Repair of the Sony Television Chassis AA-2B SMPS.
The AA-2B chassis comprises the KV-2970T models KV-3470T, KV-29V55 and KV-34V55, but other frame, such as BA-1, BA-BA-3B and 3C-1 and CA there are few differences, maintenance procedure applies to them similarly.Many technicians, or most of them, "turn up their noses" when they encounter a source of this short. The procedure however is not complicated, on the contrary, since that done correctly. And this is what the technicians go missing.
The source of these chassis is inverting, and sinusoidal oscillators of LC type high power rating. It has two transformers: the PIT (Power Input Transformer) which is responsible the current supplied by the source and the DTC (Detect Transformer Control) by the oscillation.
The first step after opening the
cabinet is to do a visual analysis of the chassis in question, observing the
PIT and DTC transformers if their nuclei are not broken or loose.
Make re-solder all plate, giving
special attention to the driver circuit, horizontal output, vertical and source
because any bad contact after maintenance can burn all again.
Deflection circuitry
The first thing to be observed
after the finding of the converters transistors shorter the horizontal and
vertical circuit. Static measure the resistance in (Ohm) of the main lines feeds to ground
should be checked for the presence of short and obviously its main components:
Fly-back, transistors, IC Vert, Yoke, etc..
To confirm the correct operation
of the horizontal and vertical step, it is highly advisable to test with an
external source. For this isolate the circuit by removing the original diodes
secondary rectification. Use a stabilized voltage source to provide 120VDC,
14VDC and 8VDC replacing the original.
The 15VDC voltage of the audio
circuit does not need to be fed now, because it is not essential to the operation of
the circuit.
Leave the device under test at
least 20 minutes and see if there is no closure or intermittent displacement of
the frame. Confirmed this, remove the external voltage source and replace the rectifier
diodes. This procedure is easier and
faster technical pass the correct budget customer, it has a clear and objective
sense of what stage really is in trouble, no need to spend money on the
components of the source.
Now we can start to maintain the
proper source.
Circuit
A critical point is the voltage
doubler, making the switching erratic, or doubling the network voltage 220V
where it was to trigger only on 127V. In that situation, which should have
close to 300VDC, we have voltage above 400Vdc. This causes excessive current
consumption by the circuit and as a result the burning of pair of transistors.
You can check if the folder is properly acting doing as described:
Open the line from the folder
until the main source;
Put the analog meter in the
extremes of the C607 and C608 capacitors.
Quickly connect the networked machine 220V, and notice the tension on the voltmeter.
It should not exceed 380VDC, if high, should be checked The IC603 components (STR81159A), C632 (2,2uFx400V) and C633 (100uFx16V) if there are still the cited documents, should be replaced by new codes.
The reading on the voltmeter
should be as fast as possible because the capacitors have risk of bursting if
the folder is not working properly.
The switching of the new STR83159
is faster, with less chance of transistors being damaged at the time of
switching.
Source Overview
After removal of the pair of transistors, one should check any source statically for short or leakage.
Attention: The Zener diode (120V) protection in the + B-line, in 90% of cases, come into Total short when transistors burn.
Primary
Close attention to the replacement of the Q601 and Q602 transistors (2SC4834MNP), the ORIGINAL should be purchased at an authorized manufacturer indicated by the code in Service Manual and not the component. because there are several denominations (Final letters) for the same transistor. For example, there are four 2SC4834 different denominations: MNP, E-MNP, NP-F09 and P which one does not replace one another in any circumstances. So very careful. Along with the exchange of the pair of transistors is required the replacement of polyester capacitors connected at their bases (C609, C610, C611, C612), which at the time short, they are fed three times more pressure than support, and will be sure to escape (in most cases the capacitance meter does not measure). Also change the Special fusistores (R651, R652) and varistors (VRD602, VRD603). Never replace these fusistores (Fuse resistors) by wire resistors, because they cause inductance changing the oscillation frequency, and transistors, again, come in short.
Secondary
Done maintenance on the primary
and revised, we will isolate the + B line that feeds the Horizontal circuit and
put a load resistor with a value of 560R / 10W.
With a lamp inserted in series with voltmeter and B+ line, connect the TV and verify the correct
operation of the source. If you find that the main voltage is high, low or
unstable immediately unplug the machine. In this case, we realize that even
there are problems at the source, more precisely in the regulator circuit.
Therefore we must analyze the R635 components (470R), R647 (22K) and IC601
(DM48).
Final test
Complete maintenance, we can now
take charge and make the device test.
However still advise to keep the lamp in series. Turn on the TV and check all the voltages are
correct, always observing the temperature of the transistors. Occurring all
usually remove the lamp and let the test at least 5 hours and may then be
delivered to the customer.
