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Author Topic: Trying to fix a universal 25-29 china chassis  (Read 647 times)

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perjmolsen

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Trying to fix a universal 25-29 china chassis
« on: January 10, 2019, 04:44:25 am »
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



« Last Edit: January 10, 2019, 05:02:56 am by perjmolsen »

princess prin prin

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Re: Trying to fix a universal 25-29 china chassis
« Reply #1 on: January 10, 2019, 09:31:33 am »
Given the lack of schematics a shot of the underside would help.

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #2 on: January 10, 2019, 02:21:41 pm »
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)
« Last Edit: January 10, 2019, 02:34:25 pm by perjmolsen »

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Re: Trying to fix a universal 25-29 china chassis
« Reply #3 on: January 11, 2019, 09:16:40 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

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.
« Last Edit: January 11, 2019, 09:20:05 am by princess prin prin »

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #4 on: January 11, 2019, 02:08:22 pm »
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.
[/quote]

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.. :)


« Last Edit: January 11, 2019, 02:21:59 pm by perjmolsen »

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Re: Trying to fix a universal 25-29 china chassis
« Reply #5 on: January 11, 2019, 02:35:02 pm »
The big 3.3K resistor (orange/orange/red).

And pin 2 of the flyback, not 3 and of course the lightbulb needs to be connected to the trace not to the pin itself. Or desolder the HOT and connect the bulb to the middle pad (in this case you don't have to lift the resistor).

Is that isolated or an autotransformer?
« Last Edit: January 12, 2019, 02:22:24 am by princess prin prin »

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Re: Trying to fix a universal 25-29 china chassis
« Reply #6 on: January 11, 2019, 03:11:31 pm »
The "Pin3 flyback" photo above appears to show a tiny glob of solder shorting the pin 4 and pin 9 traces below pin 4 and some residue on/near several solder joints that looks like flux.

It would be a good idea to clean those areas with some isopropyl alcohol and a Q-tip.   :cheers:


Scott

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Re: Trying to fix a universal 25-29 china chassis
« Reply #7 on: January 11, 2019, 04:18:00 pm »
i would like to add that the flybacks used on these chinese clone chassis are pretty rubbish, i had a weiya clone less than a year old come in for repair with flyback bad- cheaper to buy a replacement chassis

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #8 on: January 12, 2019, 07:42:45 am »
The big 3.3K resistor (orange/orange/red).

And pin 2 of the flyback, not 3 and of course the lightbulb needs to be connected to the trace not to the pin itself. Or desolder the HOT and connect the bulb to the middle pad (in this case you don't have to lift the resistor).

Is that isolated or an autotransformer?

Hi

Oh yes pin 2 ;) a typo and a drawing error from my side.
I have removed the one leg of the big 3.3K resistor
One wire from the 40W bulb to pad of pin2

And i have 115.4V at the pad - I assume that is good :)
Note that the D1138 is not put in yet.

About the transformer, i do not know if that is an isolated or autotransformer... it only says step up or down transformer and it gives 110V at one outlet and 220V at another.
Any conserns?

What to test next? I assume some low volts ex at LA7840 at pin 6 - datasheet says recommended 24V

Another thing...
The best monitor for this chassis - I guess that i should be one as close to the vertical and horizontal ohms reading.
Chassis:
VR: 9.6 ohm VL: 24.6mH
HR: 1.3 ohm VL: 1.5mH

I got a monitor with this reading
VR: 12.9 ohm (yellow -green)
HR: 2.1 ohm (red-blue)

I guess that is to high - for best compatibility is is best to be under og over?

Thanks again  :cheers:

« Last Edit: January 13, 2019, 07:33:32 am by perjmolsen »

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #9 on: January 12, 2019, 07:43:37 am »
The "Pin3 flyback" photo above appears to show a tiny glob of solder shorting the pin 4 and pin 9 traces below pin 4 and some residue on/near several solder joints that looks like flux.

It would be a good idea to clean those areas with some isopropyl alcohol and a Q-tip.   :cheers:


Scott

Hi Scott

Thanks for the notice :)
I have now removed it  :cheers:

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #10 on: January 12, 2019, 07:47:53 am »
i would like to add that the flybacks used on these chinese clone chassis are pretty rubbish, i had a weiya clone less than a year old come in for repair with flyback bad- cheaper to buy a replacement chassis

Hi

Yes I do not either have any hopes for this repair - it is just trying to learn, and get some "universal" fault finding tips from you great guys, and kill the time waiting for my last parts to my Nano MS9-29SU repair for my scud race.

I have order the last caps in UK - i do not want to wait any longer for china  ;D

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Re: Trying to fix a universal 25-29 china chassis
« Reply #11 on: January 12, 2019, 09:24:37 am »
Another thing...
The best monitor for this chassis - I guess that i should be one as close to the vertical and horizontal ohms reading.
Chassis:
VR: 9.6 ohm VL: 24.6mH
HR: 1.3 ohm VL: 1.5mH

I got a monitor with this reading
VR: 12.9 ohm (yellow -gold)
HR: 2.1 ohm (red-blue)

The values on the chassis label are for a common yoke type used for 25"-28" TV tubes. A Hantarex Polo has a tube with those readings.

What tube (type number) are you using?

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #12 on: January 12, 2019, 10:18:14 am »
Another thing...
The best monitor for this chassis - I guess that i should be one as close to the vertical and horizontal ohms reading.
Chassis:
VR: 9.6 ohm VL: 24.6mH
HR: 1.3 ohm VL: 1.5mH

I got a monitor with this reading
VR: 12.9 ohm (yellow -gold)
HR: 2.1 ohm (red-blue)

The values on the chassis label are for a common yoke type used for 25"-28" TV tubes. A Hantarex Polo has a tube with those readings.

What tube (type number) are you using?

Hi

I have found an old TV and pulled it apart, to see if i could use it for testing chassis, it has the same pins at the neck, but only a 20"
I guess that i need to find a 25-29 inch real arcade monitor to use for test.
I did not want to use one of my working arcade monitors if i break something :)

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Re: Trying to fix a universal 25-29 china chassis
« Reply #13 on: January 12, 2019, 10:25:53 am »
Yeah, you need an A59 or A66 tube. It doesn't have to be from an arcade monitor.

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #14 on: January 12, 2019, 11:25:12 am »
Yeah, you need an A59 or A66 tube. It doesn't have to be from an arcade monitor.

OK :) Thanks

I had another old TV - pulled it apart
It had also the correct neck pins but still high reading, but a bit lower..
Tube A48 i guess..

VR: 11.2 ohm
HR: 1.9 ohm

« Last Edit: January 12, 2019, 01:39:28 pm by perjmolsen »

perjmolsen

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Re: Trying to fix a universal 25-29 china chassis
« Reply #15 on: January 12, 2019, 03:51:59 pm »
I think that i will put this chassis to the side until I get a compatible monitor to test on.

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Re: Trying to fix a universal 25-29 china chassis
« Reply #16 on: January 14, 2019, 06:33:46 am »
Forgot to say - Thanks for all the help until now!  :applaud:
 :cheers:

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Re: Trying to fix a universal 25-29 china chassis
« Reply #17 on: January 27, 2019, 03:10:25 am »
any luck with this monitor yet ?
this looks to be a cheaper version of the Wei-Ya 429 chassis, these are both very close to the Wei-Ya 825HR & Wei-Ya 826H schematics
These are a very versatile chassis with only changing a few components   ;D
tube size selection is done via C909 0.01uf & C912 0.0047uf .. these are very wide & you may find the adjustments at the max
you can change C912 down to 0.0022uf to make dialling in a 29" easier.. & the 0.01 uf is best suited to a 27" tube.. about center of the H. Width potentiometer   :)
So you now want to know what to use to suit any tube ?
C909 0.01uf  &  C912 0.0022uf will give best for 27" & 29" tube
C909 0.0082uf & C912 0.0022uf will give best for 25" & 27" tube
C909 0.0068uf & C912 0.0015uf will give best for 24" & 26" tube
C909 0.0068uf & C912 0.0022uf will give best for 23" & 25" tube
in short, you need about : 0.0072uf suit 23" , 0.0082uf suit 25" , 0.0104uf suit 27"  &  0.0122uf suit 29"
Horizontal adjustment range is done via C908 0.33uf & C908A 0.1uf..
you then may need to change or trim C908 from it's 0.33uf.. so 0.47uf will give smaller picture & 0.22uf a larger picture, you want to aim for about center of the H. Width potentiometer   :)
also trimming C912 either up or down from the values i gave above will a tighter tolerance on tube size..
P.S. i am yet to try these down to a 20" size tube.. but thinking C909 to 0.0047uf & C908 to 0.47uf will suit.. not sure.. but will up-date..