Author Topic: Tv test without load?  (Read 939 times)

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Offline The_Todd

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Tv test without load?
« on: August 05, 2018, 05:46:05 am »


Hello everyone,

I’m in the process of repairing a CRT tv drive board out of and arcade machine, as it kept shorting out. I believe it’s fixed but Will it be ok to run this separately from the unit without a load to test. Also it’s 125v

Cheers.


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Offline boB

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Re: Tv test without load?
« Reply #1 on: August 05, 2018, 05:53:06 am »

I would make sure the HV anode connector is away from the other parts so it doesn't arc and hurt those OR you first.

If you don't have a HV meter but want to make it arc for testing, just use a ground lead a few mm away from that anode lead.

Otherwise, it should be OK.

boB
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Offline The_Todd

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Re: Tv test without load?
« Reply #2 on: August 05, 2018, 05:57:04 am »
I just want to make sure the fuse doesn’t blow when plugged in. That’s all.


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Offline Gyro

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Re: Tv test without load?
« Reply #3 on: August 05, 2018, 07:16:19 am »
Operating the line output stage and LOPT without the deflection yoke connected might cause problems.
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline james_s

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Re: Tv test without load?
« Reply #4 on: August 05, 2018, 04:48:21 pm »
I wouldn't recommend it, the deflection yoke is an integral part of the resonant tank that is part of the HV circuit.

You should be able to test all the usual culprits with a multimeter. If the fuse blows it's normally because a shorted HOT. If you replaced that make sure you installed it with a proper insulator if needed.
 

Offline The_Todd

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Re: Tv test without load?
« Reply #5 on: August 06, 2018, 08:45:39 am »
yeah it was a failed rectifier, the thing was from the 80`s and has been sitting there powered on for countless hours, so that's not too bad. the cabinet is at a customers house witch is a bit of a drive, so was hoping to test to see if the short has been fixed. but have not had much experience with CRT circuits, i know some high voltages needs a a load so it doesn't destroy itself.
 

Offline james_s

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Re: Tv test without load?
« Reply #6 on: August 06, 2018, 03:06:33 pm »
Well if you replaced the shorted rectifier then it's probably ok. You could connect it to a variac and bring the voltage up slowly to about 24V, if it isn't drawing excessive current by then it's probably ok. Keep in mind that most arcade monitors use a hot chassis design and *require* an isolation transformer. If you connect one directly to the line the chassis ground will be floating at a substantial voltage.

You should also replace the electrolytic capacitors, usually the big bulk filter is ok but there are almost always some bad capacitors on these. There are cap kits available for most arcade monitor chassis' which simplify things.

Normally when I service arcade monitors I pull the whole monitor and take it to my bench, unless I have an identical monitor in one of my games to test the chassis in.
 
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Offline SeanB

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Re: Tv test without load?
« Reply #7 on: August 06, 2018, 07:06:40 pm »
With CRT drivers there are high value resistors in the CRT base side, on the brightness and G1 controls, that tend to drift high in value, and thus you get either a smeared image or flyback lines, depending on the resistor. Suspect any resistor in the high voltage area around the tube base and around the LOPT that is over 47k, and which is discoloured, and check the value of them. Many will have drifted high.

Any electrolytic values 22uF to 220uF, 100V or higher replace on sight, they will be high ESR by now having been run nice and toasty for years. They are on power supply rails, and generally have a high ripple current through them from being part of the LOPT tank circuit. Beware of bipolar electrolytics that are often used to couple the scan coils, they can leak or go bad as well, and are not replaceable with a regular electrolytic, though I have replaced a good number of them with 10uF 160v metal film capacitors mounted up off the board to get the space to fit the few in.
 
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Offline The_Todd

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Re: Tv test without load?
« Reply #8 on: August 06, 2018, 11:31:45 pm »
well thanks to all.

on other idea, whats the chances or putting this into a shallow ultrasonic cleaning and alco bath to clean it up?

it literally is covered in a brown sticky layer of filth.

cheers
 

Offline james_s

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Re: Tv test without load?
« Reply #9 on: August 06, 2018, 11:38:15 pm »
I normally give them a rinse in the sink with warm soapy water and a soft bristled scrub brush before I even start working on one. Do try to keep water out of the focus and screen adjustment pots though, if it gets inside that assembly it will cause focus and brightness problems and takes a long time to dry out.
 

Offline The_Todd

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Re: Tv test without load?
« Reply #10 on: August 07, 2018, 12:28:41 am »
ihve the alco bath and a heated plate to dry out, im more worried about the sonic bath cavitation will wear away the insulation on the winding and stuff. 
or even make miro cracks on the old traces.
 

Offline james_s

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Re: Tv test without load?
« Reply #11 on: August 07, 2018, 03:21:29 am »
I seriously doubt you'll have any problems with that, I've washed small PCBs in an ultrasonic cleaner several times and never had any trouble.
 


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