RE: [TCML] vttc issue power supply arcing during staccato mode

[earl rhodes <earl_1975@xxxxxxxxxxxxx>]

hi john

having a good old think about it and also consultng the uk list it seems we may have narrowed it down to a faulty mosfet 
we deduced  that it was connecting and disconnecting the filament rapidly like at rf sort of speed and causing a sort of contact bounce effect 
this would be really the only thing it could have been....
the staccato timing is fine  i have a scope and theres nothing wrong with it the pulse starts on a positive going zero crossing and ends just after the end of the peak ..the position of the pulse does not change at all with the number of pulses being made...im getting more parts tomorrow and should know if its still doing it pretty soon... no doubt i will post again with the result
earl


> To: tesla@xxxxxxxxxx
> Subject: Re: [TCML] vttc issue power supply arcing during staccato mode
> From: futuret@xxxxxxx
> Date: Thu, 4 Oct 2012 08:53:03 -0400
> 
> Earl,
> 
> Staccato tends to give more kickback than normal operation especially
> if the switching occurs when it shouldn't... if the timing is not just
> right.  The pulsing should sound "solid", not "snappy".  Adjust the
> timing pots to give a solid sound.  I'm not sure
> if this will fix the problem though.  I've never had the transformer
> arcs in my VTTC's.  In one early staccato design, the switching
> transistor was burning out so I used a series choke to protect it.
> Eventually I went to using a triac and later an SCR.  SCR's are
> more robust in general.  I used a 1200V, 50amp SCR but that
> was overkill.  I used it because that's what I had handy.  
> 
> Cheers,
> John
> 
>  
> 
>  
> 
>  
> 
> -----Original Message-----
> From: earl rhodes <earl_1975@xxxxxxxxxxxxx>
> To: tesla list tesla list <tesla@xxxxxxxxxx>
> Sent: Thu, Oct 4, 2012 6:54 am
> Subject: [TCML] vttc issue power supply arcing during staccato mode
> 
> 
> 
> hi 
> i spent all last night sorting out my staccato for my coil tested it first thing 
> this morning and it does indeed work !!...but  every time theres a spark or 
> enough voltage coming in to cause the smallest discharge there is arcing between 
> the primary and secondary of my power xfmrs 
> both of them in exactly the same place  they are going between where the 
> filament winding for the magnetron is and the primary  
> it is not a destructive arc it is id say about 10 maybe 15kv by the looks of it 
> and it is very persistent it happens every time theres a spark
> a look at the output waveform of the coil on an ungrounded probe to air it is 
> what you would expect really a smooth gain in voltage followed by a quick 
> dampening down  no unusual sighns
>  no spark on the output but i could still hear the arcing going on on the other 
> side of the xfmrs so it has no effect on the output  
>  siince the test my switching device (a mosfet) has died a quiet death  due to 
> testing after checking out the connections to ground and checking my rf bypass 
> was ok which it all is  and the fault was still present
>  so the mosfet was probably badly over voltaged but since the mosfet went and 
> normal operation was restored the fault dissapeared  so its only happening when 
> the tube connects or disconnects i have no way of telling which way round it is
> can anybody help 
> earl
> 
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