Newsgroup: it.hobby.elettronica.riparazioni
Titolo: come testare un fbt - parte III

Ulteriori test avanzati Parte III
Advanced Testing
When the Basic Tests Don't Reveal Anything
Also see the section: Flyback Testing Equipment. 
There are several ways of testing flybacks (assuming you do not actually
have special test equipment for this purpose). Here are two
possibilities. The first is easier if you have a scope but the second is
more fun.
Method 1
The following technique, called a 'ring test', works for flybacks,
chopper transformers, motors, mains transformers, deflection yoke
windings, VCR video and other magnetic heads, and other transformers,
coils, or inductors.
However, note that it can miss certain problems like open windings (if
they are not used for the test) as well as shorts or opens that occur
only when the flyback is driven near full voltage. Thus, do the basic
tests FIRST and don't assume that the flyback is 100 percent good just
because it passes the ring test (though the likelihood of this is very
high).
(Portions from: Gabe (
gg...@mcs.com
).) 
This is called a 'ring test' and is the method often used by commercial
flyback (or other coil/transformer) testers. The theory is that a faulty
flyback (which cannot be found by simple resistance measurements) will
have shorted turns in one of the coils. In such a case, the 'Q' of the
transformer is greatly reduced. If excited by an impulse, a faulty
transformer will resonate with a highly damped oscillation while a good
one will decay gradually.
Connect a high quality capacitor across one winding of the suspect
device. Hope for a resonant frequency of a few kHz. You may need to
select the capacitor value for best results. I have found that a
capacitor in the .001 uF to 1 uF (non-polarized) will usually be
satisfactory.
Note that it doesn't matter whether the excitation is applied to the
shorted winding or any other one. However, you should avoid trying to
connect the generator to one of the very small windings like those for
the CRT filament which may only have 2 or 3 turns.
Apply a pulse waveform to the parallel resonant circuit. In 1960, most
scopes had a 'sync out' on the timebase that provided a few 10s of volts
at enough current for this. A circuit in "Television" magazine a couple
of years ago used a BU508, a 12 V power supply, and a small oscillator
built from a 4011 chip. A function generator or a 555 timer based
circuit will also make a satisfactory stimulous. Also see the section:
Flyback Testing Equipment.
Look at the waveform across the resonant circuit with a 'scope. A good
unit will give a nicely decaying oscillation, of at least a few cycles,
possibly 10's of cycles. If there is a shorted turn *anywhere* in the
device, the oscillations will be seriously damped, and you'd be lucky to
see 2 complete cycles. Experience and/or comparison with a known good
device will tell you what to expect.
                                       Scope
                       _                 o
          Pulse or   _| |_               |      Device under Test
          function o---------------------+-----------+    +---
         Generator                       |            )::(
                          High Quality  _|_           )::(     All other
                          Non-polarized ---           ):: +--- windings
                              Capacitor  |            ):: +--- left open
                                         |            )::(
            Ground o---------------------+-----------+    +---
(From: James Elliott (
jell...@stlnet.com
).) 
I tried the Q evaluation method using the 100 volt CAL voltage pulse
from a Tektronix scope. It worked best when I used a series 200 pF
capacitor. I got maybe 100 pulses before it decayed to zero. If I
shorted two of the primary pins, the decaying pulse train went to zero
almost immediately. So it works!
I thought of another method. The Q of a resonant circuit is equal to the
center frequency divided by the half power bandwidth. I applied an audio
generator through a 22k resistor, found the peak frequency, then went
off that frequency to .707 of that amplitude. Double this would be the
bandwidth. I got Q's of 26 and 16 for two I tried. (Editor's note: This
appears to be a valid approach.)
Method 2
The circuit below excites the flyback in much the same way as in normal
operation. The only caution is that this tester probably does not put
enough stress on the flyback to find an intermittent that fails only
under full operating conditions. However, most flyback failures are
solid - once a short develops, there is a meltdown of sorts and it is
there to stay.
You will require a 12 V power source of at least 2 or 3 amps capacity
(regulation is not important - I just use a simple transformer,
rectifier, filter capacitor type of power supply).
The circuit is shown below. None of the component values are critical. 
    +12     Q1   +----------------+
     o           |                 )::
     |       B |/ C                )::         <-- Flyback Under Test
-->
     |  +------|    2N3055         )::
     |  |      |\ E            5T  )::            +------|>|----------o
+HV
     |  |        |                 )::         ::(   HV Diode(s),
     |  |       -_-                )::         ::(   usually built in.
     |  |                          ):: +-----+ ::(      
     +--|-------------------------+ ::(       )::(
     |  |   Q2  _-_                )::(  10T  )::(
     |  |        |                 )::(  each )::(
     |  |    B |/ E            5T  )::(   _   )::(
     |  |  +---|    2N3055         )::(  _|_  )::(
     |  |  |   |\ C                ):: +-- --+ ::(
     |  |  |     |                 )::  Switch ::(
     |  |  |     +----------------+ ::         ::(
     |  |  |                        ::         ::(
     |  |  -----------------------+ ::            +------------------o
-HV
     |  |                      2T  )::
     |  |             +-----------+ :: (Numerous other windings not
shown.)
     |  |             |        2T  )::
     |  +-------------------------+     Note: :: denotes ferrite core.
     |                |
     |          R1    |    R2
     +--------/\/\/\--+--/\/\/\---+
                110        27    _|_
                2W         5W     -
Note: if the circuit does not start oscillating at about 5 volts or
less, interchange the two feedback connections to the transistor bases.
The tester is just a chopper feeding the salvaged core from an old
flyback (I removed the inductance control spacers for this core). The
drive (5T+5T) and feedback (2T+2T) coils can be wound from hookup wire
(#14-#20) and well insulated with plastic electrical tape. Connect the
center taps directly to the coils - do not bring out a loop of wire.
Make sure all the turns of each coil are wound in the same direction.
Wind the feedback coil directly on top of the drive coil. The secondary
of this core is a 10 turn well insulated coil similar to the other two
wound on the opposite side of the ferrite core.
You will need to remove the suspect flyback from the TV or monitor.
Another 10 turn coil is wound on the suspect flyback core anywhere it
will fit. Connect one end of this coil to one end of the 10 turn coil on
your old flyback core. Use a wire nut or twist together securely.
Provide an easy way of connecting the other ends momentarily - a
pushbutton comes in handy.
Make sure you locate the HV return lead on the flyback and use that as
the return for the arc. Otherwise, you may puncture the insulation when
the high voltage finds it own path to ground.
Identifying the High Voltage Return on a Flyback
It is essential that this be correctly connected or else the high
voltage *will* find a suitable path to ground - and it may not do the
other circuitry any good!
There are several approaches that can be taken - possibly in
combination:
Process of elimination - the HV return will often be an isolated pin on
the flyback not connected to anything else. Therefore, if you test
between all combinations of pins on the flyback (removed from the
circuit board) and find a pin that appears open to all other pins but is
connected to a pad on the circuit board, it is quite likely the HV
return.
Check all connections on the circuit board and identify those that go to
ground. One of these flyback pins will be the HV return. It will do no
harm to connect them all to ground during testing.
Use a 100 VDC or greater power supply and high value resistor, say 100K.
Connect the power supply negative output through this resistor to the HV
lead on the flyback (suction cup connector):
                100K
   PS- o--------/\/\--------+--------o CRT (suction cup) connector on
flyback
                            |
                            o
                            -
 100 VDC          Measure voltage here
                            +
                            o
                            |
  PS+ o---------------------+--------o Probe to pins on base of flyback
Check each pin on the base of the flyback with the probe. Touching the
return pin will result in the voltage reading dropping to perhaps 50 or
60 volts. This is the forward voltage drop across the high voltage
rectifier stack inside the flyback. All other pins will result in it
remaining at the supply voltage (except for the ground connection to the
F/G2 divider if it is separate - then it may drop a fraction of a volt).
Note that if you cannot locate the HV return, your flyback may indeed be
defective; it may have an internal bad connection, open HV rectifier, or
burnt out HV winding. Or, if other pins drop the voltage, you may have
already found shorts in the flyback!
Method 2 Testing Procedure
Once everything is wired and double checked, turn on the juice. 
If the flyback is good, then with the coils connected there will be
several kV at its output - enough to create a small arc (1/8" typical,
up to 1/2" for color flybacks).
The load imposed on the oscillator will be modest (the frequency
increases in response to load).
If there are any shorted windings, then there will be no significant HV
output and the load on the oscillator will increase dramatically.
If you get arcing or corona from *under* the flyback - at the pins -
either did not locate the correct HV return or there is a short inside
resulting in HV arcing internally to the low voltage windings.
I have used this 'tester' on a dozen or so flybacks. It has never been
wrong (though I have opted not to believe it and gotten screwed).
a presto ciao 
Luca Lavinci
in...@lavinci.it
-- 
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in...@lavinci.it
Ottimo servizio Luca!
Grazie mille per queste utili informazioni, da salvare
e conservare.
Stefano

e conservare.
Bastava postare il link a 
www.repairfaq.org/
Dove ci sono molti di questi documenti anche in italiano.