another heatsink, use it with an adapter plate and heatsink compound or return it. Have to think about it for a bit.  

Bob

[KC2ZFA]

couldn't one fashion 4 Z brackets with the / of the Z as | to mount the board to this sink ?

I may not know what I'm talking about here but isn't it true that the only points of contact between the board and the sink are just the 4 corner holes ? with the transistors bolted to the sink and their legs bent to be soldered into their board holes ?

[W1RKW]

Peter,
Yes. That would work.  

Also, one could get away with just 2 brackets on one side of the heat sink.  One side of the board can mount to the heatsink and the other side of the board to the brackets which are fastened to the heatsink.

I was hoping to get away with the least amount of fuss and essentially have a symmetrical assembly and have a clean install so having not to worry about how  I'm going to fit and mount it into my modulator.  

But using your idea, I think I have in mind a variation of yours that could work in my situation.  I'll keep you posted.

[w9jsw]

Bought a pair of these for the power supply.

https://www.ebay.com/itm/283384262755

John

[W1RKW]

I have 2 on the way also. Thanks for the tip, John.  Nice find and nice price!

[W1RKW]

Peter,
I took a piece of 1/2" x 3/4" aluminum angle stock and cut it to the length of the EBay heatsink. I drilled 3 holes for the mounting screws along it's length. Then drilled and tapped 3 holes in the heatsink.  The 3/4" side of the angle stock bolted to the heatsink.  The 1/2" side will be used to attach the PC board which is mounted so the surface is flush with the surface of the heatsink.  I attached the angle stock to the heatsink then proceeded to drill and tap the holes for mounting the board.  

Be aware of one thing, the fin spacing of this heatsink is not ideal.  When you drill and tap the holes for the transistors, only one transistor hole will line up between 2 fins if you align one side of the board mounting holes  to fall between 2 fins on an outer edge of the heatsink.  The other 4 transistor holes will graze a fin and there is risk of binding and breaking a drill bit.  I jammed a bit and broke it.  Go slow. Let the bit do the work. Highly recommend using a drill press with a new bit vice using a  hand drill.  Can be done with a hand drill if careful.

In the end, everything came out good.  

couldn't one fashion 4 Z brackets with the / of the Z as | to mount the board to this sink ?

I may not know what I'm talking about here but isn't it true that the only points of contact between the board and the sink are just the 4 corner holes ? with the transistors bolted to the sink and their legs bent to be soldered into their board holes ?

[KC2ZFA]

hi Bob, can you snap a couple of pics ? something about pics and 1000s of words.

[w9jsw]

Impossible to build a board to match unknown fin spacing unless this is a commercial operation where I can spec the heatsink as well. Best to go slow and worry your way thru it.  

I finally got some time to go do some drilling and wouldn't you know it, not a 6-32 tap to be found. My plan is 6-32 on the mounting holes and 4-40 on the FETs. These fets are nice! They have a built in screw insulator. All you should need is the silpad.

Frank got his running for some QSOs. No smoke so the basic board is good to go. Of course, there may be some fine tuning of component values necessary depending on what you are driving.

John

[W1RKW]

hi Bob, can you snap a couple of pics ? something about pics and 1000s of words.

Peter, The pix are a little blurry but I think tell the story nonetheless.

John,  I expected that there might be issues with this but it is what it is.  So many different heatsink with various dimension, who would have known.  The key here was to go slow.  In retrospect, I think I would have aligned the transistor screws so that they would hit the spaces between the fins on the heatsink.  I think it was doable and not necessarily worry about 3 of the board mounting holes aligning between the fins.  The one thing I did not think of until after the fact was the transistor holes in the PC board left some wiggle room. I didn't take advantage of that.  It's possible I wouldn't have snap a bit had I thought of it.  But all in all, it worked out. Everything is tapped and ready to go.   

[KC2ZFA]

nice Bob ! and thanks for the pics.

Please let me know if this newby got this right:

The transistors are mounted on the heat sink
so that the mounting bolt is accessible through
the board hole and the transistor legs are bent
up and matched to their respective board solder
points. At the end of the day there is a separation
between the board and the sink of size equal to
the thickness of the transistor plus its silpad.

Peter

[W1RKW]

Peter, you describe what needs to be done perfectly, so a resounding yes.

Frank/GFZ is using 3/8" spacers to mount the board so that will give the transistors a little air gap between them and the PC board.  I will do the same.  I've ordered some nylon spacers for that purpose via Digikey.  If you need the part number, let me know.  I'll dig it up and send it along.

Bob

[KC2ZFA]

excellent ! me no big dummy :-)

I've got tons of nylon spacers from Lowe's...thanks for the offer. And I need to get a
thread tap kit.

I have a drill press for my Dremmel, at highest speed if I go slow, would that suffice ?

all my past aluminum work is only with chassis and panels which I clamp between my
knees to drill or punch holes. I've never built anything with transistors...

Peter

[KD6VXI]

In the electric world we now have combo tap drills.

Drills the hole and taps it for the correct screw all at the same time.

It may take some practice, but they do work.

--Shane
KD6VXI

[W1RKW]

Peter,
I'm not sure a Dremel will work well especially were torque may be required such as in my experience with the drill bit grazing the heatsink fin.  I suppose if you get the holes to line up in a manner where they don't hit a heatsink fin it will work.  It will take some drilling time.  If you have a hand drill you can do this. The key will be to make sure you're drilling as perpendicular as possible. Most important, have a nice sharp drill bit.  Get a new one if necessary.

When you get to tapping the threads for the screws, let us know. We can walk you through it but first if you can get yourself a scrap piece of aluminum that is 1/8" thick or so you can practice on that to get the feel for tapping.

Bob

[KC2ZFA]

thanks for the tips Bob.

But why do we have to drill into the fins ?
The thickness of the sink plate, from flat to
the valley floor between fins, is 7/32”...
shouldn’t that be the drilling depth ?

Peter