Sample and no longer on hold

Sometimes things don’t go so well…

Also uncompleted on my workbench was the PMFoundations sample and hold module. This was my first surface mount project aside from the little SMD tutorial kit, and last spring things were going kind of okay with the build, though a lot of resistors ended up at a distinct angle away from the intended one, but then my new soldering iron which had been misbehaving occasionally to that point crapped out completely while I was trying to get the last electrolytic cap mounted. I got the iron replaced but didn’t get back to this build until recently.

One minor irritant: I know I ordered the specified LED in a board mount housing, and I’m pretty sure I must have received it, but… no idea where it went. Actually it’s a minor miracle that was the only part I misplaced in the interim. I soldered in a different LED temporarily. Also the 100k pot I had on hand had a knurled shaft, while the 1MΩ pot had a D-shaft. Awkward. I used them anyway.

I put everything together and powered it up. No smoke, no bang, no excessive heat. But the LED lit up and stayed lit up rather than flashing along with the internal clock as it’s meant to, and I found +10V DC on the output jack regardless of what I plugged in and how I set the controls.

I contacted John at PMFoundations and got some help. Looking at a photo he noticed I’d… well… installed the IC backwards. That’s on me, although in my partial defense, it was oriented the same way as in the photos in the assembly instructions. That’s because those photos were of an earlier version of the board, on which it was oriented the opposite way from the current layout. But the markings were there on the board to show the correct orientation, and I got it wrong.

I had very little confidence I could remove the IC and solder in a new one with the nearby electrolytic cap, power header, and jacks in place, but on the other hand the worst that could happen was I’d destroy a module that would be useless if I didn’t, so what was there to lose? As John advised, I cut the IC legs with a hobby knife, removed the IC body, and then used solder wick to get the legs and solder off the pads.

Then I got the spare IC (I’d ordered one or two extra of everything) and installed it. Tricky given the cramped quarters, but it seemed to go okay. Minor cosmetic damage to the power header shroud but no big deal.

I powered it up. No smoke, no bang, no excessive heat. But the LED lit up and stayed lit up, and there was a DC voltage on the output. A different DC voltage this time, but.

There followed two or three days of occasional messages back and forth as John suggested voltages and resistances to check, and I found some of them wrong on my board, and got some of them right again with some solder touch-up. But then I found IC pin 9, which was supposed to have connectivity to one of the resistors and a finite resistance to pin 8, was instead an open circuit to everywhere. It seemed likely I’d done something like destroy the pin 9 solder pad while replacing the IC.

At that point I said, “Starting to think it may be time to chalk this up as a learning experience and review SMD techniques before trying again with a new board.” But John replied he’d be happy to take a look at the board if I wanted to send it to him. So I mailed it out on Monday. Early Thursday morning I got a message saying he’d gotten the board, checked it, touched up a couple of cold-looking joints, found the pin 9 trace was indeed compromised, soldered a wire from pin 9 to where it was supposed to go, and gotten the whole thing working!

And then said he’d send it back but if I wanted to buy anything from his Tindie store, he’d send the S&H board along with it for nothing.

For.

Nothing.

Now, how can anyone possibly run a business if they’re spending, probably, a couple hours or more of their time helping out a $15 customer free of charge? Insanity!

Needless to say, I wasn’t about to not throw some more business John’s way. You may recall (well, you probably don’t, but you can look it up) the S&H was a near-tie with a dual ring modulator when I was choosing what to buy. So, you guessed it, I sent an order for the ring modulator PCB and panel. I mean, clearly I need to practice my surface mount techniques on something, right?

Unless it was all the fault of that dodgy soldering iron. (It probably was to blame for getting the IC turned around. Must’ve done it while I wasn’t looking.) Or, I now realize, maybe I need to set my iron temperature higher; I’ve been reading and seeing most people seem to go for about 350°C or higher. I was working lower. And getting away with it on the through hole stuff, apparently, but I’m trying working hotter from now on.

Saturday the S&H along with the new board and panel arrived. It works! Well, mostly it works, and maybe entirely. I haven’t been able to get anything out of Clock Out but maybe that’s just me. I’m not going to worry about it for now.

I still need to do something about knobs. (I have a cunning plan.) But I’m sampling and holding.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s