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Old 30th Aug 2017, 11:40 pm   #1
MrBungle
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Default The perils of homebrewing equipment

Well I had an interesting couple of days. I approached, well bungled appropriately, three projects I have on the go.

The first, an upgraded frequency counter to replace the old one I made about a year ago, ended up blowing my Arduino based PIC programmer to bits because I wired up the programming voltage wrong.

The spectrum analyser project, I decided to try and check the calibration of the power meter and managed to blow up a rather expensive AD8307 log amplifier by overloading it due to knocking the load resistor off the input.

Now I decided that things weren't going particularly well and to walk away for a bit. This got me thinking about trying to get 13dBm out of an amp for a double balanced mixer in a receiver I've been working on for about 2 years. Brilliant, nice and easy I think. So I sat down, did the calculations, put it together and instantly forgot that I'd got the bench supply running in parallel so the 12v that went into it was actually 42v. This caused the transistor to explode instantly. After removing the charred remains of the transistor and replacing it, it did work as intended at least.

I have put all the equipment away and will have a couple of days off

Each failure is a lesson
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Old 31st Aug 2017, 6:11 am   #2
Bazz4CQJ
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Default Re: The perils of homebrewing equipment

I hold the view that Ham Radio is actually quite stressfull hobby, more so as I get older. I almost envy my pal who sits and reads Dickens' novels all the time.

Perhaps this is why my list of 'projects started' is very much longer than those completed . Skywave and myself had a terrible time last winter building endless published designs of RF Voltmeters, none of which worked to their claimed specs. The therapy bill was enormous.

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Old 31st Aug 2017, 7:03 am   #3
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Default Re: The perils of homebrewing equipment

I may have a couple of AD8309 log detector devices on some scrap boards. They're essentially the same thing, but with differential outputs of the limited signal. Any use?

David
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Old 31st Aug 2017, 9:03 am   #4
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Default Re: The perils of homebrewing equipment

Thanks for the offer but I have ordered another one from RS. They really sting at around 10 a go.

Quote:
Originally Posted by Bazz4CQJ View Post
Perhaps this is why my list of 'projects started' is very much longer than those completed . Skywave and myself had a terrible time last winter building endless published designs of RF Voltmeters, none of which worked to their claimed specs. The therapy bill was enourmous.
This is inline with my experience as well. I have more failed projects than working ones for certain. I'm going to jump back on it today and build a simple SA612/LM386 receiver because they usually work, don't take long and are satisfying.
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Old 31st Aug 2017, 10:12 am   #5
Lloyd 1985
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Default Re: The perils of homebrewing equipment

What a round of bad luck! They say that these things come in 3's, so you should be safe now...

I once built myself a little valve superhet, using some bits from a 3 band cossor, it worked really well! That was until I decided to tidy up the wiring under the chassis.. it never did work again! Maybe time it made it back to the bench for another go.

Regards,
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Old 31st Aug 2017, 10:45 am   #6
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Default Re: The perils of homebrewing equipment

Fingers crossed!

My father told me once to "let working dogs lie" i.e. don't meddle with them. I've made the same mistake a number of times so I agree . Then he died and left me a whole pile of homebrew equipment that doesn't work any more. He built a DVM, power supply, function generator and a frequency counter. All of them were built by ripping off Heathkit designs with even more cost savings (i.e. buying hooky bi-pre-pak transistors and reject ICs). I am slowly going through rebuilding them and they still don't work! I've reverse engineered the frequency counter so far so I'm just going to admit defeat and build a copy of it. Seems to be loosely based on the PW September 1971 design but with LED displays.
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Old 31st Aug 2017, 11:44 am   #7
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Default Re: The perils of homebrewing equipment

I've made a couple of howlers over the last couple of years. I tend to build 'dead bug' style on plain copper board with just a few lands cut for the main ins & outs. Keeping track of the pin order with the IC's lying on their backs can be confusing! I always mark pin 1 with a dot on the board, but sometimes the dot gets obliterated by a solder joint.
I recently returned to a half-built project (new IF receive IF strip for my homebrew Topband transceiver) and was horrified to see I'd got all the pin connections to the sync demod chip (a venerable TBA120) backwards.
So I sighed heavily and got stuck in, putting it all right. A bit of a mess compared to the original clean construction but at least it would now work......
Of course it didn't work and the PSU went into current limit, but even the limited current was enough to see off my last TBA120.
Needless to say I'd 'corrected' the wiring when it was correct in the first place. Now I'll have to do a new build as the board will be looking very scruffy after the third attempt. This is what you get for NOT THINKING!!
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Old 31st Aug 2017, 3:38 pm   #8
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Default Re: The perils of homebrewing equipment

My most-recent upcockage was when wiring the voltage-selector switch to the primary of a mains transformer.

One of the type that has 0 230 250V tappings.

Of course I managed to wire it 'backwards' so on the "230V" setting instead of the mains being applied between 0 and 230V it was applied between the 230V and 250V taps.

Bang!

The transformer survived, but the secondary-side rectifier-diodes and fuse didn't.
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Old 31st Aug 2017, 8:08 pm   #9
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Default Re: The perils of homebrewing equipment

Quote:
Originally Posted by Lloyd 1985 View Post
I once built myself a little valve superhet, using some bits from a 3 band cossor, it worked really well! That was until I decided to tidy up the wiring under the chassis.. it never did work again! Maybe time it made it back to the bench for another go.
A few years ago, we had a student join us for a summer placement. He was given the task of building some kind of a bridge unit of unusual specifications. He was doing physics and had no constructional skills, but after a week or two he had built the rattiest rat's nest ever seen...and it worked really well.

So, big pat on the back for him and he was asked to re-build it in a chassis. Long story short... he stayed with us for about another 8 weeks, and we all joined in on the cause, but that thing never worked again. He really did need therapy by the time he left!

B
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Old 1st Sep 2017, 2:33 pm   #10
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Default Re: The perils of homebrewing equipment

Took me several hours to trace an intermittent fault on a Vero creation recently. Magnifying glass out inspecting for any little shorts between tracks. Flexing the board I could induce the fault. Now are all the legs of the IC I had to change making good contact? Yes!

Eventually I spotted that a naked ground wire on the non-track side was able to contact a little monitor wire I had soldered from the track side because it protruded onto the ground.

Peter
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Old 1st Sep 2017, 6:25 pm   #11
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Default Re: The perils of homebrewing equipment

I have had difficulty getting AD8307s in DIP package and ended up buying surface mount devices on a dip header pcb from China, I now the feeling when damaging components due to simple mistakes, I manged to damage a series regulator transistor when I accidently shorted the 124V supply in the spectrum analyser I recently built and also manged to get the pinouts wrong on 2 pnp transistors in the deflection circuit.

Dave

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Old 1st Sep 2017, 7:05 pm   #12
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Default Re: The perils of homebrewing equipment

You can still get the DIP ones from RS for about 1 more than the SMD ones. I'm buying the SMD ones though. Roth Electronics sell some 1.27mm pitch prototype boards which are conveniently the same pitch as SOIC packages. Add 0805 parts and stuff works out pretty cheap and small and not that hard to work with. I'm using stripped solid copper telephone wire for bus strips and slug tape as a ground plane. Only problem is 0.125W 0805's let the magic smoke out quicker than my usual 0.6W metal films.
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Old 1st Sep 2017, 9:17 pm   #13
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Default Re: The perils of homebrewing equipment

In nearly forty years "doing" electronics, I have made a few things, and very few of them have actually worked. Things I made from magazines hardy ever worked, or the components were unobtainable/there was an error that was corrected (sometimes) in subsequent issues. I have had more success with things I have designed and built myself, but even then that wasn't 100%. I have much more success repairing things.
I must have made at least half a dozen crystal sets, and couldn't get any of them to work(including, much to my shame )the "Ladybird book" radio.

Oh, yes I have done the "upside down" chip thing as well!
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Old 1st Sep 2017, 9:41 pm   #14
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Default Re: The perils of homebrewing equipment

As well as the "upside-down chip" thing, I've more than once got the wiring of an 11- or 12-pin valve/CRT base rotationally off-by-one and then wondered why nothing sensible seemed to be happening.

And has anyone been 'bitten' by the HT because a design using Octal valves has used the classically 'unused-on-glass-valves' pin on the valve-base as a tie-point for HT but you've fitted a metal-cased valve which expects this unused-on-glass pin to be grounded because it's connected to the metal shield?
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Old 3rd Sep 2017, 9:11 am   #15
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Default Re: The perils of homebrewing equipment

I have had several fails on those cheap n nasty Chinese 1 and 2 stage IF pocket radios. Then, when they do go they sound awful.
First radio I made was a Electron-Coupled TRF: EF39 plus 6J5 and 6V6 o/p. Used to squeal if you tried to wind up the volume to max.
I wouldn't class that as a fail, because it worked as soon as the HT came up for the first time.
It was a rats nest underneath the chassis. I was surprised it worked as well as it did.
I don't know Forum member Heatercathodeshort made his 'schoolboy' style radio work so well with the same line-up of devices. Even when I tidied up the underside of the chassis it never worked as well.
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Old 3rd Sep 2017, 1:27 pm   #16
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Default Re: The perils of homebrewing equipment

As a young kid, I asked for money at Christmas and at my birthday. I was saving up for a radio kit I'd seen in PE. The Laskeys Skyrover 6-band job. Despite living just across the town from the BBC's main northern MW transmitter site, I could only get a peep out of the thing. I tried and tried. It had to be me! So I read everything I could get my hands on for years. Saving continued and eventually yielded an AR88 which really did work. It took a long time to get my confidence back and to convince myself that it wasn't me.

Roll on several years, chuck in some rolled-up pieces of paper with big words on them, and I found myself with a job designing things. I built breadboards of my designs, then prototype PCBs. It all had to work. Failure wasn't an available option. Some things worked first time, a lot didn't. I had to learn how to debug problems, to diagnose and understand them, and to find ways to circumnavigate them. That's what the pay was for.

One thing I learned was that if things weren't making sense, I should look at it from another direction. Sometimes it took several directions. But somehow, I got there. My stuff was going to work. My stuff was going to work properly. I was not going to sink to the level of that kit. I suppose I'd quite like to find one nowadays and give it a serious looking over with a pair of much more experienced eyes. I'm not sure what I'd do if I ever met the guy responsible for it. I'd like to throttle him, but I suppose I've really got a lot to thank him for. How does that saying go? That which doesn't kill you makes you stronger. (I've never figured out zombie movies. Freshly raised form the dead, I can't see where the superhuman strength comes from. By my logic they ought to be feeling very poorly)
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Old 3rd Sep 2017, 3:31 pm   #17
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Default Re: The perils of homebrewing equipment

I guess we all have bad days sometimes! Having a couple of days off is probably a good idea and you will come back to it refreshed. I've been quite poorly for a few days now which is why, to fill the time, I have been taking the opportunity to learn more about SPICE simulators using LTspice. Yesterday I thought that I would have a go at replacing the electrolytic caps in my Fluke 8062A yesterday which all arrived - or so I thought - during the week. Not only was I one short of one 100u because I had been supplied 4 instead of the 5 ordered, but the ones I already had to hand were physically maybe 1mm too big to fit into any of the existing positions. Then due to a momentary slip up with the soldering iron in a tight spot I ended up melting the top of one of the other caps which now also needs replacing. With hindsight perhaps I should have stuck to the simulator!

I am curious about whch SA design you based your project on? I came across this one, but it looks quite complex:

http://www.qsl.net/n9zia/spec/
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Old 3rd Sep 2017, 3:53 pm   #18
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Default Re: The perils of homebrewing equipment

Well I just received an amplifier kit, came with just a circuit diagram and a note to say that layout was up to the builder. I have designed a layout but as you can see the build did not progress very well. I must remember to place plain washers underneath the shake proof ones when using ceramic bases. Never used these before and did not see the problem coming, thankfully they are easy to obtain but yes, you always seem to be on a never ending learning curve with home construction
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Old 3rd Sep 2017, 5:58 pm   #19
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Default Re: The perils of homebrewing equipment

I always have a few projects on the go at the same time. When something doesn't go right, I tend to get frustrated with it so it goes back in the box for six months while something else takes my interest. By the time I get back to the first project, everything seems clear again and quite often I finish it with out too much further head scratching. RF instability is always the big one to sort out. Often I have ended up with the inevitable rat's nest of 0.01 capacitors tacked onto everything till it calms down. Sometimes I never bother with finding out why, I am just happy that the capacitor soldered between x and y solves the problem.
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Old 10th Oct 2017, 3:03 am   #20
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Default Re: The perils of homebrewing equipment

It's often the simplest circuits that frustrate the most! Last week, I designed and built a complex, auto-balancing telephone coupler for a radio station. It has audio processing to overcome the worst excesses of the telephone network, it automatically adjusts itself for maximum send to receive attenuation ratio and was all built into a 1U 19" rack box. It worked first time and the customer is delighted with the job (and has ordered three more).

The other project last week was a small aluminium box with a couple of XLR connectors (send and receive) and a pair of 3.5mm jack sockets for connection to a computer sound card. It's entirely passive, using little 600Ω : 600Ω audio transformers and a couple of preset resistors for minor tweaking on installation. It allows a telephone channel on an Allen & Heath broadcast desk to connect to a computer for use with Skype. Trivially simple you'd think..... It's been driving me mad for days!
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