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Toshiba CX2780A Vertical Deflection problem

Hi,

Id like to get some advice to repair this set. The problem is: bright horizontal line in the center of the screen and then picture above it, which occupies about 25% of the screen. Picture compressed at the center and almost normal at the top of the visible rectangle. So picture expands gradually in the 25% height of the screen. I'm familiar with electronic repairs, but not TVs though. I have digital multimeter and reading different groups already changed all electrolythics with numbers 3xx and IC301 LA7837, - no difference whatsoever. Numbers of capacitors are not typical on this set. I would throw it away unless this repair was for person who cannot afford professional repair and to buy a new TV. Also to my desire to continue investing time in this TV set adds that this person has another old Toshiba CF27??? TV (same size, one year difference) with exactly the same problem, so after figuring out this one I might as well easily fix another for her. I also checked/replaced one diode in pump up circuit (between pins 8 and 13) - again - no difference. Nothing looks wrong on PCB, also checked without taking them out several resistors, mostly big ones around IC301 - all seem good.

Schematic diagram would help, but I missed one recently sold at eBay for CX2780A & CX29801 in the same book.

I'm observing 9V on pin 1 of IC301 and 0.9V on pin 12. Some other TVs have 9V, but typical is 12V on pin 1 and on pin 12 I think should be around half of supply voltage, which is 30V. (Several other toshibas i found have 27V on schematic diagram).

Signal to IC301 comes from IC501 TA8801N and I'm wondering is there any sense in replaciing this one or maybe some non-electrolythics. Reading groups about same fault I see that some people fixed it with capacitor changes while others with similar issues didn't get any answers beyond to change el. capacitors.

Any help will be very appreciated

thanks, Vict

Reply to
vict
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You missed at least one capacitor. Go through the set again and look for either a couple of 1uf or 2.2uf capacitors that use to be red in color.

Reply to
dkuhajda

Thanks for helping. I seriously doubt that I missed any electrolythic capacitor. All of them are black hi-temp, only some dark-brown. There is one red capacitor immediately very close to IC301, but it is not radial electrolythic, looks like something about 0.1 mF in value. Should I change this one? thanks!

Reply to
vict

Thanks for helping. I seriously doubt that I missed any electrolythic capacitor. All of them are black hi-temp, only some dark-brown. There is one red capacitor immediately very close to IC301, but it is not radial electrolythic, looks like something about 0.1 mF in value. Should I change this one? thanks!

Reply to
vict

C305 and C342 are the typical culprits in addition to C308 in that chassis. One is next to the jungle ic.

Full repair typically takes only a few capacitors in the power supply, and the vertical circuit. The semiconductors in that series of Toshiba rarely if ever fail with the exception of the vertical output due to people continuing to watch the set with distorted vertical deflection.

Reply to
dkuhajda

From my own experience! If this does not do it , change the IC.

--------------------------------------------------------------- Text pasted from WinSTIPS 2.0

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TOSHIBA CX2780A

Vertical Stretched

Jeff Stielau : 6/21/03 : Shoreline Electronics Repair

Change C305 & C341 In The Vertical Amp Circuit. Change C308 For Good Measure.....220uf 35v

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-- ========================== Jeff Stielau Shoreline Electronics Repair

344 East Main Street Clinton,CT 06413 860-399-1861 860-664-3535 (fax) snipped-for-privacy@snet.net ========================
Reply to
Shoreline Electronics

Thanks for all help, but picture stays exactly the same. Here is what I've replaced so far:

IC301 LA7837 C308 220.0 mF 35V 105C C305 1.0 mF 50V 105C C306 2200.0mF 25V 105C - this one has the same suspicious voltage as output pin 12 = 0.9V instead of about 14V C310 1000.0mf 35V 85C - sits away from IC301; C311 100.0mF 35V 105C; C330 10.0mf 16V 85C non-polar - without plus or minus signs C342 1.0mf 50V 105C; C344 2.2mf 50V 105C,- this is the one near IC501 TA8801N; C345 100.0mf 35V 105C; Changed D301 too between pins 8 and 13.

I don't see no more 3xx electrolythics to change and I can not locate any(not electrolythic or electrolythic) C341. Seems that feedback circuit might severely reduce pin 12 DC, because not only it low on pin

12 0.9V instead of 14V; on pin 7 IC301 where feedback returns one of the (other toshiba TV) schematics I have indicates 5.0V and I have 0.2V. Maybe some resistor or non electrolythic capacitor shortened?

thanks

Reply to
vict

OK....check the value of the fusible resistor that supplies B+ to the LA7837

--

================================= Jeff Stielau Shoreline Electronics Repair

Reply to
Shoreline Electronics

I don't know how fusible resistor looks like and what is B+, sorry. There are 2 bigger resistors close to IC301. One in sequence Pin12 - L301 - R336. Took out, measured 270 Ohm, same as label on resistor. Another big resistor with one end grounded is R305. Measured value 1.6 Ohm.

Supply pin 8 of IC301 has about 30V and pin13 is 29.3. I already disconnected all parts from pin 12, still output DC voltage

0.9V way lower than expected.

thanks a LOT for help> OK....check the value of the fusible resistor that supplies B+ to the LA7837

Reply to
vict

Missed C301, also red by the end of IC301. Again another 1uF 50V. This is part of ramp generator circuit inside that IC301.

Cheers, Wizard

Reply to
Jason D.

Wizard,

thanks for the advice, unfortunately I cannot find C301. Do you have schematics or just speak from experience? The problem is that CX2780 has somewhat different numbers than other sets. Also there are no red electrolythic capacitors in this set except one on the opposite end of pcb in the power supply block. All electrolythics are black, only 2 or

3 are blue and I've replaced two dark brown ones. I cannot locate anything labeled C301. There is still small chance that I could've missed some C3xx capacitor, because at least three were quite far from IC301 and overall there are maybe 50 of them on PCB, but nothing is nearby IC301 was missed. BTW another set with the same problem, which I don't want to open without success on the CX2780A, is called CF2768B. Any new advice where or what to look for will be highly appreciated!

thanks!

Jas>

Reply to
vict

Hi, All!

I have to conclude this weird attempt to repair the set. After getting the schematics for CF2768B similar TV SET and researching it I figured, that eventually I've run out of ideas. So, I decided to assemble non working TV and put it somewhere in garage till I get oscilloscope (maybe never) I have to mention that I've disconnected output circuit from pin 12 to research voltage dependency on non loaded circuit, and some electrolythics I've replaced were soldered on reverse side of PCB. So, I restored output circuit and moved some capacitors from reverse side to where they supposed to be. In the process I put back original C306

2200 mf, because new one was slightly bigger than needed. After TV was restored to "before repair" state with some new capacitors and IC301 out of the blue it started to work again. All voltages on IC301 are about the same except pins 7 and 12 of course. So it seems output-nad feedback circuit might have been a problem, but what was fixed I don't know and don't understand why it didn't work after my replacements immediately.

Thanks to All will> Hi,

...........

Reply to
vict

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