Is it safe to power up with yoke disconnected?

[berniedd]

Unfortunately, I couldn't refuse a friend who asked me to look at her dead 15" CRT monitor. She said for a few minutes she felt the top of the cabinet very hot and smelled something burning in it just before it died. I plugged it in and got nothing, not even an LED sign of life. No high voltage static, but the primary storage capacitor in the main power supply shows correct voltage. Hmmm, if it burned something, I should see the scorched component, I tell myself. But close inspection of each individual component showed nothing unusual. I had the flyback tested at a local shop and it delivered the high voltage at the anode cap. The LOPT was also good, and so were the snubber capacitors and diodes. All power handling components raised above the PCB tested good. Since something definitely burned and I can't see it, maybe it's in the yoke? One winding is completely covered by a thick plastic sheet. My next step is to power her up with the yoke disconnected from the circuit. Would this be a safe move, or might I burn something up with part of the normal load on the output stages disconnected and cause voltages to rise? I must explain that I'm not a service tech, just a hobbyist with a little service experience.

 

[gugs]

hi, why not bring the yoke still attached to the crt to the same shop that you brought the fly to and tell him to ring it for you. dont take the yoke off the crt, youll have convergence problems if its a color monitor.

 

[things]

so the flyback still works your saying? maybe you could disharge the board, then have a sniff around and see where the burned smell is coming from?

 

[ronsimpson]

It is not good to power the horizontal section up with out a yoke for many reasons. That will cause damage.

It you had tools; I would have you measure the inductance and resistance of the horizontal yoke coil. We do not know the correct values and you probably do not have an inductance meter. What to do? The coil should not measure open! It should measure 5 ohms???? That is a guess. I do not see a way for you to measure a shorted turn in the coil. Can you find a second CRT monitor of the same type? Compare the resistance of the yoke coils.

 

[Thunderchild]

perhaps the vertical and horizontal yoke are identical ? try comparing the resistance of the two, frankly I would recommend a nice new 15-17 inch LCD monitor (or who knows one going secondhand) if it's something tricky it is not worth repairing and the LCD will use much less power

 

[Nigel Goodwin]

perhaps the vertical and horizontal yoke are identical ? try comparing the resistance of the two

They are COMPLETELY different.

 

[Thunderchild]

oh well no go there then I only used to pull em off and get the wire off

 

[gugs]

there is a procedure thats used to excite the coil with a hi freq pulse. the more oscillations produced the better. a defective yoke would show very few oscillations.

 

[Nigel Goodwin]

Deflection yokes (scan coils) do go faulty VERY occasionally, as a full time service engineer I might see one every ten or twenty years?.

It's far more likely to be something else, you need to fault find the circuit to try and find what it is - the LOPTX is a MUCH more common fault (I change probably two or three a WEEK).

BTW, the only way to test a LOPTX is to put it in a known working TV and see if it works! - you can do simple tests for shorted turns, but this only finds a very small percentage of faulty transformers.

 

[berniedd]

Thanks all, for your input. Yes the flyback (or LOPT) checks out OK on the tester (good high voltage output obtained from the anode cap). Unfortunately, they don't have a yoke tester there. I've done the "sniff test" but don't detect any odorous component. I've also checked the horiz and vert yokes' resistance, and they are about 5 ohms each.

 

[Nigel Goodwin]

Like I said, the LOPTX tester only tests for shorted turns, and this is a fairly low percentage of failures in modern transformers. The same tester will also test the scan coils as well, it just rings the windings.

But you appear to have no idea how to repair the set, you don't just randomly try and test different components - you fault find!.