Success feels so sweet

[peter.coombe]

Feeling good today. Yesterday was a success day.

About 20 years ago I made a signal generator from a kit from Jaycar. It is an old Silicon Chip design from around 25 years ago, and is a brilliant piece of kit. Frequencies from 0.1Hz up to 500kHz, sine or square wave, LED digital display to 0.1Hz, volume control. Everything you would ever need in a signal generator. It is analog, based around a 555 timer chip, so has lovely knobs on the front panel. None of this stuffing around with laptops. It has been worth several times it's weight in gold over the years. I have used it to test power amps, preamps, speakers, mandolin plates and guitars. Now about 8 years ago it developed an intermittant fault. It would work for a while and then intermittantly cut out. I must have taken it out of the case and tried to fix the fault 10 times, but every time the fault would disappear for a while and then come back. These sorts of faults a buggers to nail. In the end I gave up and just lived with it. Now about 2 weeks ago it got worst. It was silent more ofen than working and no amount of tapping, thudding, growling, hissing and no amount of very bad language would make it work. If it was not such a valuable piece of hardware, sadly no longer available, I would have kicked it out the window, stomped on it, cut it into little pieces with the chainsaw, and throw it on the bonfire. So, calm down, wait until tomorrw and let's have another attempt to fix it.

Anyway, tomorrow was yesterday. I took the cursed thing out of the case once again, put the reading glasses on and peered at the innards, and poked and prodded various bits with the multimeter. Nothing. Everything looked fine. OK, lets try a magnifying glass. Nothing, can't see anything suspicious. Right, this requires the heavy weapon. Rummage, rummage, search, search, FOUND IT. "It" was a 10x Entomology lense from my undergraduate years at University. Must be at least 40 years old. Anyway a couple more minutes peering through the lense and ureka, GOT YOU, YOU BASTARD! A dry solder joint on one of the pins of the volume control. Out with the soldering iron and dry joint no longer. Put it back in the case and it is now working perfectly. Finally after 8 years, problem NAILED. Sweet, sweet success. Feels real good.

Peter

[Trevor Gore]

Nice one, Peter!

Now I know the kit isn't available any more, but how about the component list and, most precious, the circuit diagram?

Spending $500 for the nearest alternative with knobs has never appealed to me, even though it is a good piece of kit.

[peter.coombe]

Yep, still have the instructions with the component list and circuit diagram. There are distinct advantages to making these things yourself. If they go kaput you can usually fix them if the components are still available or you kept spares. I made the power amp that I use with the signal generator as well. It has blown up twice, both times due to a capacitor going open circuit. Bloody Jaycar supplied 2 caps that were under rated for the voltage produced by the amp at full power. Someone in Jaycar who did that in a power amp kit should be lined up and shot. The first cap literally blew up, the second failure caused the amp to become unstable and oscillate at 4Mh at full power. Only noticed by pure luck that the heat sink was getting extremely hot! Dangerous.

[Trevor Gore]

So Peter, is there any chance of you posting the components list and circuit diagram for those of us who dabble in these things? I dread the day that my old Mini-Lab finally carks it, because I have no circuit diagram or component list for that, so fixing it when it fails to switch on is as much luck as judgement!

[peter.coombe]

Trevor

I have photographed the article and will put it on my web site in the next couple of days. It was published in 1990, and the back issues of SC for 1990 are no longer available.

Peter

[Trevor Gore]

That's great, Peter. Thanks. I'll keep an eye out for it.

[pat foster]

Congratulations! Triumph is sweet.

Those dry solder joints can sure be a bugger. They're sometimes invisible, as was yours, Peter, and make you want to tear your hair out with their intermittence as they degrade over time. I had quite a run-in with cold joints, a bit different but with similar symptoms. At a computer company where I worked, we had gotten a call from an irate customer whose computer had filled his kitchen with smoke after being left on overnight. He'd been given the runaround on the phone when he called the corporate office and was hopping mad about by the time my boss ended up on the line with him. He threatened to put a full page ad into a major newspaper if his problem wasn't made right, so we figured appeasing him as soon as possible would be a good strategy, not to mention that there was definitely something wrong with the product. I went out to the assembly line and grabbed a new unit, hopped on a plane, and hand-delivered the replacement before lunch.

I brought back the defective unit, which we immediately dismantled. We found a cold joint on a high voltage connector in the power supply. We then went out to the factory floor to look at other power supplies and confirmed our worst fears. Most of them had cold joints on the same connector, but so far, only this one had shown up as a problem. We had to stop the assembly line, and get in touch with our power supply supplier. Turned out we were very lucky in that we had received this lot of power supplies only a few days before, so there weren't very many systems in the pipeline. We switched to our other source's power supplies and put out bulletins to dealers to send back their stocks on recall. Cost us a full day of production, and a lot more for rework! All for one little solder joint.

What we discovered was that along with causing intermittent open circuits, bad joints have some resistance and with power passing through that resistance, oxidation occurs, which further degrades the joint's integrity, increasing resistance, generating heat, more oxidation, more resistance and pretty soon you get smoke! On this one, the joint had gotten so hot the solder in the joint melted and migrated out of the joint, making it a dry joint, which fortunately broke the connection, essentially powering the unit off.

Despite the cost, were very lucky and thankful the damage wasn't worse. Our little cold joint could have killed someone.

Pat

[peter.coombe]

The Silicon Chip article is now accessible. Go to -

http://petercoombe.com/SC/signal_generator.htm

Peter

[Allen]

Page 10 is showing a broken link on my web browser Peter.

[peter.coombe]

Page 10 is showing a broken link on my web browser Peter.

Oops, should be fixed now.

Peter

[Trevor Gore]

Thanks for posting that, Peter. Looks like a nice bit of kit. I'm just hoping, now, that I don't have to actually build it!