Electronics > Beginners

repairing crt tv, start with recapping

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MosherIV:
Hi.
Agree with Gyro. That 330uF sounds ok.
Generally speaking, most electrolytics caps have esr less than 2 \$\Omega\$ when good.
If you measure more than 10 \$\Omega\$ then it is bad.

Also agree with Benta. Your primary problem is likely to be bad solder joint some where.
I did not know about tin depletion.
I did as Benta suggested, I removed the old solder and re-soldered with fresh 60/40 solder.
I did not do the whole HV section, just found the single faulty joint and re-soldered it.
TV worked for at least 7 years before the next fault. Could have repired it (total loss of vertical scan, just left with bright horizontal line) but it was 20 years old, 4:3 aspect ratio so decided to get shiny new flat screen  :D

2takt:
so esr meter had a really bad cold solder point where the probe attaches. need to clean it tomorow, has a lot of loss solder on the pcb. but seamed to work fine.


i did discharge the tv befor doing anything :) the caps seem empty to me(had a philips cdi psu that shocked me already)

i already ordered a desolder station should arrive next week
my solder is:
https://www.rapidonline.com/stannol-535236-solder-wire-hs10-sn60pb39cu1-0-5mm-100g-50-3997
is this ok to use? what flux do you guys recommend?

i did notice that when i reflowed the neckboard that the solder points didnt look as shiny as normal.



i will keep the 10ohm in mind when further testing. maybe i should ask a friend to look for the joints, 4eyes see more then 2.
same thing happened to my sony 10years ago :P

Benta:
That solder is fine, there's no need to use additional flux, it already contains colophonium. You'll get wonderfully shiny joints after removing the old solder.

Don't forget to discharge the CR tube, I'll give you one hell of a whack if you don't (up to 1 J)

james_s:
Problem that improves as it warms up is a classic symptom of failing capacitors. The most common fault like that I see in old CRT displays is a worn capacitor in the vertical deflection circuit causing foldover when cold.

vk6zgo:
Another common cause of distorted ("folded over") vertical scans is that the vertical (& audio) output stages are fed with a highish voltage (usually around 150v) derived from an overwind on the Horizontal output/ EHT transformer.
The 15kHz output of this is rectified using a halfwave rectifier, then filtered by a fairly small caoacitance, but high voltage rated Electrolytic capacitor.
(This same system was used in the valve days, when it was called "boost HT")

If the Capacitor fails, the filtering is radically degraded, affecting the vertical scanning.
If the rectifier fails, the same thing happens, but more so, as the vert stage supply reverts to the quite low "start up" voltage, & will be unable to perform its function.

Fix?----Easy, replace the rectifier or capacitor, or both.
But wait! there's more!

A trap for young players exists, in that the capacitor has to be a "high ripple" type.
(Well, that's the old terminology, which I guess, translates to a very low ESR in today's talk!

The rectifier is a "fast recovery" type.

If you fit ordinary old caps or rectifiers, they will work OK for a few minutes then let the magic smoke out! 
No prize for guessing how I know this! :-[

Strangely, I first came upon this problem not at work, but on my Mother in Law's  TV. :o
It had the classic "fold over" symptoms, but my old Fluke 77 read the largely unfiltered output as around 103 v DC-----Not the 150v it was supposed to be, but, I felt  "not low enough to cause the problem".

When I dragged an old BWD home from work, I was confronted with a "copybook" display of a unfiltered half wave rectifier output.

From this, I learnt  another useful lesson.
DMMs can be fooled by strange waveforms, although more modern ones may be able to tell the difference.
I'm not sure, as I don't have a modern DMM.

 

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