A word on vertical collapse

Mongo

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So a buddy and I were working on a couple of modern WeiYa style replacement chassis last night. Both had vertical collapse. In these cases I usually stampede directly to the transistors in the circuit, pull, test, and replace. On these monitors, that didn't get me the quick results I was looking for.

After some time probing around with a scope, the problem was found near the flyback on both monitors. Both of these semi-modern monitors have 1/2 watt 2.2 Ohm resistors in several of the traces coming off the flyback which then feed diodes which feed the rest of the circuit. It would appear the purpose of these resistors is to act as a fuse to protect the flyback in the event of a short in the circuit. One chassis had a visibly burnt resistor. The other had a resistor that looked fine physically.

So... if you still have collapse after checking the "usual" suspects, keep these little "fuses" in mind and remember that the can be very open without showing any outward damage..
 
Fuse?

A fuse has zero ohms across it the resistor

has 2.2 ohms across it so how can it be a fuse???

if so then why does the resistor have a ohm reading

of 2.2 its because its dropping the voltage to the diode

and then rectifying for dc , when it was a ac pulse out of the flyback.

Its not a fuse!!!

It may seem that way but its not!!

When the diode get leaky and shorts it often burns

up the resistor..

when the transistor short it draws alot of current and can short

the diode but the resistor is the weakest point and you may fine it open

before the diode shorts because of its higher current rating..

Another comment to your comment about protecting the fly

when the fly needs protecting a certain winding in the fly

sends pulse to a shutdown circuit

again thru a diode and to the Horizontal chip and disable's

the horizontal drive pulse which turns off the HOT and the load

is wrong then you'll see the B+ or Horizontal scan Voltage go up

in DC.. the fly protects itself!!


Tip a good repair log has model and chassis numbers otherwise

it seem like a scenario not real fact's but a scenario of what could happen

to any general circuit no details like resistors location..

Next time check for some voltage on the supected parts

in the vertical circuit...if none then look what feeds the parts..

follow the trace its easy to follow the trace just

ohm the circuit and follow the trace to the resistor

all circuit require voltage..

Odds are the transitors were bad and the current draw open the resistor!!

and you did replace the transistors so thats why the resistor didnt burnup again!

You dont keep your fuse's where your resistor's are do you?

Not trying to give you a hard time just offer some tips for next time!!

and common sence trouble shooting..
 
I contend with your assertion that a resistor cannot be a "fuse". I point you to FR401 on the G07 (I believe that is the part location). FR stands for "fusible resistor". I would also point to the ignition system on many American cars that have a "Fusible Link" going to the distributor. It is just a wire with a resistor in it that is meant to be sacrificed in the event of a problem.

Why use a resistor in a circuit where you want a fuse? Because they are cheap. A 5 cent resistor can perform the function of a $1 fuse.

I have never seen a "leaky diode" and can tell you that was NOT the cause of either of these failures nor did any of my troubleshooting or parts replacement result in damage to any other parts. The circuits were both "open" due to the resistors.

Chassis number 1 (WeiYa 2820H) was the victim of an un-insulated screwdriver being jabbed into it (not by me) in an effort to adjust a pot who's knob had fallen off.

Chassis number 2 (still unknown to me) was the victim of something else that was user initiated.

My thread was not a "repair log" or it would have included O-Scope readings and a step by step process of elimination as well as a schematic and my methodology for troubleshooting. The purpose of my thread was to remind users (not experienced techs) that vertical collapse is not always the end result of a bad transistor although that is generally the fix for most older monitors......
 
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