[Milsurplus] [ARC5] Fall Project: Navy ATD Transmitter, Part 2

[Michael Hanz]

I put this together to send to the two lists, then saw who I was 
answering and realized that it would just prolong the agony of debate, 
so I changed my mind.

Anyway, you are right - far lower voltage cap required for C-107.

I always enjoy your reflections on these sets.

Have a great weekend,
  - Mike

On 10/11/2017 2:27 PM, AKLDGUY . wrote:
> On Wed, Oct 11, 2017 at 6:55 PM, David Stinson <arc5 at ix.netcom.com 
> <mailto:arc5 at ix.netcom.com>> wrote:
>
>     Re:  Fall Project: Navy ATD Transmitter Part 2
>
>
> > The dual .1 uFd, C-107, leaked
> > badly.  This cap is rated at 600V, but it's across
> > the filament of the MO, V101 and the common of the
> > two caps goes to the tap on the MO tank, not
> > ground, so a smaller DC voltage rating will work.
>
> The 600V rating may have been chosen because there may be relay 
> windings across
> the filament line, just as there are in the Command transmitters. The 
> capacitors may be
> intended to suppress the back EMF or voltage spike when the relays are 
> turned off.
> If the capacitors are not connected or are non-functioning, there may 
> be a risk of heater
> to cathode breakdown in the tubes.

Looking at the schematic at http://aafradio.org/docs/ATD_schematic.png - 
since C-107 is only protecting the M.O. tube in the manner you surmise, 
and in fact works in tandem with the inductors in L-103, would they not 
also be needed for the other 6L6G tubes if relay coil suppression was 
needed?  In point of fact, AB5S is correct in his assessment for this 
particular cap.  C-107, L-103, and C-108 would certainly cut down on any 
relay voltage spike, but its design intent is as follows:

 From para 2-1-5-2(c) of the manual - "The cathode and heater circuit is 
maintained at an RF potential above ground by the heater reactor L-103.  
Cathode bypass capacitor C-107 maintains the cathode and the heater at 
the same RF potential.  Heater bypass capacitor C-108 bypasses the low 
[DC] end of the heater reactor to ground."

The manual gives us a hint for the higher voltage, but it is not really 
C-107 that is the issue. A closer look at the parts list reveals that 
this .1/.1uF capacitor is also used for C-123, C-130, and C-201.  C-123 
bypasses the plate supply for the microphone amplifier, fed from the 
380v winding of the dynamotor.  C-130 is the keying relay bypass for the 
K-103-G contact.  C-201 is a transient filter for the 28v bus in the 
dynamotor.  In view of its use in a variety of circuits, it would appear 
that they simply chose 600v as a figure that allowed its survival in 
those locations and reduced the spare parts count for the supply chain.

  - Mike  KC4TOS

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