Nanao 25FAR vertical collapse then protection

[LarsVegas]

Fired up my Madonna tonight and then picture for a minute and then came a horizontal line picture back and line again, then the protection kicked in and a black screen. Let it rest and picture for a sec and again horizontal line and protection. I’m not a pro to hunt this down myself. If someone can help I would appreciate it.

Greets Lars

[grantspain]

ms8-25?
any fault on the vertical circuit will cause this, normally its a bad solder joint on the vertical ic or a bad vertical ic

[LarsVegas]

Yes MS8-25FAR. Okay, this seems like a manageable operation. How can I spot the area of the vertical deflection?

[grantspain]

its the LA7832 ic, be aware the tracks on these chassis are delicate - manual solder sucker on these is a no no

[LarsVegas]

Okay, thanks. I don’t use manual solder sucker anymore. Only desolder gun and braid. But unfortunately it will take some time, for me doing this, cause lack of time. Will report back, when I’ve checked it. Thnx grant.

[LarsVegas]

Does someone have a good source for a LA7832 IC, just in case? The shops I found do not ship to Europe. Or should I search for an old consumer crt in my size?

[LarsVegas]

So, back again. Replaced the LA7832 IC. I got picture and the monitor running again. I watched for some minutes stable. Left it on running. When I came back the monitor was dark. When I fire it up again, dark picture without neck glow or after a while running and then white horizontal line and protection. Where can I check next?

[grantspain]

maybe a fake la7832, the majority of ic and transistors on the market are
otherwise your vcc to the ic maybe spiking, i imagine you replaced all the caps in the power supply section and vertical section

[LarsVegas]

I’ve 3 spare ICs from donor chassis. Replaced all caps, except the big one and another one somewhere, cause the kit hadn’t the right ones. I imagine that I had once dark screen but the game played blind before the cap kit.
Am I right, that the fault has to be in this section(Foto)?
When I read older threads it’s most of the time a dry solder joint.

[grantspain]

check zd903 and zd904, looks for any broken traces on the vertical ic

[LarsVegas]

Checked the diodes, both OL in one direction and 0.7…V in the other, out of circuit in diode test mode. Reflowed some suspicious joints. Nothing. Changed the last cap I didn’t, except the big one, tryed another IC. Still nothing.

[LarsVegas]

I will bring up this thread again before trying the MS9 Chassis swap.
Chassis had been at Technoservice two times, new Flyback, cause the old one didn't Focus anymore.
There the chassis is running for hours. When i put at on my tube the problem still remains. after some secs or minutes picture is gone, but i can see neck glow. No static on the tube and the high pich noise from the HV disapers. I think that the monitor is loosing B+ Voltage completely. I’ve taken a picture from the shutdown.
Is something wrong with my Yoke? I dont think it’s the tube or maybe it'S the tube itself? Or the power supply from the cab? Without load it’s giving me constant 100V.
Thanks for help guys.

Greets Lars

[LarsVegas]

I don’t want to give this chassis up.
Next I would check if my power supply on the chassis is working. Therefore I would try the light bulb test, just to be sure. So take out the HOT and one wire to the collector the other lead somewhere to ground, right? Then check B+. How do I do this on this chassis?
What I didn’t found out, is the heater voltage coming from the horizontal deflection as well or does it have its own circuit?

[grantspain]

what concerns me is why the width is so narrow, i would expect full horizontal deflection even with a vertical collapse

dosolder HOT, 40 watt mains lamp to collector pad ( centre ) and other side of lamp to capacitor ground, test point the fuse F902 94VDC

[LarsVegas]

Yes, that's what is wondering me too. But when my B+ Voltage is completly gone, should i have nackglow anyways?

[grantspain]

i guess it depends on where the fault is
you need to prove b+ is stable first, my bet is that your b+ is fine and either a glass diode, possibly zener is breaking down. I also have r562 392k in my repair notes as a hv shutdown fault

[LarsVegas]

ordered a component tester. Will check B+ and while im on it, will check those components as well. Then see further if i can track this fault down.
Thank you a lot grantspain

[grantspain]

a good quality multimeter is all that's required
that resistor i mentioned, r562 is a critical value - it can't be 390k or 395k - it has to be 392k 0.1% tolerance

[nem]

A component tester won't necessarily show anything wrong with a diode that's breaking down. Something to keep in mind when you test those.

[LarsVegas]

Thanks guys for all the tips. I will keep this in mind. The tester wasn’t so expensive and maybe comes in handy for some components. What values does the zd903 and zd904 have? I can’t find a schematic for this chassis.