Intermittently Dead Monitor

[wallybob]

I recently got a Double Dragon II cabinet that has an intermittent WG 7901.  The main correlation I have been able to make is that the filaments in the CRT neck don't glow when it doesn't work.  From the schematic, the filaments circuit appears to be basic doorbell circuitry and I have done some continuity checks in this area and everything appears reasonable.  The unit makes a sound when power is applied that makes me believe that the anode is getting high voltage which make me think that the flyback primary is getting what it needs.  Anybody have any other ideas?  I plan on bringing a scope home this weekend to investigate further...

[Jess--]

not sure on the tube make on your screen, but a few years back there was a big problem with mullard tubes failing internaly on the heaters (guns)

quite often a tap on the side will kick it back into life

[Ken Layton]

You have a k7000 series monitor. They are well known for many bad solder joints (mostly on the main board). Bad electrolytic capacitors can also trigger the monitor into false shutdowns (intermittent operation).

Check closely for bad solder joints and install a capkit right away.

[wallybob]

Thanks for the responses guys.  I have looked into this further and suspect that the high voltage shutdown circuit is kicking in.  I have filaments and horizontal drive for a couple of seconds after power up, then it goes away.  When it works, voltage on neckboard P202 pin 3 is around 129 volts; when it doesn't work this voltage is 162 volts.  The trouble shooting chart says that I need to fix whatever is causing the high voltage being too high and suggests replacing C36.  Do you know of any other typical components that could be causing this?  Should I try readjusting the shutdown pot or is this a no no?  Any info is greatly appreciated!

[Ken Layton]

My advise is to install a capkit first and foremost before doing anything else. Like I said in the earlier post, a bad electrolytic capacitor will trigger a false shutdown of the monitor. I've repaired enough of these monitors too know.

If you want to be cheap about it, capacitor C57 (47 uf @ 160 volts, but it needs to be upgraded voltage-wise to at least 200 volts) is the one that causes the problems, but I would do a full capkit anyway.

Don't even think of touching the HV shutdown pot!

[wallybob]

Ken, your advice was right on.  The cap kit cleared up the shutdown issue and the monitor is working like a champ!  Thanks for all your contributions to this forum.

[Ken Layton]

Excellent! 

Glad to have helped out.