Author Topic: Help with old CRT repair - Red colour predominant  (Read 319 times)

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Offline munnaz

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Help with old CRT repair - Red colour predominant
« on: March 31, 2019, 11:40:16 am »
Hey all,  I have been given an old CRT (Model: JVC 7295au) from a good friend to simply get a dvd player to connect to it so that he can play a slideshow at his wedding. It only has an RF input so I just used a modulator to get it up and running. I got it going, but after about 30 mins (and no interference) the screen went red and the picture became barely visible. This is persistent on every VHF and UHF channel, with or without the modulator plugged in.

I have a feeling it may be a capacitor gone bad but I am no expert and not sure if this issue can be caused by something else (I.e. the tube going bad)?

I am aware of the dangers of working on a CRT and am also capable of replacing capacitors, but that's about as much knowledge as I have.

In summary, my question is if anyone can point me in a direction as where to look for an issue or have heard of this issue occurring before? Also, if it may be a capacitor, which board should I start replacing capacitors on?

The wedding is in 3 weeks so I want to either try and fix it over the next few days or buy another.

Any help would be greatly appreciated!

Ps. dont have any photos at the moment and have not visually checked the capacitors (as it just came to mind) but can get some tomorrow if it may help diagnose.

Cheers!
 

Online Whales

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Re: Help with old CRT repair - Red colour predominant
« Reply #1 on: March 31, 2019, 11:54:29 am »
Red predominant or red only?

It's worth checking the simple things like faulty adjustment wheels first (especially if you don't think the unit has any, they might be hiding under a flip-up panel or on an edge).

The R G & B guns are all separately controlled, but it sounds a bit odd for both the G & B to go simultaneously.

Offline Ian.M

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Re: Help with old CRT repair - Red colour predominant
« Reply #2 on: March 31, 2019, 12:13:38 pm »
A fault affecting one colour is most unlikely to be a bad capacitor.   The possibility of a bad tube is fairly high as you've just described the classic symptoms of the heater-cathode insulation on the red gun failing leaky. If it had actually failed shorted, it would probably have tripped the excess beam current protection circuit.

Try isolating the red gun cathode at the CRT socket by desoldering the pin.  If the excess red (and the red in the picture) all goes away leaving you with a G+B picture, the fault's probably the drive circuit, which can be confirmed by isolating the blue cathode and  patch wiring the red drive to the blue cathode - if the fault moves to blue you then know the CRT is OK.  If with the red cathode isolated, the red is still flooding the screen, that's confirmation te tube is bad.   That normally means its B.E.R. but if a customer was particularly attached to their CRT TV and couldn't afford a new tube, we used to fiddle around with an isolated heater supply which would let the heater follow the leaky cathode, so it did't increase the beam current, at the expense of poorer horizontal resolution due to the increased capacitive load.

If its *NOT* the tube, try to find a copy of the service manual online, you *WILL* need it.

Betting on getting it going well enough to trust for a wedding in three weeks is *NOT* a good idea.  Start by asking the venue if they have video projection facilities - it will be a lot easier to run a slideshow on their kit using your own laptop or DVD player, but for ****'s sake *TEST* it ASAP, with the actual laptop or DVD player you are going to use, and if you stick with a slideshow on DVD, with the actual slideshow DVD.  Not all DVD playes are equal and if the slideshow hasn't been properly rendered to video a name-brand player may well not play something a particular off-brand player is fine with.
 

Offline munnaz

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Re: Help with old CRT repair - Red colour predominant
« Reply #3 on: April 01, 2019, 07:41:35 am »
 You can still see a picture. I found all the adjustments on the front and tried them all. Nothing changed.
 

Offline munnaz

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Re: Help with old CRT repair - Red colour predominant
« Reply #4 on: April 01, 2019, 07:43:46 am »
 Thanks for the suggestions I will try and let you know how I go.
 


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