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Testing a TV with Yoke Removed

Over the years I've used this trick numerous times to diagnose problems, with good success (and no burning a dot in the face of CRT's!). Recently though I ran into something quite unusual. The set is the 31" Hitachi I mentioned in an earlier post. After replacing some leaky caps, the set appeared to work great. But Q752 (pincushion amp) was still overheating. I replaced every diode, cap, transistor, choke etc in the horiz. output area, with no luck. In desperation I tried powering the set with the yoke unplugged - and the HOT fried within 2 seconds. Previous experience leads me to suspect a flyback problem, but right now I suspect there are some gaps in my knowledge that are confusing me. Any ideas?

Reply to
Chris F.
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That model is notorious for the little round blue capacitors in the horizontal deflection to go OPEN, change value. Causes all kinds of parts failures, especially the pincushion and HOT.

Chris F. wrote:

Reply to
dkuhajda

If there is a polarized capacitor in the horizontal drive, replace it. Some units will "blow" the output, without the yoke, & some can burn the crt badly. Use caution. Unplug the CRT board, to keep from burning the CRT. Rono.

Reply to
Rono

hmm, unplug the CRT board?>, since it looks like there are some dangerous idea's being past around! what happens to the voltage on the anode while you have this CRT board unplugged? i mean, disconnected the yoke and firering it up don't seem to be a problem here! i can only assume your going to fire up the TV with the anode voltage on the CRT while the CRT board is off?, i wouldn't want to be the one working in the back of that set at that time!

Reply to
Jamie

Nothing at all happens, with the neck board unplugged the CRT cathodes aren't heated, no current flows, the pins are dead. If you *do* get any arcing then the tube is up to air.

Reply to
James Sweet

Well you keep on doing that pratice, i know far tomuch about CRT theory.. i service irradiation units designed to cross link polymores which are nothing but huge CRT's with a titanium window and steel cone scan section. and one of the golden rules is not to apply Voltage (which reaches in the 1.5 M range) unless the hot cathode is connected to the path.

i know your not working with irradition units but the theory of operation is the exact same thing, just in a much larger scale.

Reply to
Jamie

theory..

You're comparing apples to oranges, 1.5M? We're talking 15-35 KV here, the pins on the neck are connected to transistors rated at 250-300V. Comparing a TV CRT to an irradiation unit is like comparing a hand crank dynamo to a 350 megawatt turbine, the basic principal may be similar but in practice the difference is night and day.

If you ever have a chance, flip through the factory service manual for any projection TV they'll almost always tell you to remove the neck boards before powering up a set with deflection problems, forget to do it while you've got the protection circuit disabled for testing and you've just burned up $900 worth of tubes before you realize what happened.

Reply to
James Sweet

Jamie !! What are you smoking ?

kip

Reply to
kip

Rats: Just give me a nice clean magnetron, about -1.8Kv and a wave guide. Opps, K-mart will sell me one for about $40.00 US. Just to cook food. Of course this has little to do with service of teles, however the voltages present within the microwave ovens can be lethal, even the current stored in the HV cap with the flippin thing unplugged can hurt.<

Reply to
Art

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