[Hallicrafters] HT-33B HV problem

[Grant, Ken]

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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