[Milsurplus] ARC-5 Dynamotor

[mac]

DeWitt,

Changing the .01 to .001 doesn't make a dramatic difference.  Don't  
recall the voltage rating of the original cap (C66) but have run 274N  
xmtrs plate modulated with it in place and gotten away with it.  Many  
commercial ham rigs hung fixed caps across various xformer windings in  
the mod circuits.  Common failure points in my experience and I  
usually clip them out.  And yes, the paper caps in the 274N (xmtrs,  
receivers and other) do go bad in time like all paper caps of the  
period.  Haven't run into the VFO problem you mention.

73,

Dennis D. W7QHO
Glendale, CA



On Sep 9, 2010, at 7:04 PM, DeWitt Clay wrote:

> Interesting. Never thought about the SCR-274N HV plate circuit being  
> designed
> for screen modulation. How much of a difference does it make on the  
> audio
> quality/modulation results? Changing C66, the .01 to .001 uF I mean.  
> Might save
> the orginal .01 cap from shorting out from the higher peak voltages,  
> hihi.
> Heathkit used .02 uF on the secondary of the modulation transformer  
> in their
> DX-100 rigs. The original .02 cap had  already been replaced in my  
> DX-100 with a
> .01. I have gotten reports that the signal is good and has plenty of  
> highs.
> Somehow I didn't think .02 was neccesary so I left it at .01 uF.  
> Lots of audio
> availible in the DX-100. How about the other caps in the SCR-274N  
> transmitters?
> The paper caps no doubt need changing. Any recommendations there?  
> The rigs I
> have fired up appeared to have a strong (VHF?) parasitic oscillation  
> in the 1626
> VFO stage.
>
> 73, DeWitt  N4QNX
>
>
>
> ----- Original Message ----
> From: mac <w7qho at aol.com>
> To: WA5CAB at cs.com
> Cc: milsurplus at mailman.qth.net
> Sent: Thu, September 9, 2010 1:23:09 PM
> Subject: Re: [Milsurplus] ARC-5 Dynamotor
>
> I've plate modulated  274N transmitters with 500 - 550 volts into the
> PAs with no C65/67 arc-over problems.  Works better, though, if one
> swaps out C66 (.01 mfd) for something around .001 or so.
>
> Dennis D. W7QHO
> Glendale, CA
>
>
>
> On Sep 9, 2010, at 10:11 AM, WA5CAB at cs.com wrote (in part):
>
>> If you run modulated B+ to the plates of the series fed
>> units you might exceed the voltage rating of C-65 & 67.
>> None of the various
>> parts lists give any electrical rating information for the  
>> capacitors.
>
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