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Author Topic: Electronics question  (Read 1110 times)

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Martin_1962

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Electronics question
« on: 11 February 2010, 21:56:28 »


Two components on a board are missing, both in bottom right, the component just below and the component just to the left of the yellow dot just under the battery holder hole.

What are they and how do they effect the RF output - which is missing?

I have a few PCBs to cannibalise but not sure how to measure what should go there.

Thanks

Martin
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Gaffers

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Re: Electronics question
« Reply #1 on: 11 February 2010, 22:17:03 »
looks like it might be a transistor of some sort.....the upper and lower most yellow dots look inconsequential to me, esp the lower which looks like some kind of emf input/output
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Kevin Wood

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Re: Electronics question
« Reply #2 on: 11 February 2010, 23:20:19 »
I could tell you if I understood the question. ;)

Kevin
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Kevin Wood

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Re: Electronics question
« Reply #3 on: 11 February 2010, 23:29:40 »
Are we talking about the two rectangular components in kind of pastel shades? One green, one pinkish?

If so, they are capacitors. They are in what looks like it might be the RF output part of the circuit given the inductor on the PCB next to them.

Difficult to know what value they were if missing, short of checking components on a good example.

Consequences of them not being there depends exactly what they do. Might be decoupling in which case you may get away without them.n the other hand they might be part of a resonant circuit in which case you'll get very little RF out of it.

Kevin
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Mr Skrunts

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Re: Electronics question
« Reply #4 on: 11 February 2010, 23:45:17 »
I know what the problem is.   ::)

It's FUBAR.  :-/ :D :D :y :y

Only joking, am in one of those moods.
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Marks DTM Calib

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Re: Electronics question
« Reply #5 on: 12 February 2010, 08:35:13 »
If that R and C are missing, you would get no output as they look to form part of the main current path through the small RF transistor which just happens to drive the aerial
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Re: Electronics question
« Reply #6 on: 12 February 2010, 08:37:57 »
Kevin, they look to form the emitter leg of a basic amp....an R to give controled gain at DC and a bypass C to up the gain for AC
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Kevin Wood

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Re: Electronics question
« Reply #7 on: 12 February 2010, 09:51:21 »
Quote
Kevin, they look to form the emitter leg of a basic amp....an R to give controled gain at DC and a bypass C to up the gain for AC

Yep, agreed, looking at it. :y

Kevin
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Martin_1962

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Re: Electronics question
« Reply #8 on: 12 February 2010, 13:30:21 »
Quote
Are we talking about the two rectangular components in kind of pastel shades? One green, one pinkish?

If so, they are capacitors. They are in what looks like it might be the RF output part of the circuit given the inductor on the PCB next to them.

Difficult to know what value they were if missing, short of checking components on a good example.

Consequences of them not being there depends exactly what they do. Might be decoupling in which case you may get away without them.n the other hand they might be part of a resonant circuit in which case you'll get very little RF out of it.

Kevin


Yes thought they were.

Bits of track are missing too

Whole other side covered in superglue

battery holder bodge repair had claimed those components
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Re: Electronics question
« Reply #9 on: 12 February 2010, 13:42:37 »
I dont recognise the markings on the SOT23 but it looks like an oscillator, using the printed inductor as the tank, driven by the collector of the SOT23. 180R is for bias, the two Cs are feedbackand resonate the L. Likely to be a few 10's of pF only.
Caps dont carry value markings unfortunately and if it is the main RF osc the value has to be correct for it to transmit on the right frequency.
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Kevin Wood

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Re: Electronics question
« Reply #10 on: 12 February 2010, 13:54:28 »
Quote
I dont recognise the markings on the SOT23 but it looks like an oscillator, using the printed inductor as the tank, driven by the collector of the SOT23. 180R is for bias, the two Cs are feedbackand resonate the L. Likely to be a few 10's of pF only.
Caps dont carry value markings unfortunately and if it is the main RF osc the value has to be correct for it to transmit on the right frequency.

I wonder what that ceramic device is to the left of the transistor? I wonder if it's a crystal or an oscillator module or something? The transistor and printed inductor stage could well be a frequency tripler hanging on the output of it?

Either way, I agree that it won't work well without the right value caps in it. :y

Thinking about it, I've got one of those in my pocket. Perhaps I'll have a probe around...

Kevin
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Martin_1962

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Re: Electronics question
« Reply #11 on: 12 February 2010, 17:56:35 »
I have three flip fob PCBs to borrow components off!
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Martin_1962

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Re: Electronics question
« Reply #12 on: 12 February 2010, 17:57:46 »
Quote
If that R and C are missing, you would get no output as they look to form part of the main current path through the small RF transistor which just happens to drive the aerial


The 2 caps and part of the track are missing
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Martin_1962

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Re: Electronics question
« Reply #13 on: 12 February 2010, 17:58:20 »
I have two random capacitors on but still not working, I think a bit more track needs reinstating
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Martin_1962

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Re: Electronics question
« Reply #14 on: 12 February 2010, 19:28:32 »
First ones failed

Tried another couple - matched the colours off mine ie a brown one

Worked :y :y
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