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PSA: MS293X, faulty chassis can break the tube

When I was testing my two chassis with two bad flybacks I am fairly certain I saw arcing (bright blue or white) down in the yoke. I chose not to believe it because I had just read this thread at the time and didn't want to lose hope. Thankfully the tube is still good and running with one of those ebay flybacks now.
 
nem said:
I would like to remind everyone that getting a replacement tube for a MS2930 is a huge PITA. Best guess right now that for this chassis the problem is the two SMD caps on the MSPAC004 sub-board:

mspac004_1.jpg



If you've had your MS2930 "serviced", more than likely they haven't done jack shit to the SMD caps. Get them replaced!

Thanks to mufunyo on Arcade-Otaku for the tip.
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I replaced those earlier this year when working on my Blast City. They weren't part of the capacitor list on Arcade Otaku wiki when I checked at the time, and I doubt are being included with "cap kits". They're easy to miss if you are only focused on the through-hole electrolytics. The original SMDs are likely good quality but there were some really bad ones going around in the 90s and again in the early to mid 2000s. Definitely a good idea to replace while replacing all the other capacitors.
 
Hatsune Mike said:
I can't help but notice that that pinhole is right below the enormous resistor that goes from the anode/HV to pin 4 on the neck. This further supports my theory that an arc is happening through the neck to the yoke. Perhaps this is something that happens as a result of a spike in HV.
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There's this post on ArcadeOtaku by PrincessPrinPrin:

Newer Toshiba tubes (families KZN, LBT, LTF, etc.) have a unique double focus design. Grid 4 (which together with grid 3 forms the focusing lens), which is at fixed anode potential on standard tubes, here is adjustable as a voltage divider has been incorporated in the neck. The ground end of this divider is coupled to pin 4 of the CRT socket (an unused pin on standard tubes). To make it adjustable, pin 4 is connected to ground through a pot. On the Nanao MS2931 chassis it's not adjustable as a fixed 10 Mohm resistor was used instead.
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All of the blown tubes have been Toshiba brand. The Blast tube is a KZN or a LBT (and possibly KSM?), the PF tube is LTF. They all have that voltage divider in the neck.

There's also a gnarly video in that thread where you can see what happens when you connect one of these tubes to a chassis that doesn't have pin 4 connected.
 
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I got to find out what happens if it’s disconnected by using an MS9 on a tube with that pin - nasty arcing sounds, though no actual damage at that time.
 
He didnt go into specifics buy when I spoke to Jomac about having an MS2931 repaired he agreed to repair it on the condition that it wasnt going into HV Shutdown. Both himself and Sonya who works in the office stated that when they are going into HV Shutdown they can blow the backs of tubes.
 
Been a while since I was hands on with my chassis, can anyone point me in a general direction as to where I'll find the MSPAC004 sub board on the chassis?
 
I was doing monitor work today and the MS2931 has the same MSPAC004 sub-board at U701 as the 2930.

MS2933 also has U701, but it's much smaller than MSPAC004, with only miniature SMD components on it.

MS2934 doesn't have U701 at all.
 
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nem said:
MS2933 also has U701, but it's much smaller than MSPAC004, with only miniature SMD components on it.
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Yes, looks like ceramic components only. On mine at least.
 
Are we thinking that Nanao updated this board and phased out the previous design after it blew out a few tubes?
 
A few more.

1) MS-2930, with XM29 tube (is a rebranded A68***696X, I am 99% positive)
nw5dFyb.jpg


2) MS-2930, service / life history unknown
RXJxVCp.jpg


3) MS-2930/2931 from arcade in Tokyo; chassis saw partial recapping but the original orange capacitor was there. I received this alongside some blasts. The arcade is frustrated with the Blasts being this way and the maintenance needed for them, so they are switching to Vewlixes.
uN6GsMp.jpg

JllkIeW.jpg


All of these have a pinhole from an arc starting with that metal "cup" piece at the end of the electron gun assembly, not the big resisitive divider forming more grids. I'm not sure this is the tube's fault at all, and may just be the chassis and HV circuitry alone.

To date, have we ever heard of an MS-2933 (the one with a different flyback and HV driver) cause this?
 
I was of the belief that the 2933 fixed a number of design issues/flas that the 2931 had.

That being said, all necked tubes I've heard about has been with a 2930 or 2931
 
And one PF.

All of the blown tubes have been the ones with the resistor in the neck. Admittedly, they're the ones normally paired to 2930/31, but still.
 
Generally speaking you're not going to find a Ceramic with the same capacitance, tolerance, and voltage needed to replace an electrolytic, if you're looking for non-leaking replacements for electrolytic then you'd want to look at Tantalum Caps, they're more expensive, and slower (which might not work for this application) but that's typically what gets used in place of electrolytic

those ceramics you linked are rated for 22µF ±20% 6.3V and 2.2µF ±10% 50V respectively, what are the voltages and tolerances of the caps being replaced?
 
The pb-7534 has a similar board also.
 

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