[Hallicrafters] HT-33B HV problem

[rrkrr]

Ken,
I may be stating the obvious here, so please ignore if that's the case.

The 0.9x RMS input figure for the output voltage of a choke input filter 
applies under current load.  The choke acts to reduce the ripple current 
in the filter by inducing series voltages that oppose the ripple 
current, but the output voltage for a choke input filter will normally 
rise as you describe if there is no current flow.  The usual method of 
preventing the output voltage of a choke input filter from rising too 
much under no load conditions is to add a high power bleeder resistor 
that draws a significant amount of current.

I'm not familiar with the HT-33B specifically, and your discovery of the 
resistive leakage path in the inductor sound like it is a probable 
cause, but if not I would check for an open (or non-existent) resistive 
bleeder.  Also check the filter output voltage under the rated current 
load, if that is possible.

Bob
K4ERR

Grant, Ken wrote:
> Hi guys,
>
>  The plate voltage meter on my friend's HT-33B is showing 4200V rather than
> the specified 2700V. This was verified with an HV probe and DVM.
>
> At first, we thought that the primary winding on the plate transformer might
> have developed a partial short, effectively raising the transformation
> ratio. Hence the request for some solid numbers on the DC resistance of the
> transformer windings. 
>
>  The filter circuit used in the HT-33B is a choke input. Reading up about
> them in an old ARRL Handbook, I found that the DC output of such a circuit
> is about .9 x the RMS input. Thus,  .9 x 3000 gives the expected 2700 VDC.
>
>  If the circuit were a capacitor input filter, you would expect to see 1.4 x
> the RMS input. This would yield 4200 VDC, exacting what we are seeing. Aha!
>
>  The filter choke, L3, is wired from the center-tap of the transformer's
> secondary winding to ground. This puts less of a demand on the choke's
> insulation since the 'hot' end of the choke is at a relatively low voltage
> above ground. Nonetheless, my friend Ian ( VA3IC )checked L3 and found a 400
> Ohm leakage from one of it's leads to the case ( which is grounded )! 
>
>  The circuit had, therefore, been turned into a capacitive input filter and
> the filter capacitor, C25, could charge to 4200V. Luckily, C25 was a 5000
> VDC part ( even though the schematic shows a 3000V part ).
>
>  We'll keep the reflector posted as to our progress with the repair.
>
> Ken, VE3FIT
>
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