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Chat Area => General Discussion Area => Topic started by: Martin_1962 on 11 February 2010, 21:56:28

Title: Electronics question
Post by: Martin_1962 on 11 February 2010, 21:56:28
(http://i91.photobucket.com/albums/k286/martinimber/kebfob/Megamos2.jpg)

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
Title: Re: Electronics question
Post by: Gaffers 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
Title: Re: Electronics question
Post by: Kevin Wood on 11 February 2010, 23:20:19
I could tell you if I understood the question. ;)

Kevin
Title: Re: Electronics question
Post by: Kevin Wood 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
Title: Re: Electronics question
Post by: Mr Skrunts 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.
Title: Re: Electronics question
Post by: Marks DTM Calib 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
Title: Re: Electronics question
Post by: Marks DTM Calib 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
Title: Re: Electronics question
Post by: Kevin Wood 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
Title: Re: Electronics question
Post by: Martin_1962 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
Title: Re: Electronics question
Post by: Matchless 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.
Title: Re: Electronics question
Post by: Kevin Wood 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
Title: Re: Electronics question
Post by: Martin_1962 on 12 February 2010, 17:56:35
I have three flip fob PCBs to borrow components off!
Title: Re: Electronics question
Post by: Martin_1962 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
Title: Re: Electronics question
Post by: Martin_1962 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
Title: Re: Electronics question
Post by: Martin_1962 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
Title: Re: Electronics question
Post by: Turk on 12 February 2010, 23:34:48
Reduce the downfall of the uptake on the downside uploader and re-calibrate the flux capacitor.  :D
Title: Re: Electronics question
Post by: Entwood on 12 February 2010, 23:47:31
Quote
Reduce the downfall of the uptake on the downside uploader and re-calibrate the flux capacitor.  :D

Och .. dinna be so stupid Cap'n ......   

The Dilithium Crystals canna take that sort of punishment...

Title: Re: Electronics question
Post by: Radiomarko on 13 February 2010, 09:11:50
My eyes wont allow work on such smd stuff anymore   :-X

About time these low power devices were moved out of my 70cm band   ::)
Title: Re: Electronics question
Post by: Kevin Wood on 13 February 2010, 09:54:09
Quote
My eyes wont allow work on such smd stuff anymore   :-X

About time these low power devices were moved out of my 70cm band   ::)

I don't know.I quite like having a chinwag in the car park and watching all the drivers who have to wait until the end of my over to unlock their cars.  ;D

G7BCS
Title: Re: Electronics question
Post by: Radiomarko on 13 February 2010, 11:25:39
hehe  agreed Kevin, 'cept there's very little 70cm activity down here, so I'd be talking to myself - which happens increasingly often without the radio.....

G7LJN