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Author Topic: LC584AL arrived today but channel 4 is not working :(  (Read 961 times)

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Online tautech

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Re: LC584AL arrived today but channel 4 is not working :(
« Reply #25 on: Yesterday at 03:27:29 PM »
is this the type of material that is on the board covering those diodes and surrounding area?
http://www.ebay.ca/itm/Kafuter-100g-K-5902T-transparent-silicone-industrial-adhesives-waterproof-glue-/292277339286?hash=item440d164496:g:IicAAOSwaB5XvAD3
If what's on the board is pliable and not hard, yes. That ^ seems ideal.  :)
If it is hard it may be hot melt glue.

WRT BAV99, have you looked up any spec to indicate that is indeed temp sensitive ? Maybe there's a temp graph in the datasheet.
I'm struggling too to get my head around why it's where it is and covered in silicone ?  :-//
Yes, point C should be 0V, sorry for the previous confusion.
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Online analogRF

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Re: LC584AL arrived today but channel 4 is not working :(
« Reply #26 on: Today at 01:24:04 AM »
yes, I did take a look at BAV99 specs. They are labeled as ultra-fast silicon switching diodes
which I don't know why it should be the choice in this particular circuit. They dont seem
particularly more temp sensitive than other diodes. There are a couple of graphs showing the
VF or IF vs T but they look like any other diodes to me, not particularly temp sensitive.
For example in one graph from Fairchild, Vf drops from 0.75V at 25C to 0.70V at 50C
and then to 0.60V at 110C (all at 10mA forward current which is close to the 11.3mA used in this circuit)

yes, the silicone on the board is kinda soft and I can poke into it by multimeter probes
but not that soft which can be removed by hand, for example.
 

Online analogRF

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Re: LC584AL arrived today but channel 4 is not working :(
« Reply #27 on: Today at 01:35:21 AM »
...but the thing is if the two diodes are matched, even if their Vf changes with temperature still their sum will be zero!

besides, is this really a good idea for protection against overload? I mean, if overload happens
it takes a while for the diodes to get warm and do their thing and by that time the damage is done!
there must be another path for overload detection, too, I guess but I cannot see it from the schematics yet...

LC684 is using the same concept as this one except it uses LM60 temp sensor instead of BAV99 diodes.

I am not even sure if juts purely resistance measurements + multimeter diode measurements on board is a reliable way
to tell me this diode pair are at fault here except that the values I get are different that the other three channels.
besides, when I did the in-circuit diode measurement, the forward bias was OK and exactly as other channels
the reverse was off but in-circuit diode testing on reverse bias is kinda meaningless and totally depends
on the surrounding elements...so concluding that the diodes are at fault here may not be correct at all...

 

Offline nctnico

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Re: LC584AL arrived today but channel 4 is not working :(
« Reply #28 on: Today at 01:49:17 AM »
If you see different readings between working and non-working channels you are very likely in the part of the circuit where things are wrong. There may be multiple components which are defective. If you have good soldering skills you could replace all the semiconductors in the front-end (presumably some diodes and transistors/fets).
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 


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