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
2) Resonating capacitor(s) too low in value or open.
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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Received on Wed Feb 25 22:34:02 2009
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