It should be pointed out that the test technician side of me comes up
with stuff like this, I doubt any of it is in a manual anywhere. Most
of this is more theory than anything I've ever witnessed. You may very
well just have a batch of bad rectifiers.
John Robertson wrote:
> 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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Received on Thu Feb 26 11:30:30 2009
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