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Help fix my Domy Theater! Taxan GNN-33J Fault

kasaski

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Hey all,
Having an annoying issue with the chassis in my newly acquired domy theater. works great on a cold boot, image looks good (albeit with some video noise I'm still trying to work out..) but after its warmed up, the chassis will lose sync and the 24khz auto switching relay will begin clicking, as if I've sent the wrong signal to it. If I shut it off than on again it'll continue to click. After waiting a few minutes to cool off it will start up again fine but repeat the issue.

Its pretty easily triggered by neo geo attract screens, Once the attract loop starts over itll trigger immediately. However I've managed to keep the chassis working fine for like 30ish minutes on a still screen like a test grid.

I know its a fairly niche chassis and no one knows much about it but pretty much any advice on how to diagnose this will be greatly appreciated. Otherwise i may just attempt a shotgun approach with caps.
 
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These can be hard to fix.

Easy repair would be reflowing solder joints and replacing caps.

Hard repair will be to find a component that breaks under stress (a transistor or diode).

Ideally you would have and know how to use a scope.

See here for a similar repair on a different chassis:

https://www.arcade-projects.com/threads/nanao-ms7-18-sync-issue.5257/#post-419580

Personally, I would ask Jomac if they can take a look at it.
 
thanks for the thread ill have a read through it.

I hit up jomac last night, and unfortuantely not something he can work on, since he has no tube to test it with.
 
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Finally have had some more time to try diagnosing this, heres an update:

I ended up recapping the horizontal IC area and replacing a few transistors there in hopes for a quick easy fix - no luck.
Pulled the chassis again and performed an extensive reflow - I still get the same issue.

Despite the awful sound of relay clicking I decided rather than panicking and shutting it off immediately id check a few more things to ensure my fault description is accurate.

Turns out it probably has nothing to do with sync so i misguided myself a bit there.

- CRT turns on perfectly happy on cold boot and looks great, I also no longer see as much video noise after a big clean and new psu in the cab (still very feint), but this could just be because i pulled out the fan that runs on ac mains connected after the noise filter lol.
- After about 10 minutes the crt shuts off, relay starts clicking and neck no longer glows, I've lost HV? Now im assuming the relay clicking is actually just power cycling and the chassis is in current protection.
- Shut off the screen and im able to turn it back on immediatly and its all good again for about 5-10 minutes before it faults again.
-As noted in my first post, its seems like a static non stressful screen like a test grid lets the chassis last longer before shutting off

Whatever is causing the issue is cooling down/recovering very quickly, so im guessing its something with a heatsink - maybe the hot, the voltage regulator itself and so on. Its quite cold atm so those heatsinks would be cooling everything off very fast. Or maybe the filter cap is right at the end of its life, that could also explain the video noise?

Ill be testing the hot and other transistors, but If anyone has any pointers of first things to check please let me know.
 
I know you said you recapped the horizontal section but I would check the power supply section. It's possible B+ is going too high after warm up and the monitor is shutting down. I assume there's no manual or schematics for this so you might be able to check the voltages at the diodes on the secondary side and see where they're sitting and if they change over time.

Might also be helpful for anyone else who owns this rare monitor to make a cap list as you go through it.
 
I actually ended up finding a very dead capacitor near the flyback that I'm waiting on a replacement for, so will update when i swap that out and give it a test
 
The replacement arrive and.. Good news! Replacing the 100uf 50v capacitor beneath the large ceramic resistor next to the flyback resolved the issue! All the other caps on the main pcb tested perfect so i wont replace them. Still have a tiny hint of video noise but i can live with that, probably a cabinet grounding issue somewhere.

My take away from this is to buy an esr meter of my own.

Been running it for over an hour without a hitch :D. Very relieved that it’s fixed for now, I can move on and stop thinking i bought a giant paperweight.
 
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