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Offline rockchuck

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Building OP sound cards
« on: January 10, 2022, 06:03:19 AM »
I thought I would share my experiences with building the op sound cards. I had all the parts for a while before I decided to brave the build. I read many articles and watched a lot of you tube videos about using an oven for the reflow process. I did try to build a soundcard with a teensy and a cheap chinese 10 watt amp module-- which did work but very noisy and a bit of a mess with wiring etc. So that is when I committed to building the OP sound card and it was a big project for me as I intended to build 15 sound cards.

I bought the boards and a stainless steel stencil from OSH and all the other parts from various suppliers. Being in Australia the freight costs have been costly but just couldn't get everything from the one supplier as stocks weren't always available, some items being last time buy ie the SD card reader for example.

I have a pretty good workbench setup with various soldering iron stations, hot air rework system and also a  desoldering vacuum pump station and being a retired comms technician I know my way around electronic circuits etc.

So to the build. I use a ice cube tray to put the components for one board in it as I found it easy to get the components out and have it close by the board. I set the pcb up on a wooden bread board and stick it down with the supplied aids that OSH provided. The stainless steel stencil is easy to clean which is something very important as the amp chip is the problem device to get right with the reflow process. This component is the only item that has caused me problems. Getting the spread of solder on the board is not to hard but the amp chip is really a bit hit or miss. The main thing is not too much solder on it and maybe very little in the centre of the chip and then cross your fingers and hope for the best.
So I have had a 20 % failure rate as three of the 15 boards have no sound working. All the other functions work as per the LEDs but no sound. I did remove the chip from a board using my reflow hot air, then used the vacuum desoldering gun to clean the excess solder then fitted a new chip and reflowed with the hot air gun , but the device still didn't work. I checked all the supporting circuitry and everything buzzed out ok so just can't tell what is going on underneath the chip !!

I did look at making an arduino control heating unit for the oven so it would follow the reflow curves for the solder, however I watched a video where someone was just using a timer and a temperature probe and turning the oven off and on manually and they got good results.  So this is what I did and got good results for 80% of the time!! So I have three dead boards which are not repairable so I will strip what parts are reusable and throw the rest in the bin.

So was it worth it??
YES it sounds fantastic. I have been using the sound files from this site and with a decent speaker in a good box the tank roars to life!! Luke has done a great job with this device as it has loads of potential for you to customise your tank models. Just a bummer about the amp chip-- I think with more practice I could get better with the solder placement etc-- you really need to be precise when you drop the device onto the board. You can't push it around too much as you  will create shorts under the chip.
So I am short a couple of boards for my TCB units however I have got a few Taigen sound modules so I will use these in my Panzar 3's and also fit these with a Sabretooth serial dual motor drive units (5amp) as these should be ok for the smaller tanks.

cheers and happy tanking.
Building OP sound cards IMAG0279.jpg
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Building OP sound cards IMAG0275.jpg
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Building OP sound cards IMAG0277.jpg
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Offline LukeZ

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Re: Building OP sound cards
« Reply #1 on: January 12, 2022, 08:16:20 AM »
Hi RockChuck, great work! I'm very impressed. I don't think you should feel bad at all about the 20% failure rate. I've assembled dozens and dozens of boards (the sound card but also TCB and Scout) and my average overall success is 2 out 3 that survived, so I'd say you did very well!

I agree that a special oven controller to get the heating curves just-so is not necessary, and wouldn't have made a difference with the problems you had with the amp chip anyway. I didn't even use an oven, just a hot plate. Then a temp probe like you had and I adjusted the temperature manually. The advantage of the hot plate is that it's much easier to see what is happening, but I suppose the disadvantage is that we are just heating the bottom of the board rather than the air all around. I've definitely burnt some.

The small chips are just difficult to get perfect for the reason you described, having the exact correct amount of solder paste and no more. I have found for the fine pitch parts it is better to err on the side of not enough paste than too much, but no matter what it's just difficult to do by hand with a stencil (the factories have a special machine that injects paste with a tiny needle and are much more precise). The amp chip is probably the hardest of all, but the processor on the TCB can also be a pain. I found it was usually easier to just do the processor by hand with a soldering iron, but that is not possible with the amp chip.

Most likely the problem with your amp chips is that some of the edge pins ended up bridged. If you have a magnifying glass you could inspect them, though it is sometimes hard to tell. If one pin is bridged with the pin next to it you can sometimes correct it with a fine tip soldering iron and some flux. I used MG Chemicals 8341 which comes in a syringe but I decant it into an even smaller syringe. However if a bridge occurred between one of the pins and the center pad underneath, then there is really no way to fix that. For that reason after wiping the board with solder paste I would usually remove most of it from the center pad of the amp and leave just a little dab in the very center.

Anyway you've done well. It's gratifying to see someone make and enjoy this project!
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Offline rockchuck

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Re: Building OP sound cards
« Reply #2 on: January 13, 2022, 07:59:10 AM »
Hi Luke
thanks for the vote of confidence!! It took me a while to work up the courage to attempt the reflow process. In the end that wasn't the hard part!

The critical points are getting the right amount of solder paste on the pcb using the stencil ( very important that it is clean first) but you can still get a bad result if you are a bit shaky getting the stencil off as you can smudge the tracks and get bridged joints.
 Your points about the amount of solder paste etc and scrapping the heat sink pad under the amp chip is very important for good results. As I said I think with a bit more practice I could get a better result.

I did have some bridge pins that I was able to fix with a fine tip soldering iron and a solder sucker. However it pays to check the circuit layout first because trying to fix bridged pins can sometimes make things worse. For example there are some adjacent pins on the amp chip that wouldn't matter if you did have a bridged joint as they are connected together via the rest of the cct so it would be best to leave them and not tempt fate.
For instance pins 1 & 2 connected together and then to speaker terminal and 17 & 18 to the other terminal. Pins 23 & 24 go to GND and pins 21 & 22 the same to GND.

So the boards that don't work I don't know if the amp chips are ok as the chip does have some protection in built against shorts etc. But at less than $4 a chip it is not worth the hassel reusing them to find out that they are stuffed.