K4915 - Strange power problems

oliviarium

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I have a WG K4915 medium res monitor I'm trying to fix for a friend. Before we got it it was recapped, and we've checked polarity on all of the caps.

Symptoms --

No picture/No raster, did adjust brightness at flyback
0.5VAC for heater voltage at the neck
B+ measures 123v @ TP91 and at R503, which I believe is correct for this monitor.
After about 20-30 seconds of operation, one of the resistors will get very hot. I can change which one gets hot by changing the h-center jumper. In center position, the resistor that is piggy-backed on a couple others (I believe R394 on the schematic) will eventually start to glow. Moving the jumper will cause a different resistor to overheat (but testing the resistors once they cool down seems to indicate they are not damaged).

Things I've tried --
Checked R416 at the neck, tests good, voltage coming in there is about .7VAC, the .5VAC measurement above is at pins 9/10 on the neckboard.
Checked for yoke shorts
Looked for any continuity problems in the heater circuit
Replaced HOT
Swapped Flyback
Bypassed input protection board
Looked for any continuity problems in horizontal circuit

Looking for ideas or suggestions, thanks!
 
Last edited:
Hows it going with this repair?

you might disconnect the yoke

and see it either resistor is still OH, under the same time as with the yoke

connected.

another would be the yoke return cap or width cap.

bring up the schematic so we can look at it too.



I have seen more 4900 yoke issues

if you have any other yoke within the same resistance range for testing

I would swap it in the connector and can leave the neck board off

so not to have a dot on the screen if chassis has hV and nothing getting hot

then I would suspect the yoke.

Most of the time you will see a burn on the windings on the inside of the yoke.

so to know for sure you have to remove the yoke and look..

Measuring the ohms is not a way to tell if its bad.

I'm not saying its bad, I would need more details, but you could check these parts.

But before you do that I would remove the H.O.T and monitor the dc and resistors

still getting hot. to isolate any secondary shorts off the flyback

R503 will be hot but theres a limit to how hot..

need to see where the other resistor is in the circuit.
 
This one turned out to be stupid.

So the partial collapse was fixed by the vert transistor replacement once we got it working again.

With the overloaded resistors near the h-pos jumper, we figured something in that area must be shorting to ground.

We found that in all of the handling of this chassis, one of the wires that jumped across it to the coil & cap on the left side of the chassis had broken off and was touching the frame. Ugh.

We fixed that and everything magically worked again :)

We fixed a friend's K7000 at that party also. Would not sync. Found the jumper between test and normal mode was missing. Added one. Now it syncs.

We are super geniuses :)

--O
 
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