[Boatanchors] Advice Request on Heathkit HP-20 Power Supply

[Glen Zook]

Are there bypass capacitors from each side of the filament windings to ground?  If so, there is a very good chance that what you are seeing is just like the old bypass capacitors from each side of the AC line to ground when the chassis is not grounded.

In most radios, one side of the heater ("filament") voltage is connected to the chassis and that should eliminate this voltage problem.  You can remove the bypass capacitors and that shouldn't hurt anything.

Also, installing a 3-wire cord should definitely be done for safety reasons.  Connect the green wire to the chassis, connect the black wire so that it goes through the fuse and then to the power switch.  The white wire goes directly to the "other" side of the primary winding.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: D C _Mac_ Macdonald <k2gkk at hotmail.com>
To: "boatanchors at mailman.qth.net" <boatanchors at mailman.qth.net> 
Sent: Tuesday, January 22, 2013 6:46 PM
Subject: [Boatanchors] Advice Request on Heathkit HP-20 Power Supply
 

I just bought a Heath HP-20 power supply on the bay. 
It arrived today and is in beautiful physical condition. 

I opened up the bottom and found a lot of stuff that 
was NOT in the Heathkit manual. 

While I didn't try to draw a schematic for it, what 
was in the manual was mostly correct for the high and 
low B+ and C- supplies. 

The filament supply was something else!   It appears 
that a prior owner tried to rectify the filament supply 
to provide DC to "something." 

I removed all of that stuff and rewired the HP-20 to 
original wiring status. 

Next came a test of the outputs (no load).  Both B+
and the C- bias supplies had reasonable voltages. 

The rewiring of the filament section gave me a bit over 
14 VAC, no load. Again, reasonable without load. 

Then I decided to check the filament windings against 
ground.  OH, OH ! !  About 48 VAC from one side and 
about 36 VAC from the other side to ground! There is 
NO wiring from either side to ground. 

I'm guessing (never ran into this in 59 years as a ham) 
that the power transformer is leaky.  The HP-20 has 
had a two-wire cord installed; the known dangerous 
in-line fuse plug is gone. 

What are the dangers?  Could installing a three-wire cord 
help out here?  I sure don't want to risk electrocution!