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Nanao MS-2931 - immediately goes into shutdown.

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Title says it all really at this point. Didn't do anything at all, recapped it, now it goes into shutdown about 2-3 seconds after being plugged in.

Not sure where to start.

I have two others that are even worse off, but going to tackle one at a time.
 
The service manual actually has a map of the protection circuits in the middle pages (Page II-3 specifically). Are you sure the B+ pot isn't just way out of whack after possibly being turned up to compensate for weak caps?
 
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Do you get any signal before it shuts down?

If so, does the video look normal, or do you see sync issues?
 
ebzero89 said:
The service manual actually has a map of the protection circuits in the middle pages (Page II-3 specifically). Are you sure the B+ pot isn't just way out of whack after possibly being turned up to compensate for weak caps?
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I had guessed at that, so I've tried lowering it about 10% at a time until I hit the bottom end. Still trips the shutdown even at minimum.

I'll check AO for the manual.
buffi said:
Do you get any signal before it shuts down?

If so, does the video look normal, or do you see sync issues?
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No video.
DecepticonZero said:
Did you swap the HOT?
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I pulled it, and it tests good.
 
kuze said:
At least the MS2931 chassis out there with dead flybacks are fixable with the Chinese replacements now.
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Very true. I just didn't want to fork out for new flybacks without /knowing/ it's the problem, you know? The work is... fine. I just don't want to do it if it wasn't actually necessary.
 
rewrite said:
kuze said:
At least the MS2931 chassis out there with dead flybacks are fixable with the Chinese replacements now.
Click to expand...
Very true. I just didn't want to fork out for new flybacks without /knowing/ it's the problem, you know? The work is... fine. I just don't want to do it if it wasn't actually necessary.
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Yeah from what I've gathered it is the typical failure point on these ms2931 chassis though.

If you fork out for one and it doesn't fix the issue well then it doesn't hurt to have a spare!
 
Okay, manual in hand I can do a bit more.

C905 sees a constant 167V rather than the 130V it should.

You can see C921 get 180V as expected, but when the shutdown kicks in and it trickles down to 0 fairly steadily.
 
I found some old AO posts saying that the vertical transistor will also trigger shutdown and to test that if the HOT is fine.

Any guesses as to which one that is?

Second question : on NPN transistors, Google says 0.45 - 0.90 are good readings on base to collector and base to emitter. How exact does that need to be? Is 0.44 fine? 0.42? 0.41?
 
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Vertical deflection is handled by a LA7846N at U401. And HOT readings seem normal to me; any discrepancy could just be down to your DMM.

I had a 2931 a while back that I couldn't get out of shutdown; I'm pretty sure the issue was U451 and I could never find a replacement part. Scrapped it and sold it for spare parts.
 
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@benime thanks. At the moment I have 4 MS2931's on hand. 2 won't come out of shutdown, 1 just does nothing at all, and the last one I haven't cleaned up yet.

1 bad HOT out of the three non-working ones. The other two seem to be fine.

My understanding is that you can't test LA7846N, the only thing you can do is swap in a known good one?
 
You could swap a known working one, but the pads on that are pretty delicate and I wouldn't recommend it if you can avoid it.

It sounds to me like the problem is in the Primary side of your power supply if you're getting over-voltage at C905.
 
benime said:
You could swap a known working one, but the pads on that are pretty delicate and I wouldn't recommend it if you can avoid it.

It sounds to me like the problem is in the Primary side of your power supply if you're getting over-voltage at C905.
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I bought a handful on ebay just in case.

There's not a whole lot going on over on the Primary side of the power supply.

I *believe* I've properly checked Q901.
 
Okay, so per the manual...

The protection circuit has 3 parts.

So I decided to start with part 1 which is:
Q901 over-current/voltage protector by U901.

If U901 sees 4v+ on pin 7 then it freaks out and shuts everything down.

In my case, U901 sees 4v+ (and as high as 5.1v before it's through), and guess what? Shuts down.


There only seem to be a handful of parts in the primary power circuit prior to the U901 check:
U923, PC901, Q901, and R907.

Next step in my mind is to see what voltage U923 is actually receiving (it expects 180, shuts down at 220+). But that's for tomorrow me, I suppose.

My AC in is 104v so well inside of 100-120v.



Maybe Q901 /is/ bad, and I just misunderstood how to test it?
 
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Anyone happen to have the pinout for T901? I googled around for the part number but have come up empty :/.

ETS39AF2Z5NC
 
Following this. I too am in a similar boat. I have a 2931 that has no neck glow, just makes loud popping noises.
 
@radiantsvgun I've got a bunch of broken ones to mess with, so hopefully whatever I find should help?

Current one clicks on, then when it would normally static up the screen just clicks into shutdown. No pops here.
 
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