[AMRadio] Too much HV

Wed Oct 19 07:05:29 EDT 2011

The Collins 516F-2 power supply has this same problem.  The input choke 
changes value (resistance) and then the voltage increases significantly. 
Probably shorted turns in his case.  Did you measure the resistance of the 
input choke with the original value?  The comments from  Kim (below) are 
right on.

David Knepper, W3CRA/W3ST
Collins Radio Association - Join Today
www.collinsra.com
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----- Original Message ----- 
From: "Kim Elmore" <cw_de_n5op at sbcglobal.net>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Tuesday, October 18, 2011 11:57 PM
Subject: Re: [AMRadio] Too much HV

>I think the silly part comes fundamentally from blaming the HV
> increase on the SS diodes. At most, expect to see 15-20 V increase in
> the HV using them. But that's not what we are told is observed. Thus,
> it can't be the SS diodes. What might it be? Let's work the problem:
>
> Most of the older HV PS used choke input filters and a few used
> resonant filters, wherein the inductor and capacitor offered an LC
> circuit resonant at about 120 Hz (for a FW rectifier). Based on all
> the books in which this stuff is discussed, we should expect a
> capacitor input filter to generate approximately sqrt(2)*VRMS, or
> 1.414*VRMS, where VRMS is the RMS AC voltage applied to the rectifier.
>
> So, as an example if we neglect voltage drop in the rectifiers, if
> 100 VRMS AC is applied to the rectifier and then followed with a
> capacitor-input filter, we may expect the output voltage to be
> somewhere in the neighborhood of 140 VDC. If the filter is replaced
> by a choke input filter, we can expect the output to be about
> (2*sqrt(2)/Pi) * VRMS, or about 0.9*VRMS and is ~ 90 VDC.
>
> Let's look at our example: if we have a 3 kV CT secondary and use a
> FW rectifier with a capacitor input, we should expect a no-load DC
> output voltage to be about 1.4*1500=2100 VDC. That same transformer
> feeding a choke input filter will produce about 1350 VDC open-load.
> In the original posting, we're told that the rectifier is fed 1900
> VRMS. We also know that originally, the PS used a choke input filter.
> So, we expect to see 1710 VDC. Yet, we're told that the output is
> about 2300 V. If the input choke is shorted, then we have nothing but
> the capacitor and so a capacitor-input filter. We expect it to
> produce about 2660 VDC. We're told it's really 2300 VDC, which is a
> bit lower than we might expect but in the ballpark. So, a shorted
> input choke is strongly suspected.
>
> 73,
>
> Kim N5OP
>
> At 10:01 PM 10/18/2011, you wrote:
>>I have found the discussion in this thread interesting. What's silly about
>>it?
>>
>>Brad K4RT
>>
>>------ Original Message ------
>>Received: Tue, 18 Oct 2011 09:27:26 PM EDT
>>From: "Bernie Doran" <qedconsultants at embarqmail.com>
>>To: "Discussion of AM Radio in the Amateur Service" 
>><amradio at mailman.qth.net>
>>Subject: Re: [AMRadio] Too much HV
>>
>>look up the voltage drop for Vacuum tube rectifiers, mercury vapor, and
>>diodes. VTs drop around 50 to 60 volts, MV tubes 15 volts, almost 
>>identical
>>to a string of diodes. each diode is about 1/2 volt or so.    This is
>>getting silly, just what is on the currant amateur radio tests?    If the
>>voltage changes a measurable amount, something else has happened. not the
>>diodes
>>original Message -----
>>From: "CL in NC" <mjcal77 at yahoo.com>
>>To: <amradio at mailman.qth.net>
>>Sent: Tuesday, October 18, 2011 1:52 PM
>>Subject: [AMRadio] Too much HV
>>
>>
>> > Well, anytime I ever replaced a tube rect. with a diode, I always ended 
>> > up
>> > with a higher B+.  The HA 10 I am working on does not have the choke in
>> > it.  It arrived with a 1900VCT xfmr, a bridge rect. and  a cap string
>> > totaling 16uf.  Unloaded, the plate volts rests at 2300, to much for
>> > idling 811's.  I removed the bridge rect. used only half the diodes in
>> > full wave configuration and get 1700 no load, which is OK.  I picked up 
>> > an
>> > original Heath 3kv CT to put in it.  But in conjunction with Bill, who 
>> > has
>> > the an original HA10 with correct PS plate xfmr, swinging choke, and 
>> > oil
>> > caps, gets 2500 with a W2AU diode block.  I expect to end up with the 
>> > same
>> > problem if I switch out the xfmr,  So I'm looking to solve Bill's HV
>> > problem and avoid creating a similar one.  I am putting inrush 
>> > protection
>> > in it since the original HA10 plate xfmr was a known weak point, along
>> > with an internal  ant/amp bypass relay.  Guess the choke will not make 
>> > a
>> > return since it is
>> > obscenely expensive when newly manf., so upping the filter to a string 
>> > of
>> > 6  450uf at 450vdc.  While the amp is not needed for use with the Heath
>> > Apache, it does come in handy with the C Line when running it on AM. 
>> > One
>> > fellow questioned the xfmr, both xfmrs have the proper 3kv AC out of 
>> > the
>> > secondary, with 1500 each side of CT, no imbalance, so does not appear 
>> > to
>> > be a xfmr issue.
>> >
>> > Charlie in NC
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