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G8MNY > TECH 10.03.09 08:27l 139 Lines 6325 Bytes #999 (0) @ WW
BID : 20120_GB7CIP
Read: GUEST
Subj: Faulting Tips
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<GB7CIP
Sent: 090309/2311Z @:GB7CIP.#32.GBR.EU #:20120 [Caterham] $:20120_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Nov 07)
(8 Bit ASCII graphics use code page 437 or 850)
When faced with a PCB with 100 components it can be a daunting task to find
the one that has failed especially if there is no circuit diagram.
Here is some tips I have used.
SKILL SET
So you need to use what clues are there, & make some VALID assumptions. Where
there is obvious functional blocks to a device, e.g. PSU, Display, Rx, Tx, etc.
one needs to be able to "read the PCB" & identify the relevant areas. This is
an ever changing skill as components change from the Valve era, to Solid state,
to Chips, LSI, & now SMD (surface mount).
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LOGICAL ORDER
I always start with the PSU on most things, as a duff power rail or hummy /
spiky power rail can cause all sort of problems with modern electronics.
Duff capacitors are a primary cause of faults nowadays, with over stressed hot
electrolytics drying out to be less than 1/100 of their original value in just
a few years. N.B. there is no such thing as a high temperature capacitor, only
one that dries out just after the G'tee.
ÚÄÄ´>ÃÄÄÂÄÄÄÄ ³ _ _ ³ _ _
||( ³+ Normal ³/ ""--..__/ ""--.. Fault³/ \ / \
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On faulting suspect stages, I again start with the DC circuit, as incorrect DC
will always cause AC problems. Using a scope often means you can check both DC
& AC status at the same time.
SAFETY
With mains circuits there is L>Äo-oÄ¿ ÚÄÄÄ
nothing to beat an isolation fuse )||( Floating
transformer on the mains. or )||( Mains
Mine is a 1kW Constant Voltage trip )||( Safe to
type that limits current to 5A. )||( Earth
N>ÄÄÄÄÄÙ ÀÄÄÄ
With a floating supply I can earth with a scope probe before or after a mains
live bridge rectifier to see what is going on. My transformer also makes a buzz
noise under various load is also an instant indication to mains circuit
function.
Remember though that most small scope probes should not be used on the high HF
switching voltages of a SMPSU, because they are not rated at 600V AC @ 50kHz!
Dump caps with a resistor as there is always a danger from them still being
charged.
Tape over hazard areas, e.g. exposed live mains fuse.
Earth is a dangerous as live is!
One hand behind the back is also a good safety rule!
CIRCUIT PROTECTION
For circuit protection I often L>ÄÄ 275W ÄÄÄÄÄ¿
use a lamp or power resistor in lamp kit
series with the supply either, under
@ mains, or on the power rail. test
N>ÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
Load )||(
Transformer)||(
Sometime I even use one in )||
series with the collector in ³__Protection
a line out stage or SMPS when ___ Resistor
it likely to blow up another Line or ³/ 47R 30W
transistor or shut down. Pulse Ä¿ ÚÄ´
width )||( ³\e
drive )||( ³
ÄÙ ÀÄÄÄÁÄ
PARALLEL FAULTS
With shorts on power rail or data buses, find which part is at fault can very
difficult. You need to break the circuit if possible, by removing jumpers,
links, or cutting tracks of you have to. Remember ohms law & what may be inside
an IC...
____mV______
³ ³
PowerÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄÂÄÄ By passing a safe current of 1A via
Rail+ 1k 470R _³_ 10R 220R 47R a car stop light lamp, this circuit
³ ³ \_/ ³ ³ ³ will have voltage drop of a few mV
³ ³ ³ ³ ³ ³ down the +ve track. This will be seen
³ ³ 0R ³ ³ ³ as an increasing maximum to the fault
0V ÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÁÄÄ then little increase after.
Faulty
IC
Another approach is to see what gets hot, this method may be the only one
possible in a multi layer PCB, where an ICs/Cap has gone short to an inside
in accessible track layer.
INTERMITTENT FAULTS
These are a large waste of time. Some method of putting on, or removing the
fault is needed to be certain you have found the cause. Heating & cooling
components can be useful, but other faults can show (red herrings) that are not
the cause!
Flexing the PCB, leads & tapping components (insulated screwdriver handle) may
give the tell tail cause. I have found a hair line cracked track by passing 5
amps down an isolated PCB track & looking for the spark while flexing the PCB
before now.
But often just a good inspection of solder connections & resoldering may be the
most successful answer.
REMOVING COMPONENTS
Always identify leads, & plugs etc. (mark up) so you can put them back. If the
component is definitely dud, then cutting the legs off can often speed things
up. Solder sucking, solder pumps & solder wick can be a great help. But don't
forget that just heating all the legs at once with a pool of solder will let
the component just drop out of the PCB with NO PCB track damage. You will have
to remove all the surplus solder & clean out the holes for the new component.
For bulk stipping of components off scrap boards try a hot air gun (paint
stipper), & most parts will fall off very cleanly, for parts like IC holders it
may be best to leave the IC in place to maintain the shape & sping parts.
Remember the PCB & parts will be very hot!
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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