Nanao MS8-25FTB vertical collapse / shutdown issues

[MagicianLord77]

Hi, I've got a Nanao MS8-25FTB chassis that has vertical collapse, tube is Toshiba A59JMZ193X, yoke readings (not perfect) are:

• 0.4 ohm / 0.360mh
• 6.4 ohm / 25.47mh

While I'm trying to repair the chassis (difficult), could something worse than odd geometry happen if try the tube with a Kortek KTX26 chassis? (game is 15khz)
The usual Zenith monitor yoke readings that work well with Kortek are:

• 0.7 ohm / 0.750mh
• 27 ohm / ??

Investigated old threads but couldn't understand if lower impedance could mean risk of burning the yoke or something?
Thanks!

 

[trashedcabs]

Do not try to use the Kortek chassis, it is made for a standard NTSC yoke (high inductance), where the Nanao uses a high-res yoke (low inductance). The resistance doesn't matter as much as the inductance value, as far as "blowing up my chassis". If you try it, your HOT will fail or worse.

Dead vertical deflection issues are usually pretty easy. If you've done caps, you can look at replacing the vertical IC, looks like an LA7832 in this. Common failure and cheap part to replace.

 

[MagicianLord77]

Got it, I won't try the Kortek then, I guess I'll try it in the future with a TV tube+yoke since you mention it's could be a good fit for standard NTSC yoke (measuring inductance before), but that'll be a different topic, thanks!.

About the MS8-25 vertical collapse issue, this is what I had done with it before bringing it again to the workbench:

• Chassis looks in good shape, no signs of anything broken or previous repairs, haven't recapped it yet. Besides the vertical collapse it was shutting down by itself after some seconds - not sure what's the cause of that.
• Replaced IC401 LA7832 with LA7833 - arcadepartsandrepair says it's compatible.
• Installed socket and replaced IC103 HA11423 just in case (not sure if it could be related)
• Measured a bunch of components around the vertical area (ZD401, ZD501, both 6.2v zener are good), and after desoldering and closely measuring 2 transistors that seem related to vertical, multimeter displays some voltage (diode test) between C-E, got replacements, they don't have that weird reading, put them into the chassis but nothing changed:
  • Q402 PNP 2SA1408 Y - only found an O one to replace it with but hFE looks quite similar, maybe it doesn't even matter.
  • Q401 NPN 2SC3621 Y

So now I brought the monitor again to the workbench, and this time it doesn't even power on, I hear a slight "psu on" noise, but it is clearly not on, like if it was on shutdown protection or something?, hopefully after recap it'll work again.

Now I was doing a bit more of research since it seems the MS8-29 is quite the same (unlike the MS8-26), and there are an amount of threads about it. Found this one https://www.aussiearcade.com/topic/98425-nanao-ms8-29fsg-no-hv-or-shutting-down/ , mentions the same odd transistors readings that I had, and then his issue was a bad fusible resistor R438, so went and checked mine, it is bad too!, measures megaohms.
The weird thing is that he mentions "It's supposed to be 2.2 Ω", I guess because it's a MS8-29, mine is red/violet/green/green, so 2.7 Ω? (not sure what's the green/green). I'll update once I get to replace it

Thanks!

 

[MagicianLord77]

This is more difficult than expected, I can't find 2.7 Ω 1/2w fusible resistors anywhere, tried mouser/digikey, ebay and arcadepartsandrepair. Also I'm not even sure if it is 1/2w or 1w? measures 8.5mm length, 3mm diameter, so it matches a usual 1/2w carbon but that does not guarantee it.
Also found a parts list but it's from MS8-29, and not complete: https://wiki.arcadeotaku.com/images/2/2e/MS8-29FSG_manual_partlist.pdf

Power rating	Body length (l)	Body diameter (d)	Lead length (a)	Lead diameter (da)
Watt	mm	mm	mm	mm
1/8 (0.125)	3.0 ± 0.3	1.8 ± 0.3	28 ± 3	0.45 ± 0.05
1/4 (0.25)	6.5 ± 0.5	2.5 ± 0.3	28 ± 3	0.6 ± 0.05
1/2 (0.5)	8.5 ± 0.5	3.2 ± 0.3	28 ± 3	0.6 ± 0.05
1	11 ± 1	5 ± 0.5	28 ± 3	0.8 ± 0.05

https://eepower.com/resistor-guide/resistor-standards-and-codes/resistor-sizes-and-packages/

Closest I could find is a 1W 2.7 Ω one, I wonder if it would be safe to use it? it is curious that it is 1W but smaller (6.5mm length)
https://mou.sr/4lrf9rc Vishay PR01000102708JA500
Also I don't even know if it's really fusible, only thing it says in its datasheet is "defined fusing characteristics" but not classified as fusible

Closest one that is really fusible is 2W 2.7 Ω (10mm length), so it would be clearly above the required wattage, this could mean it could not fuse when needed
https://mou.sr/3I4JQnF Vishay PR02FS0202708KR500

Could a fusible resistor be made somehow by combining a regular resistor and a fuse in series?