Yoke troubles

For monitor related issues
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mufunyo
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Yoke troubles

Post by mufunyo »

I'm still getting my Blast City monitor serviced, thankfully the guy working on it is extremely patient and careful, if a bit inexperienced with CRTs.

This monitor has had quite some history, it started off as an MS2930 that was killing tubes left right and centre, and eventually a repairman figured out that an SMD capacitor on a sub board was faulty and replacing it made it stop killing tubes. A tube was fitted and a matching yoke found, and it worked for a while, until it started developing an issue where the screen would randomly zoom in and out rapidly. I decided to source a "known working" MS2931 board for it instead, which sat under my bedroom couch for years as I tried to find somebody willing and able to work on it.

Now the MS2931 is fitted, and mostly working, however the repairman working on it now (different than the one who fitted the yoke originally) tells me the following:
The vertical output IC is burnt out, and he can replace it, but it will probably burn out again. The yoke's vertical resistance is too low, overloading the output IC.

I have a spare yoke with the right values (0.7Ω 0.20mH H, 7.2Ω 17.1mH V), but fitting it seems dangerous: there is about 4mm of play in the neck when gently trying to (re)move the yoke. He's afraid of cracking the neck while messing with it. I didn't know this was even possible without having an already cracked and gassed tube, so I don't know if this is even normal. It doesn't move side to side, but it moves backwards and forwards slightly.

Open questions are:
Is it normal for the neck to move at all?
If so, how to safely remove the yoke?
If not, what the ****? Haha
Is it possible to modify the chassis circuit slightly to accept the current yoke without burning out the vertical IC?

He mentioned carefully heating up the glue that's keeping the yoke attached to the tube, but I'm thinking that applying heat unevenly to the neck might crack the tube as well.

The yoke that's on it now reads 0.8Ω 0.32mH H, 5.5Ω 12.3mH V.
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grantspain
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Re: Yoke troubles

Post by grantspain »

the yoke you have on the tube is not matched to a 2931 chassis, its more closely matched to a sanwa pfx or a nanao ms9-29 or a weiya 3129a
its not just your vertical thats out, your horizontal is well out as well

if you want a working monitor then you have no choice but to swap the yoke with all the issues that raises or swap the chassis to something within the correct tolerence range
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nem
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Re: Yoke troubles

Post by nem »

mufunyo wrote: November 24th, 2022, 8:17 pmOpen questions are:
Is it normal for the neck to move at all?

It's not possible. The tube is a single piece of glass. If the neck moves, the tube is broken.
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mufunyo
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Re: Yoke troubles

Post by mufunyo »

nem wrote: November 25th, 2022, 2:12 pmThe tube is a single piece of glass. If the neck moves, the tube is broken.
Well, rip then. I've mailed Mo as he seems to have an A68KSM696X going off an extremely blurry pic on his website. No idea how much of a ball ache shipping would be since Brexit, so I'm also going to look on Neo-Arcadia and that Italian forum if I can remember the name of it...
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