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china circuit over voltages, standby

Technician Tonny

Aug 31, 2015
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Hey guys!
I have a problem with troubleshooting over voltage and standby in CRT China circuits,
when I take such circuits to my fellow technicians they don't allow me too see what they do to troubleshoot it

I need your help plz guys, any one with a schematic diagram showing the primary voltage, oscillation voltage,chopper b+, secondary voltages etc
Thanks....
 

73's de Edd

Aug 21, 2015
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Sir Tonny . . . . .

You think that you have a problem ?, our problem is being even GREATER, since you have provided no BRAND or MODEL number of the set that you are wanting info on.

A mere "China" referencing is too widespread and generic to suffice.

"when I take such circuits to my fellow technicians they don't allow me too see what they do to troubleshoot it"

Sounds like they are protecting their hard earned proprietary secrets, along with their evolved experience.


73's de Edd





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Technician Tonny

Aug 31, 2015
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Hey sir Edd
I mostly open different crt tvs with different brand names but with the same circuits,
They just change the p.on logo in factory setting's , even on the circuit nothing printed on it to show its model,
the chip/ic number can t be used as the model ?
 

73's de Edd

Aug 21, 2015
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Sir Tonny . . . . .

If no model numbers . . . .then SURELY you can supply some of the different brands used on the units .
Also you might pick out the most popular unit and give all of the principal IC numbers on a chassis and its power supply IC /or / transistors , the H.O.T. used , the vertical output IC.
And if any code dates are on parts, give them to determine the vintage/age of the set(s).
I thought that all CRT based TV manufactures had ceased, unless they are trying to dispose of a massive unused parts stockpile.

73's de Edd


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Technician Tonny

Aug 31, 2015
34
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Aug 31, 2015
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H'lo Sir Edd
It uses c2383 ,A1015, C1815, 1 preset 2k and C5287 (power part), the vertical ic is UTC78040,
the hot transistor is D2499,
three ics La76810A, LC863524C and FM24C08A
four output secondary diodes
B+ = 140 which is supposed to be 110 volts
Vertical = 32 volts instead of 24
CV203 sound ic = 20 volts instead of 12 volts
...
 

73's de Edd

Aug 21, 2015
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Sir Tonny . . . . .

With your list of parts and the VERY MOST important info, is the referencing of the 110VDC supply being
higher than is normal, as well as your supplies to the vertical and audio output stage also being excessive.
Lets throw in a "Generic Chinese TV with CRT " (You likee egg rolls with that . .also ?) schematic of its
power supply design which I can assume that might fit your situation.
It is using the same 2SD2497/8/9 series of a fast switching power transistor and discrete transistors for its oscillator drive.

Schematic . . .Technical referencing . . .

picture.jpg




There are two stages to this section, a primary stage power [input] and a secondary stage [outputs]
How can we check it?

Let’s take the primary stage AC [input] section first. AC power is the input to this section and passed through a line
filter [L501], a full wave bridge rectifier section consists of 4xRL 207 diodes, the smoothed by a main filter capacitor
C507; and the DC voltage is applied to the primary winding of the SMPS transformer, the other end of the winding is
connected directly to the collector of main switching transistor 2SD2498.The emitter of this transistor is pulled back
to ground via a resistor of low ohmmic value (R510).

The transistor 2SD2498 acts as a high speed electronic switch, at about the rate of 40 to 45KHz. The oscillation to
its base in order to work at this frequency is generated and supplied by two other small signal transistors. V511 and V512 and
from your given transistors a 2SA1015 is in agreement, but your V512 s probably using a 2SC815 or 2382.
In all of its simplicity, this SMPS consists of 3 semiconductors. Look Mom . . .NO IC !

Let’s go straight to the fault/s that can occur to this circuit.
Since you have given voltage readings the regulation fold back control must be the fault.
Two critical reference element s are zener diodes VD519 for a 9 volt reference source and VD561 for an ~ 6.2
reference source.
Power up and DC meter to see if they are having near that voltage across them.
The '519 is used as a threshold detecting function, while the '561 is just a voltage standard, with it being dependent on
R554 for its keep alive voltage.

The regulation is accomplished by V553's base taking a sample of the "supposedly" 110V supply line thru a voltage
bridge with RP551 being its adjustment/trimming mode.
The simple analog comparator circuit senses if the base voltage compared to the emitter refernce is in error and
compensates in varing the DC to the right light emitting diode within the N501 optical oscillator.
Its degree of illuminaton varies the resistance of the left photo transistor within the N501.
Trace that N501's left lines down to the oscillator circuiry and see that one feeds to the base of the V517 so
that it's degree of making that transistors C to E conduction is proportionatively robbing some base drive signal from V513.
Simultaneously, the photoresistance is also being a variable in the feed back loop of the V511-V512 and if the "110"V
out from the supply is high, the pulse width of the V513 driving waveform will be narrowed in order to bring the voltage downwards.

Some times there is need for quick correction . . .good example . . . of a scene/display/video going from white to pitch
black where the kine beam current and overall power consumption demand from the flyback transformer /power supply
would almost instantly drop.
Thats where the oscillators already used 1-2 feedback winding, comes further into play in using the VD518 in shifting
osc parameters to compensate . . . FAST.

Gonna stop . . . . and see what troubleshooting principles you can apply now.
AND of course, the first would be to monitor the "140 V " supply line . . .but . . . .but . . .(stutter) . .BUT .BUT.BUT
It's 'posed to be only 110 V !

Sooooooo lets just s l o w l y vary that RP551 trimmer to see just exactly much our output voltage range is ?
PARTICULARLY towards the 110V direction. If going the higher voltage direction we might make BIG BANG.
If no adjustment, mainly check VD561 as ~6VDC and also VD519 as ~9VDC .

Standing by to see what your feedback is.



73's de Edd


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Technician Tonny

Aug 31, 2015
34
Joined
Aug 31, 2015
Messages
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Hello Master 73's de Edd
Your explanation's and the schematic diagram helped me a lot through the troubleshoot,
You provided me with the best weapons and knowledge to complete my battle of the Chinese CRT over voltage
Thanks you very much sir...

The main fault was R552 resistor 100K which Connect's to the B+ 110VDC, its other end connects to trimmer RP551 and to base of C1815

At first I connected a 100w bulb in series with AC input then i changed 3 transistors V512, V511 and V553 when I powered up the high voltage 140Vdc was decreased to 120Vdc,
then I adjusted the trimmer RP551 anticlockwise and clockwise but it acted as if its in holiday i.e there was no change on the B+ voltage, then I changed the first resistor connected to the trimmer and GND but the voltage was still high, then I changed the 2nd resistor R552 whish is 100k, when I powered up the B+ voltage decreased to 107VDC, and the trimmer started to function,
then I measured voltage's across the 2 mentioned zener diodes VD561with 6.23VDC (6.2voltage) and VD519 with 6.10VDC (9VDC) ??! then I replaced the VD519,
I measured VD519 across it again with 8.95VDC (9v) then I measured the secondary supply...
B+ ~110 VDC
Vertical ~26.6VDC
Audio ~ 14.66VDC
standby ~ 9.62VDC

Thanks alot sir Edd and nice day
 
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