RCA TV problem

[tweetyhack]

Got a RCA 32" TV that will intermittently display horizontal white line in middle. Smack it a few times and the picture returns to normal.

After smacking it a few more times, it's completely dead = no video. The funny thing is, now I get no sound either! When I turn off the power, a light does show on the screen for a moment, then blank again.

I figure something was loose but having no experience in TV repair..., can anybody point me to the culprits? Also, the tube has a label that says to discharge the Anode to the CRT. The Anode is the "electron gun" right? It's been too long.

 

[TillEulenspiegel]

no msg

 

[TheOne]

The horizontal white line indicate a vertical scan collapse. This could be anywhere from the plugs coming from the scan coils on the yoke (large assembly) around the neck of the tube to an actual dry joint on the pcb containing the vertical circuitry and output stage. The fact that thumping the set brings it back to life, indicates a dry joint to me.

Now you will have 2 sets of wires from the yoke assy to the pcb circuitry. The one set will go to the line output stage (the same place the "EHT" Extra High Voltage) for the CRT is generated (look for the lead with a rubber cap that plugs onto the tube itself). This is not the section you want. The other set will go to the vertical stage. If you located this stage, look under a bright light, and when you're my age, using a magnifying glass for any hot spots (dis-coloring or browned areas) on the solder-side of the pcb. In these areas with your magnifier look for solder that looks dull and cracked around leads coming through the pcb. You are most likely to find this where components like power resistors generate a fair amount of heat. Also light tapping on the pcb with something non-conductive may help to locate the position of the dry joint/s.

Depending on the age of the set, you may have to look for dry joints in general where you notice hot spots.

WARNING: TV's operate on 20kv or more on their CRT's. Be very careful!

Read **broken link removed** for safety tips before attempting a repair

 

[Nigel Goodwin]

I've no idea what an RCA set is like?, presumably it's an American set - so probably a cheap Chinese import?.

Most common places for dry joints are the frame output IC pins, or the socket on the PCB where the leads to the scan coils plug in.

As already suggested, a magnifying glass would be helpful!.

 

[zevon8]

Depending on the chassis number, it may be one of the units that had problems with the soldering around the tuner cage and eeprom. This run of sets were infamous, and RCA ( Thomson ) produced service kits with special solder to fix the issue. All sorts of weird failures could occur, starting with random minor problems, that would progress toward total chassis shutdown if the eeprom lost its data, preventing the set from turning on. Regular solder can be used, but if the eeprom is dead, the repair can be more difficult. Eeproms with "basic" programming ( enough to get the TV to turn on ) can be purchased, then the TV is adjusted to proper operation.

Have a look here to see if your set is one of these chassis:

**broken link removed**

The chassis number is on the sticker on the rear of the set, it is a CTCxxx number.

 

[lavenatti]

I'll have to check my RCA set out now, the tuner stopped working properly about 91 days after we got it. I've been using the VCR with the TV's A/V inputs to change channels for about 8 years now.

Other than that, its been a pretty nice set.

 

[zevon8]

I would check it out, since the problem only gets worse ( more expensive ) if more of the solder joints fail.

It is a cheap fix if caught before the rest of the solder joints fail and the whole thing goes ape on you.

About a half hour with a soldering iron and magnifying glass, and you're done. If the eeprom packs up, then its about 10 dollars for a "second level" programmed chip. The problem is tho, you only get one shot with the new eeprom. It's a 8 pin chip, and it stores data for alot of things, like SMPS frequency, sweep frequency, etc, etc. When you get one, it is programmed with "average" values, enough for the set to turn on, then adjusting the TV will program the rest. There is a sequence to getting the TV to turn on with a new chip, and if you goof or there is a problem still with the TV, you just may hose the new eeprom.

All this was caused by a really poor soldering process that failed to solder the metal box and vias to the PCB.

 

[tweetyhack]

So I did a physical inspection. Looks like one of the resistor(R511) is burnt. The numbers on it are completely gone so I don't know what value it is. If anybody has schematics or the same Model(T25001GY) with a Chassis # B4UY91RVT and can tell me the value of that resistor, I'd be greatful. The resistors next to it are 3.3K so if I can't find any info, thats the value I'm putting in.

I traced the resistor and found it to go to pin 8 of the flyback and the other end goes to a diode and capictor. Sort of like this(the dots are spaces).

C510           D502                      R511
---||------+--- ---|<------+----^^^-----< pin 8 of flyback
           |               |
           |       C509   |
           +-------||-----+

Will replacing the resistor be enough or will the resistor just burn up again?? The diode seems good, tested infinity on one side and ~.4 on the other(in-circuit).

Thanks for all the great tips.

 

[zevon8]

You really cant just replace a part based on the value of the component next to it. Another thing, I would suspect that this resistor is of a very low value, a couple of Watts and should probably of the "flameproof" variety, for safety reasons. This circuit is used to generate a lower voltage supply for the chassis.

Generally, one of the reasons these resistors connected to the flyback are in the circuit is to act as "expendable" parts should something fail downstream. One cheap part saving far more expensive and perhaps dangerous failures that could cause a fire and burn the house down. I bet there is something else wrong with the chassis, like a leaky capacitor, or one with a high ESR. From the original description of the fault, the Vertical IC is suspect. Something has overloaded the circuit, and the burnt resistor is the result. You need to find the original fault, fix that, then replace the resistor.

No offence, I am not just going on a rant on you, but you need to be careful before replacing parts willy-nilly.

 

[Nigel Goodwin]

As 'zevon8' suggested, this is probably a low value flame-proof resistor, presumably the HT feed for the frame output IC. It's common to have a low value resistor between the transformer and rectifier, using a resistor that fails easily and safely.

As a TV service engineer for over 33 years I would first resolder the frame chip (as you have the intermittent frame problem), then check with a meter to see if anything is obviously short circuit. If that check is OK I would then replace the resistor (probably just hang it on the back of the board) and see if it works then.

If it works OK, I would then fit the resistor correctly.

In the (more likely) event that the resistor fails immediately, I would then try changing the frame output IC.

When you have dry joints on the frame output IC this could cause the resistor to blow, but it could also cause the IC to fail - which then causes the resistor to blow!.

 

[zevon8]

Hope this helps:

R511 is a 3.9 ohm, 2 watt flameproof
IC501 (vertical IC) check for pin 3 shorted to ground. ( IC bad if so ) RCA part no. 228444

This IC crosses to a generic Toshiba # TA8427K ( about 5 dollars )
here is the data sheet:

https://www.alldatasheet.co.kr/datasheet-pdf/view/TOSHIBA/TA8427K.html

I would replace the capacitor connected to R511, and any electrolytic cap connected to the vertical IC, likely from pin 7 to pin 3, about 100uF. Make sure to use one with the correct voltage rating.

Make sure to inspect the rest of the chassis for any suspect joints.

 

[Nigel Goodwin]

I would replace the capacitor connected to R511, and any electrolytic cap connected to the vertical IC, likely from pin 7 to pin 3, about 100uF. Make sure to use one with the correct voltage rating.

Presumably this is the bootstrap capacitor?, it's always a good idea to change it (if it's a set which still uses one?) if the IC has died for no apparent reason. In this case, it sounds probable that it's been killed by dry joints - so the capacitor doesn't NEED changing, but it's as well to do it anyway while you've got it in pieces.

 

[zevon8]

Presumably this is the bootstrap capacitor?, .

Yes, that's what it is, coming from the charge pump. I always change them in this type of failure. Cheap enough, like you say, while the hood is open..... . If they haven't started to go bad from age, then the heat from the vertical IC having dry joints, and the cap usually being nearly touching the IC, it's probably been cooked.