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MS8-29 dim picture, heater voltage half what it should be

Aurich

Aurich

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I'm trying to troubleshoot an MS8-29 with a dim picture.

At first I just figured it was an old tube. Happens. I did a full recap of the entire chassis just to be safe, no difference. We put a rejuv on it and ran the the tests, and the guns were definitely low. But after some rejuv cycles everything was all the way up in the green, so I figured we'd see at least some improvement.

Nada. Just as dim.

This made me suspicious there was something else going on, so I measured the voltage at pins 9 and 10 at the neckboard, which should be 6.3v. I was getting 2.9v. Welp, that sure seems to explain it.

I checked the B+ for the chassis, should be 94v I believe, it was dropping down to 91v after warming up. Adjusted it back up to 94, no difference.

My next working theory is that this was maybe a tube swap, or a mismatched neckboard, something where the heater resistor wasn't matched up right. Checked the resistor, it's 4.7ohms, which is standard for the MS8 as far as I can tell. But still, if there was a mismatch maybe it was still wrong.

So I picked up a handful of different fusable resistors to experiment. I'll skip to the end here, I went as low as 2.2ohm instead of 4.7ohm, and the heater voltage only went from 2.9v to 3.3v. Enough that I had to turn the screen pot down slightly to get the blacks back, but hardly brighter.

Then I measured off the first leg of the resistor to the neckboard, and got 3.8v. Which seems to confirm the issue is before the heater resistor and there's not a mismatch, the voltage is just low.

Going to try and trace back and check components, but if anyone has seen anything like this before and has suggestion for things to look at all ears. :)
 
I’m not certain if this applies to every monitor but my understanding is with the heater voltage being sourced from the flyback and the flyback running the same frequency as the scan frequency (15.72KHz standard res) that you’ll need a TrueRMS DMM to take an accurate reading, most DMM count on 50/60Hz in order to be accurate.
 
jamesv833 said:
I’m not certain if this applies to every monitor but my understanding is with the heater voltage being sourced from the flyback and the flyback running the same frequency as the scan frequency (15.72KHz standard res) that you’ll need a TrueRMS DMM to take an accurate reading, most DMM count on 50/60Hz in order to be accurate.
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Oh hmm. Okay, that's worth looking into more, I hadn't considered that I couldn't just take a reading off 9 and 10.
 
I'm just using my little cheapo DMM for this:

IMG_4726.JPG
 
Yes crappy meters are crappy (only useful in certain circumstances). Good meters can do it no problem (or at least more than usable information). I'll attach a picture of what my UT210e (crappy....well awesome mA clamp, but not high end meter) reads vs what my fluke 289 reads. You can even view freq with 289 and adjust rising/falling, etc(I have output at 15.8 on my generator). If your b+ is good and you haven't been neck or tube swapped then your heater should be good. If you don't want to go down the rabbit hole of measurement (without getting new meter or scope) you can verify that is even your issue by isolating the heater and swapping out the heater voltage for any other floating voltage really (if you have a variable voltage source like lab power supply, variac, etc). You can even manually wind one off the flyback (but then you still run into measure issues since same frequency you cant read now). At least that way you can eliminate as your culprit by If you input voltage and problem goes away then back down rabbit hole. If you swap and issue stays, you are looking elsewhere.

alternative I am selling a fluke 287 which is same as 289 except no loz and loohm :P
 

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Very interesting, thanks! Been doing research since this point came up and found a lot of conflicting info. Good to know you can accurately measure it, just ... not sure I'm itching to buy a $650 meter. I won't get enough use out of it to justify the cost realistically.

In the meantime I stuck my cheap meter on another MS8 that looks great and is tuned up and got 2.2v. I'm definitely not trusting these numbers anymore! I put a 4.7ohm resistor back in the other chassis to not screw around with it while my data is poor.

I should probably just leave it be and spend my effort trying to fix the sync issue on my spare MS9 so I can just yank this MS8 and set it aside as an emergency monitor.

( https://www.arcade-projects.com/threads/help-nanao-ms9-sync-problem.19322/ )
 
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brad808 said:
If you have another have you swapped chassis onto each tube? That should narrow down for you chassis or tube issue.
Click to expand...
Yeah, I have one more cab with an MS8, and I agree, that's the logical step at this point. Just a bit of a hassle so I haven't done it yet. But it's not that much work really, can knock it out in an afternoon. Probably do it at some point when I have a little help, prefer to take out monitors with two people.

I thought I was onto something! But instead I learned something new, so not a total loss. :D
 
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