NEC Multisync M500 (JC-1572VMA-1) Monitor

[TinnedTrace]

2 days ago got a NEC Multisync M500 (JC-1572VMA-1). Bought and picked it up without testing (though for 10 euro it's a risk I'm willing to take), but it doesn't work properly. Power LED on, it recognizes a signal (without one it drops to standby), the degauss thunk fires, the relay sounds normal, and there's a faint orange glow at the tube neck. There's no static feeling on the screen and no raster. Checking voltages on the flyback (4710-5705 / HFT2009) one pin spikes to 15V only to drop again on power up and everything else on the flyback stays low.
There are also no obvious blow/leaky caps though it's hard to properly check on close as it's now been a day and I still can't figure out how the board is held in the case, I removed all screws and can't find any latches.

Anyone any idea what it could be? Or if someone has a service manual (or a non-destructive way to get the main board out, as for the life of me can't figure that out) it would be greatly appreciated.

Here are some possibly helpful images:
https://imgur.com/a/2grwJEl

Edit:

Found a way to open it, it was held by 2 tabs that seem to require almost breaking the case to get loose.

Some pictures of the component sides of the boards:
https://imgur.com/a/gVNjPF8

[Username1]

.

Good Morning-

You will have to get a service manual to this thing and begin looking at the power supply
circuits most likely you have a bad capacitor somewhere. The NEC manuals
have strange diagnostic steps and not very informative snipits of how
it all works. I have a 15" NEC Multisync that also needs work that I
need to work on this winter.


.

[kf4rca]

I assume you're in Europe. What mains are you using? What mains does it require?

[kf4rca]

A while ago I ran into the same problem with a Dell (Sony) monitor. All the regular checks checked out. It was less than2 years old so the caps were probably still OK. I talked the CE into ordering the service manual ($20). So I'm zero-ing in on the problem area and suddenly I see it. An SMT transistor was hanging by just one lead. Resoldering the transistor fixed it. The secretary was happy to get her monitor back.
Those multisync monitors are really nice. They lock to the oddball scan rates that multimode monitors will not lock too. Sun Microsystems and Silicon Graphics computers both use oddball scan rates. Those are Linux computers. We had Silicon Graphics computers in the Weather department.
Remember this: if it worked before, it can work again!

[TinnedTrace]

Originally Posted by kf4rca

I assume you're in Europe. What mains are you using? What mains does it require?

Yea, Netherlands. We use 230 and it needs 230, it was a local second hand purchase.

I did check for obvious faults and broken solder joints but no luck, currently doing a full recap to just check things. And yea, NEC multisyncs are amazing, I do a decent amount with Japanese computers and 24Khz isn't commonly supported by anything but NEC multisyncs do even modern flat panels with a VGA connection. (Why they still support it in the current day is a mystery to me though, unless its grandfathered in)

[kf4rca]

Mitsubishi DiamondScan monitors are also multisync.
Our GM had a NEC TV in his office. It was the first one I had ever seen. It was MTS which was popular at the time. I had heard that NEC had gotten out of the American TV market as it was too competitive.
NEC also made transmitters. WRIP TV61 in Chattanooga had one. I believe it was 60KW.

[Ed in Tx]

Originally Posted by kf4rca

Our GM had a NEC TV in his office. It was the first one I had ever seen. It was MTS which was popular at the time. I had heard that NEC had gotten out of the American TV market as it was too competitive.

I had a 30" NEC CT-3000S from 1988 until 2008. Went through 1 CRT and 2 flybacks, plus resoldering the video "jungle" IC. It was a great TV in it's time. Pioneer sold several TVs back then that were made bt NEC.

[vol.2]

Originally Posted by Ed in Tx

I had a 30" NEC CT-3000S from 1988 until 2008. Went through 1 CRT and 2 flybacks, plus resoldering the video "jungle" IC. It was a great TV in it's time. Pioneer sold several TVs back then that were made bt NEC.

That was a great set!

My friend's family had one in their family room and I watched pretty much all of Transformers on that set hanging out after school

[TinnedTrace]

I made some progress.

Couldn't find the M500 service manual to save my life but the M700 seems to be it's big brother. https://archive.org/details/manual_MULTISYNCM700_SM_NEC Would still like to find the M500 service manual if it exists.

Recapped the whole board without success. But the capacitor list for the M500 I made myself is close to identical to the M700, I think about 20~25% might be different. And a lot of other parts are also the same.


Also followed the circuit and shorted Q2006 Base and emitter, which resets the latch: C5G3 rises to ~58 V, HV comes up, and after a few seconds I get a normal picture. If I hold the short until the picture appears and then remove it, the monitor keeps running normally. The latch only trips at start-up. A hot restart (off 2-3 s, then on) also trips, so it isn't a cold-CRT / no-beam-load issue.

So something is tripping the latch on startup. Don't know what yet.

[kf4rca]

NEC made computers also.
NEC monitor test software here:
https://www.softpedia.com/get/Tweak/...enerator.shtml

[TinnedTrace]

Yea, that's why I want a proper multisync. 24Khz doesn't really work on most monitors.

Some more progress, I think I've found which part of the protector is firing.

The protector has three inputs feeding the Q2006/Q2007 latch: the HV branch (D2002/ZD2001), the primary overcurrent branch (Q5F1/R5F3) and the FBT secondary leak branch (Q2001/R2005). I lifted one leg of each in turn. With R5F3 lifted it still trips. With R2005 lifted it starts every time. So it's the secondary leak branch. Its sense node (R5U3/ZD2002/C5U1) takes 1~2 s to come up to about 15 V, which is longer than the latch delay.

Then I put the scope on B+ (C5G3) at a cold start, with R2005 lifted. Right at switch-on there's a short spike to ~118 V, then it humps up to ~99 V for about a second before dropping back to ~55 V. Soft-start reference on IC5H2 pin 3 looks normal, ramps to ~4.6 V in about half a second.

The interesting part: I also scoped C2001 (the rectified flyback pulse from FBT pin 8, the same thing the HV protector looks at). That one just ramps up slowly to ~27 V over about 1.2 s and doesn't follow the B+ hump at all. So the HV isn't actually overshooting. For the first second the flyback just makes a much smaller pulse than it should for the B+ it gets, and the loop pushes B+ up to make up for it. No white flash, picture fades in normally, so I don't think it's beam current.

So it looks like something on the horizontal side is weak or in the wrong mode for the first second after power on. Next I'm going to check:
- H frequency at switch-on (is it 31.5 kHz right away or does it start somewhere else)
- Base drive on the HOT (Q561) during start-up
- The driver supply cap C554. The recap put 1000uF 50V there where the M700 has 470uF 25V, but the fault was there before the recap.
- Band switching (Q57x MOSFETs) in the first second

Smaller things I found on the way: the chopper sawtooth on IC5H1 pin 3 flat-tops at ~10 V instead of the ~7.5 V the clamp zener (ZD5H4) should give, so that zener might be open. Going to test it and maybe swap IC5H1/IC5H2 for LM393/LM358 while I'm at it.

Has anyone seen this behavior before? Or is that B+ hump at switch-on normal for these and the protection is supposed to ride it out? Still looking for the actual M500 service manual too if anyone has it.

(Only running it with R2005 lifted for short tests, not leaving it like that.)