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Trying to fix a universal 25-29 china chassis (SOLVED)
perjmolsen:
Hi :cheers:
It is exiting to learn something new/more ;D And while waiting for parts to my other repair I will try to learn
I got this 25"-29" unknown brand china chassis for free.
I want to see if I am able to fix it. :) If i can - maybe someone can use my information :applaud:
Yoke data:
VR: 9.6 ohm VL: 24.6mH
HR: 1.3 ohm VL: 1.5mH
The fault where that it where dead.
So far I have found the following faults:
Both BU2508 where shorted - also one of the two 3A 250V fuses where blown.
I have changed the two BU2508 and also the fuse.
Tested the D1138 out of circuit, its tests OK. http://pdf.datasheetcatalog.com/datasheet/HitachiSemiconductor/mXwuvvr.pdf
When i turn it on now, it seems that there is static, and the fuse does not blow, neither the BU2508 blows.
But the D1138 gets verry hot
If I follow the pcb trace from D1138 it get to the Vertical IC LA7840
With my limited skills then I think that i might be the LA7840 who causes the D1138 to get very hot
So i have orderd som new LA7840 in china
My rep log: https://www.dropbox.com/s/b2gtaciyxx86jfu/25-28%20inch%20china%20chassis%20rep%20log.xlsx?dl=0
princess prin prin:
Given the lack of schematics a shot of the underside would help.
perjmolsen:
Some pictures of the chassis
Nearly in the middle of the picture there is three holes, its there that D1138 where mounted (the component that gets hot when the chassis is turned on)
princess prin prin:
--- Quote from: perjmolsen on January 10, 2019, 04:44:25 am ---If I follow the pcb trace from D1138 it get to the Vertical IC LA7840
With my limited skills then I think that i might be the LA7840 who causes the D1138 to get very hot
--- End quote ---
I disagree. The trace you followed is not relevant as it's ground (the emitter of the transistor is ground and so is one of the pins of the vertical IC). The transistor is likely to be the pincushion correction output transistor. Its collector goes through that big black coil (the bridge coil) to the line output/horizontal deflection circuit, more specifically to the anode of a diode and the cathode of another diode, i.e. the usual damper+modulator diodes that are found in all chassis with pincushion correction.
You first need to check if you have the B+ voltage that goes to pin 2 of the flyback. Lift one side of the 33K resistor to the right of the yoke headers so the horizontal drive transformer (yellow one) is not powered and isolate pin 2 of the flyback and connect a light bulb to it and see if it lights up and how much voltage you have. You need an isolation transformer for this chassis.
perjmolsen:
You first need to check if you have the B+ voltage that goes to pin 2 of the flyback. Lift one side of the 33K resistor to the right of the yoke headers so the horizontal drive transformer (yellow one) is not powered and isolate pin 2 of the flyback and connect a light bulb to it and see if it lights up and how much voltage you have. You need an isolation transformer for this chassis.
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Hi
Well I think that you are right ;D (you always has.. so)
But you say the 33K resistor next to the yoke header...
I see the following.. (if my eyes see colors correct.. they not always do) :)
680 ohm (blue- gray-brown-gold)
2.6 ohm (red-blue-red-gold)
3.3 ohm (orange-orange-red-gold)
0,75 ohm (violet-green-silver-gold)
1 ohm (brown-black-gold-gold)
6,8k ohm (blue-gray-red-gold)
380 ohm (orange-gray-brown-gold)
1 ohm (brown-black-red-gold)
And pin 3 of flyback is this one right? ( i will isolate that, and put a 40W light to the other pad, and ground to heatsnik)
I have an 110V isolation transformer - and correct me, I do NOT need to hook the chassis up to a monitor to test the volt (when i have removed the resistor and isolated pin3 of flyback?)
Its easier to test it at the table.. :)