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G8MNY > TECH 06.05.07 12:59l 124 Lines 4667 Bytes #999 (0) @ WW
BID : 13646_GB7CIP
Read: GUEST OE7FMI
Subj: Faulting Tips
Path: DB0FHN<DB0FOR<DB0SIF<DB0GV<DB0LJ<ON4HU<DB0RES<ON0AR<7M3TJZ<ZL2BAU<
GB7MAX<GB7COV<GB7CIP
Sent: 070505/2306Z @:GB7CIP.#32.GBR.EU #:13646 [Caterham] $:13646_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (new Mar 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, eg. 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).
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 nowadays, with over stressed hot
electrolytic drying out to be less than 1/100 of their original value in just a
few years.
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 nothing to beat an isolation transformer on the
mains. Mine is a 1kW Constant Voltage type that limits the current to 5 Amps.
L>ÄÄ¿ ÚÄÄÄ
)||( Floating
)||( Mains
)||( Safe to
)||( Earth
N>ÄÄÙ ÀÄÄÄ
With a floating supply I can earth with a scope probe before or after a 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 is there is danger form them still being charged.
Tape over hazard areas, eg. 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 use a lamp or power resistor in series with the
supply either, @ mains, or on the power rail. Sometime I even use one in series
with the collector in a line out stage or SMPS when it likely to blow up
another transistor or shut down.
Load )||(
Transforemer)||(
)||
³__Protection
___ Resistor
Line or ³/ 47R 30W
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 ICs...
____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
If it 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 & clean out the holes for the new component.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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