JVC DT-V1710cg won't degauss

[grossbard]

I have a JVC DT-V1710cg that isn't degaussing. If you press the degauss button on the monitor, the screen will say "Degaussing!" but nothing happens. I'm not even sure if it's degaussing when it's turned on because I don't hear that distinctive sound that degaussing makes. I have attached the schematic.

I went ahead and ordered a new posistor but it hasn't arrived yet. I bought this monitor brand new a few years ago and the degauss button has never worked. Is it likely to have a bad posistor out of the box?

I checked each pin of the coil connector and they both measured 120 VAC...

[Electronic M]

Are the relay contacts closing? If the relay or the circuits that drive its coil are bad it won't degauss. If the relay is bad you could short its contacts for a moment to manually degauss...

[grossbard]

I measured constant 120VAC across the two hot side relay pins, so I assume it's working. I also measured 120VAC from relay pin 1 to posistor pin 2.

Is this a case of the voltage going through, but the degauss coil not receiving enough current? I can't think of what else it could be, but I'm no expert...

[zeno]

There are 2 major ways modern sets DGS.

1) a PTC resistor in series with the DGS coil. It is AFTER the
on/off switch or relay. Every cold start the set will DGS , the
PTC will heat & show an open.

2) DGS relay. Micro IC sends a brief pulse to the relay driver transistor.
A quick "click-click" is heard. Often they DGS on turn off, the opposite
of the usual.

Every PTC resistor I have seen bad has physically fallen apart. Watch
for cold joints especially on the DGS relay, very common !

73 Zeno
LFOD !

[grossbard]

I reflowed the coil connector, PTC pins, and relay pins. No dice.

The PTC is in a black box, so I can't tell if it's damaged or not. I suppose I could desolder it and see if there's anything shaking around inside....at no point did it ever feel warm to the touch, either.

When I press the degauss button, I do NOT hear a relay clicking, so is it likely the relay is bad? I already ordered a replacement PTC and finally found the correct relay after a bit of searching...maybe I should just shotgun in replacement parts for the entire circuit...

[zeno]

Nothing learned by shot gunning.

All voltages etc are typical
PTC should read apx 120 ohms cold
DGS coil is a near short.
Relay could be stuck or have an opened coil. ( tap it )
Usually there is a little diode across the coil that can short.

The drive transistor as follows
DGS OFF E= 0v B= less than .7 V C= 12 V
DGS ON E= 0V B= .7 V or more C= near 0 V
Remember the ON state only lasts a second.
Start at the base & look for the quick jump in voltage, work
your way down stream from there.

Good luck
Zeno
LFOD !

[grossbard]

Thank you for all the info! I plan on opening it back up this weekend and taking another look.

[grossbard]

I fixed it! I started removing components in the degauss circuit and testing them individually, I found that everything was testing good. Somewhere along the way, with all the shuffling around of the monitor, The monitor would not turn on and the standby light wouldn't even show up. Great.

I realized the secondary PCB with several components on it (including the front panel connector) was not making a good connection to the main board. If I barely pressed on the board, the standby light would come back and I would be able to power on the monitor, but as soon as moved the board even a millimeter, the monitor would shut off again.

I removed the PCB completely and reflowed all the pins to the connector. I reinstalled the connector and the standby light came back on. I turned on the monitor and heard that distinctive degaussing sound that I've NEVER heard on this monitor before. I waited a little while and pressed the degauss button, and it actually worked! I appreciate everyone's advice. This was a great learning experience for me.