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Old 16th Mar 2020, 3:48 pm   #1
andymcc
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Default Sony KV-28WF1U, picture gone, help please !

Greetings, my first post to be nice !

I recently dug out of storage my old Sony KV-28WF1U widescreen Trinitron TV which I bought new around 20 years ago, hoping to put it back into use. It powered up OK, and the green onscreen icons seemed normal, but after about 5 minutes warmup, I lost the picture. I didn't have time to connect an input of any sort. There was no crackling etc. indicative of a HT component failing.

I know its not exactly a vintage item, but I would like to have a stab at getting it working again, and I'm hoping folk more experienced than I, may be able to steer me in the right direction.

I've not worked on a TV before, but do have some general electronics knowledge and have done all sorts of construction and some repair work previously. I have meters and scope etc.

It's status right now:

unit powers up and I can feel and hear static at the screen so presumably HT is there
after a few seconds a relay clicks, previously I saw the green onscreen icons, now theres nothing
unit responds to control inputs from the front panel and remote (power, volume, input selection)
connecting a VHS via SCART, I am able to select that input and get audio but no picture
with the cover off and the unit on, if I peer into the tube just past the RGB board, I can see three tiny glowing filaments

Any pointers etc ?
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Old 16th Mar 2020, 4:51 pm   #2
andymcc
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Default Re: Sony KV-28WF1U, picture gone, help please !

I downloaded the service manual from Electrotanya. Then I used a multimeter to check the voltages on CN701 on the 'RGB' board = 'C board'.

I found pin 1 was at ~0v instead of 1000V.

C board, CN701 pin 1 connects to D board, CN804 pin 1

CN804 pin1 connects to pin 5, the 1000V output of the flyback transformer.

Can I safely assume I need to replace the flyback transformer ?

The service manual lists it as:
8-453-220-11 Transformer assy, flyback (NX-1670/U2B4)

Google suggests this is substituted by HR7364.

Does this seem plausible ?

TIA..
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Old 16th Mar 2020, 5:48 pm   #3
HamishBoxer
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Default Re: Sony KV-28WF1U, picture gone, help please !

Sounds like A1 feed res and diode?The res is a safety res and fails through u/s diode.
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Old 16th Mar 2020, 5:51 pm   #4
dragonser
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Default Re: Sony KV-28WF1U, picture gone, help please !

Hi,
I would agree with the last poster.
I would check the cheaper and easier obtainable items first.
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Old 16th Mar 2020, 6:00 pm   #5
Welsh Anorak
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Default Re: Sony KV-28WF1U, picture gone, help please !

To help you, the components referred to are D809 (RGP02) and R830 (1k, 1W fusible). These are in the 1000v feed to the A1s on the CRT base, so make sure the diode is adequately rated. I wouldn't suspect the LOPT just yet.
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Old 16th Mar 2020, 6:37 pm   #6
andymcc
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Default Re: Sony KV-28WF1U, picture gone, help please !

Great, thanks for that chaps. While my last post was waiting mod approval (I'm new here) I'd already had a closer look at the schematic, got the meter out and checked R830. Sure enough it is open circuit, and I've already ordered a bag of 10 of the correct spec. Checking the diode with the DMM, shows it forwarding one way and not the other, so may be OK.

So fingers crossed..

BTW, I did find the flyback transformer on Ebay for 5 plus post, which seems quite reasonable.
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Old 17th Mar 2020, 3:26 pm   #7
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Default Re: Sony KV-28WF1U, picture gone, help please !

You do need to change the diode as it's breaking down under load - otherwise you'll go through those ten resistors very quickly! The LOPTs on this chassis were pretty reliable unlike the later grey ones.
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Old 17th Mar 2020, 4:46 pm   #8
HamishBoxer
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Default Re: Sony KV-28WF1U, picture gone, help please !

Correct Glyn!
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Old 18th Mar 2020, 5:40 pm   #9
andymcc
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Default Re: Sony KV-28WF1U, picture gone, help please !

Thanks again !

The resistors landed today, and a fresh one got my picture back. I'll get some diodes ordered too.
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