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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Jun 2015, 9:23 am   #1
Newcoppiceman
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Default Musings on a Repair - and a Circuit Puzzle

I was on the kitchen floor yesterday, sifting through dust, dirt and the odd lentil looking for a tiny SMD capacitor. Three times, flea-like, it had leapt from the PCB I was trying to solder it to. Three times I was lucky to find it.

It made me think that you just don't get issues like this - and the attendant stress - with vintage electronics.

Struggling to hold the little blighter in place - and not having any suitable glue - I improvised with a mini-vice and a cocktail stick while I soldered first one end, then the other.

I've attached some pics and a circuit diagram I drew from inspection (the two relay coils and their back-EMF diodes are omitted). Anyone know what Q4 is up to? I know NPN transistors are commonly used upside down as audio mute switches, or maybe Q4 is pretending to be a zener?

The problem with the PIR-triggered light being on all the time was due to the supply reservoir capacitor C2 being completely open-circuit, by the way.
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Old 24th Jun 2015, 9:55 am   #2
Radio Wrangler
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Default Re: Musings on a Repair - and a Circuit Puzzle

Use solder braid and a sodering iron to remove the old solder from the pads on the PCB (and from the device if necessary)

With the iron and some thin multicore solder, put a small amount of fresh solder on one pad. There will be enough flux.

Hold the component in a pair of fine pointed tweezers and place it over the pads, still held. Sneak in with the iron and warm the solder letting the component sink in. Take the iron away, count to three to let it cool. Solder the other end. then re-solder the first end because it may have moved a little while cooling.

No glue was harmed in making these connections

Tiddlywinks with smt parts and tweezers is the risk.

David
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Old 24th Jun 2015, 12:41 pm   #3
kalee20
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Default Re: Musings on a Repair - and a Circuit Puzzle

No I can't work out what Q4 is up to, are you sure you have drawn it right? If it was NPN it would make sense...
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Old 24th Jun 2015, 1:45 pm   #4
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Default Re: Musings on a Repair - and a Circuit Puzzle

SMD's drive me nuts, as does losing little components/screws. I always go through my sweeping's with a magnet. No good for non ferrous. I lost a copper/brass contact from a DMM switch five weeks ago, turned the house upside down only to find it upstairs in my daughters bedroom the other day.

Re glue to stick SMD's in place;if you havn't any glue, petrol and poly- foam makes instant glue. Blue tack is good at a pinch as well and for sticking little screws to the end of screw drivers for hard to reach spots.

Andy.
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Old 24th Jun 2015, 5:48 pm   #5
Newcoppiceman
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Default Re: Musings on a Repair - and a Circuit Puzzle

Desolder braid is great stuff - I long ago dispensed with my solder-sucker as I don't like the fact that tiny bits of solder can so easily be re-deposited on a board when using one. Desolder braid is an elegant way of cleaning up tracks and pads; although it's a bit wasteful, I even use it to tin my own PCBs (these are invariably small one-offs knocked-up with etch-resist transfers).

I use the Chem-Wik stuff that comes in little blue polythene dispensers - they seem to do a good job keeping the braid "fresh". Once it ages/tarnishes it's useless. I also recently acquired a flux pen - another useful tool.

Flux removal is often overlooked. If you're doing any soldering it makes sense to remove flux so that you can properly inspect your joint - and have confidence in it - and spot any other possible problems like bridges or track breaks. Why do some people not bother?

I double-checked the circuit diagram when the puzzle became apparent, but I'm sure it's right. I will check again, but the transistor is an ST device, clearly marked, and the device data sheet is clear about the leadouts.
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Old 24th Jun 2015, 5:54 pm   #6
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Default Re: Musings on a Repair - and a Circuit Puzzle

A few years ago, I bought a little manual suction handling tool with a variety of nozzles, which works well with all but the smallest SMDs (and I can't see those anyway).
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Old 25th Jun 2015, 12:20 am   #7
Oldcodger
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Default Re: Musings on a Repair - and a Circuit Puzzle

Quote:
Originally Posted by Dr Wobble View Post
e. Blue tack is good at a pinch as well and for sticking little screws to the end of screw drivers for hard to reach spots.
I avoid SMDs like the plague. One tip I got many years ago was to pre magnetise my screwdrivers to cope with this problem.
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Old 25th Jun 2015, 2:01 am   #8
AC/HL
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Default Re: Musings on a Repair - and a Circuit Puzzle

I've only repaired one, on an Ebac dehumidifier, and was able to replace it with a standard resistor. Cheating, and still fiddly, but it got it going.
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Old 25th Jun 2015, 10:21 pm   #9
tri-comp
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Default Re: Musings on a Repair - and a Circuit Puzzle

Imho you've reversed the Q4 Emitter & Collector pins on the circuit-drawing.
A PNP-transistor won't take kindly to being reverse-polarized.
But, even if drawn with the Emitter connected to un-reg. B+, the circuit still doesn't make much sense.
A high on IC/Pin5 will just shut-off Q4
A low will turn it on but then what ever base-current is needed for Q2 to turn-on will have to come from Q4/Base.
Clearly it's a very high impedance-circuit so that might be enough even when loaded by R9 to ground (IC-Low).
It doesn't appear to be a latching, thyristor-like circuit-function in there.

And, the PIR controller IC may be a LC9560A (...for which also no data can be located)
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Old 1st Jul 2015, 12:52 am   #10
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Default Re: Musings on a Repair - and a Circuit Puzzle

I found that the CLEAR false eyelash glue the ladies use works fine to hold the SMT devices in place for soldering.
It sets quicker than the RTV does, and so far, (in 5 years) no problems using it.
I figured the black type would have carbon in it, so that was a no brainer.
When I hit upon the idea, I tried 5 different clear brands. The ones that failed went to the other half.
Not knowing what brands are available in the UK, I found Ardell brand worked best.
it only takes a very tiny bit of a dot of it too. I use a toothpick to apply it. Wait about 30 seconds and put the part on it. Easy to remove the part also, as it has not a lot of strength.
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Old 3rd Jul 2015, 10:41 pm   #11
Newcoppiceman
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Default Re: Musings on a Repair - and a Circuit Puzzle

Many thanks for your interest and comments, folks...

For Tri-comp (and other followers of the Q4 puzzle) I've attached pics of both sides of the power board so you can see the source material, also a pic of the print-side after I'd hit it with the flux remover - note the diode bands have been diminished somewhat as a consequence. Also note (fourth pic) that the main board is a nice fibreglass one whereas the power board is a cheaper SRBP (synthetic resin bonded paper), or similar, type - see how it has bent! You can see Q4 and its markings clearly in the component-side pic of the power board.

Also attached again is the circuit - note that R9 is connected to the junction of R7 and VR3 on the main board so Q3 will operate relay K2 when the ambient light level falls below a level set by VR3, as sensed by the component marked CDS on the board. I think this is a reference to an LDR (light dependent resistor) whereas the actual component fitted looks more like a photodiode. I think the CDS refers to the LDR construction - I seem to recall from my Philips "Young Electronic Engineers' Kit" of circa 1968 that LDRs were made of Cadmium Sulphide.

And yes, those resistors around Q4 are all very high in value for a bipolar transistor stage - more akin to valve/FET design.

I didn't mention it in my earlier posts but yes, I drew a complete blank in my on-line search for any data on the "IC9560A" chip (it's definitely an "I" not an "L" on the chip).
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