Re: B&W HV cages...regulation failing...

Rodger Boots wrote:
>
> Another thing that could cause HV diodes to heat is if frequency
> (actually dV/dT) is too high. By that I mean:
> 1) Oscillator frequency actually too high
Interesting point - that makes sense, I shall put a frequency counter on
it and report back - or is that in the specs in the manual (which is @
the shop and I am at home right now). It will be good to have that test
added in any case.

The HV (not really a flyback is it?) transformer also heated up quite a
bit...
> 2) Resonating capacitor(s) too low in value or open.
That was the point raised about running the HV cage without a monitor
attached to act as a load - I was not aware of that as a problem.

I learn something every day!

Thanks,

John :-#)#
>
> Either way the diode doesn't get a chance to fully turn off as the
> reverse voltage rises, it ends up pulling current and gets hot.
>
>
> John Robertson wrote:
>> John Robertson wrote:
>>>
>>> OK, I've got a couple of WG HV cages for Asteroids and both are
>>> croaking when warm.
>>>
>>> What appears to happen is the HV diode is getting hot just sitting
>>> on the bench, but not connected to anything other than the HV lead!
>>> Ihave replaced the HV lead with another from another flyback
>>> (colour) and the diode still heats up.
>>>
>>> Have cleaned the wire, and support post, yet somewhere there is a
>>> current leak. Have insulated the wire with a silicone collar where
>>> it goes through the metal cage (and the plastic bit there).
>>>
>>> Now, if I remove the outside half of the HV wiring - from the
>>> positive end of the HV diode - and simply cap the diode with a
>>> plastic pen cap, then the voltage regulation is stable, and the
>>> diode stays cool - as expected.
>>>
>>> However if I then put the HV wire back on the diode (carefully
>>> replaced insulators of course!) the sucker gets hot and the HV
>>> regulation fails (voltage drops). Note that the anode cap is sitting
>>> inside a very clean, insulated ceramic mug for isolation and safety
>>> (mine!).
>>>
>>> Very curious! Will continue working on it in the next day or two and
>>> report my findings - but this seems to have a bearing on why some
>>> cages just seem to die and yet, may be good if the cause of the
>>> leakage is found...
>>>
>>> John :-#(#
>>>
>> OK, thanks for the replys - they have helped guide me to where the
>> problem actually is - the HV diode!
>>
>> If you have a HV cage that is shutting down after a while check the
>> HV diode end caps for signs of overheating - they turn black and
>> brittle when the HV diode is bad.
>>
>> What I am now finding is some of my supposedly NOS HV diodes have
>> internal issues that make them effectively act as loads to the
>> system. The diodes - when hooked to a good picture tube to act as a
>> proper load - are heating up and then the voltage regulation gives up
>> when the load increases too much. Trying other HV diodes (all are NOS
>> H1812s), has some working fine and some failing. I have about ten to
>> test...
>>
>> So, one major rule - NEVER short the anode of a picture tube directly
>> to ground to discharge it. We stopped doing that years ago, but some
>> people still do this. This can cause the HV diode to have issues like
>> the above.
>>
>> John :-#)#
>>
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-- 
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Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
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