Nanao 2931 Not Working

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    • Nanao 2931 Not Working

      Hello everyone,

      I recently purchased two Blast City cabs from KC with 2931's in them. A friend of mine visually guaranteed they both worked before shipment, but upon arrival one of them has no image, the other still works fine but it's a bit dim and the colors are fairly muted. I pulled the tube and chassis along with all the wiring and PSU out of the non-working cab and got it setup on a table and confirmed that the monitor doesn't have any HV squeal but the degauss coil clicks whenever it turns on. As further testing, I swapped the working chassis onto that tube and was able to get a similar dim and muted picture on the tube, so it seems like there's something wrong with the chassis. Research for similar issues indicates it's probably the HOT, but I'm not sure how to go about confirming that, and even if I did confirm it I'm not super sure how to fix it. The chassis itself was extremely dirty so I've cleaned it off and removed the dripped potting compound, but still no luck. I'm a bit lost when it comes to electronics other than basic soldering and multimetering and I'm just kind of looking for what direction to go in now.

      My thoughts are at this point to purchase a working chassis (probably a weiya or something similar), but I'm not sure where to source one, and use that chassis as a backup in case I can't fix the 2931 chassis. Reason for doing that would be to learn more about the chassis repair process for when the inevitable happens on the other. The other option obviously is to just not learn and send it off to Sharp Image for repair.

      Any help or resources you wonderful people could point me towards would be greatly appreciated. If you'd like pictures or video let me know and I'll be happy to provide them. As a side note, is there anything that will actually dissolve that potting compound? There's a plastic bit that sits in the chassis area of the tube that is basically glued down by the stuff that I can't get to with acetone and a qtip/toothbrush which is making it very difficult to swap chassis(pl?) in and out.

      Thanks in advance!
    • Thanks, that was somewhat helpful. It looks like no power is getting to u901 but 160vdc is coming out of the bridge rectifier. So it could just be a cap somewhere. I was a moron and broke my multimeter trying to test it, so I'll go see if I can get a replacement and test the capacitance of c905 or c901.
    • So, I've done a bunch of testing and verified AC input is good (120v from blast psu), BD901 (bridge diode and AC/DC converter) is getting 120v and sending 160v, T901 (step up transformer 160vdc to 180vdc) is sending power to TP921 (B+ testing pin) but the voltage begins to drop off immediately after power on so something is causing U901 to clamp on power. Q901 (power circuit MOSFET) tests good out of circuit. Tested pins 10 and 7 of U901 using a jumper wire this time, and the voltages are within range (pin 10 is 30+ vdc, and pin 7 is below 0.7vdc), so there's only a couple of things left to try. PC901 and some other diodes, the HOT or the flyback. Does anyone have any other suggestions about where to look? Most of the caps I've tested have also been good, the PC901 part is so cheap I might just buy some and swap it regardless as it appears to be having a malfunction, but getting non-hindi videos for testing photo-couplers has been hard. The flyback had leaked quite badly so it could definitely be the issue, I wonder if it'd be worthwhile to try to pick one up ahead of time from the chinese site in the other thread given the good recent results from it.

      nem wrote:

      ebzero89 wrote:

      but it's a bit dim and the colors are fairly muted
      Always start with the easiest solutions. Your monitor could just need calibrating.…or_Colours_and_Brightness
      I have fairly limited time so unfortunately, I haven't had a chance to go back and tune the working board yet, but I'll let you know how that goes, thanks for the link and suggestion.
    • Short update, the HOT tested fine. Can someone help me locate other possible protection circuit triggers or give common protection circuit points? I'll be replacing the opto-coupler at PC901 tomorrow when they come in. Trying to get someone to help with purchasing a flyback from taobao but that will obviously take quite a while.
    • New

      As a short update, I replaced that opto coupler, but still didn't have any luck. Still going into protection it sounds like. I shipped it back to KC for him to attempt repair.

      Some general questions I have though. It seems the Windy II cab uses the same tube but different chassis. Is there some reason a windy II chassis wouldn't work in a blast?