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

[Michael Hanz]

I apologise for not properly implementing my intention of avoiding 
further discussion.  I thought I had removed both lists from the 
addressee bar, but obviously made an error.

On 10/12/2017 9:40 PM, Michael Hanz wrote:
> 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

Michael Hanz - KC4TOS

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