The Shop > Electronics & IC Programing
HELP!!!
John Swift:
Hi Grinder
I think I have identified your board
the capacitor is on the output of the +12v regulator
I've captured the web address from my screen
for some reason I failed to copy it to this reply
(the info is from John Gerling at Gerling laboratories)
the only problem in changing the LM2902 / LM324 quad op amps
will be if the pcb laquer has glued them to the board
John
grinder:
Hi John,
You're right about the laquer holding those parts to the board.
I found some SOIC's, but had a hard time cleaning off the gunk in order to get good thermal contact (seems like they put an extra couple coatings on this one). I think my best option is to get a replacemant board and work on this one after I get the old components off. Do you know af any solvent for this stuff? It feels like polyurethane (maybe). I wonder if acetone, MEK or even turpentine will soften it enough to wipe off?
John Swift:
Hi Grinder ,
the nearest Sieg board I've worked on is a FC250JSMD
the board looked like the laquer had been applied with a tar brush , nice and thick !
the only components I changed was some of the electrolytic capacitors
heat from the soldering iron freed the laquer so I was lucky
( 60 / 40 solder across both connections , I only use lead free when I have to)
my first thought would be to try cellulose thinners , but would test it out on an old computer board
to see what it does to the components , just incase !!!
take a picture of the pcb ,the markings may come of the board
if you remove the presets from board
you can wipe the dissolved laquer of the edge and away from the wire ended components
the only time I had a problem removing surface mount ic's
was when the red glue used to hold the ic's before soldering , worked too well,
the copper track come away with the ic
I would start with the easy bits first
change the through hole components first
the electrolytics and semiconductors in the + & - 12v power supply
since C12 decouples the +12v regulator
without the mains supply , it may be worth using a bench power supply set to +&- 15v to test the onboard regulators
and prove the ic's are faulty or not
to remove the ic's , I would cut the legs off first as close to the epoxy body as possible
with some high quality ,flush cutting side cutters or dremmel type of cutter
then you can remove the legs from the board one by one
If the laquer proves easy to remove
a soldering iron with a bit to heat all 14 pins would be quicker
the chip resistors are marked with three digits but the capacitors are not marked at all !
I'm not sure if the value of the chip capacitors is shown on all positions , on the circuits off the web
it looks like they are all 0.1 uF
NOTE :-- with your FC250BJ board
some of the resistors , setting the gain of some of the op amps will differ from the values on the FC350BJ boards
John
John Swift:
Hi Grinder
just looking at the two versions of the FC350BJ controller i've downloaded
it looks like in one version the capacitors on the input and output of the regulators
have been exchanged.
is it the one of the circuit diagrams thats wrong or one version of the pcb ??
looks like one pcb !
usually the capacitor on the input side the capacitor is the larger of the two
see the pictures
John
grinder:
Hi John,
I'm not able to look at the board now, since I'm actually at work, but I'll take a look when I get home. I've seen both combinations of capacitors on low voltage power supplies before, and I don't know that it makes an awful lot of difference, depends who you talk to. I prefer larger caps on the input side, so that the output side discharges first. These low voltage regulators work great, but they don't like it if the voltage reverses on them. So I always try to arrange it so that smaller caps (or a faster time constant) is on the output side. :beer:
Stan
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