[ARC5] Strange problem with racked transmitters

[J Mcvey via ARC5]

Heater-cathode short was a very good possibility as well as gas, but a NOS pair of 1625 did the same thing. 
I don't know the exact amount of excessive current , but it is substantial enough to drag  the 200 ma dynamotor voltage down 50v below normal. The select relay is normally open as it supposed to be on both poles in standby. 
I stuck a scope probe in the 1625 access hole to see if there was an oscillation present there. Nothing seen.The 51K cathode resistor should be more than enough to limit the current as well as producing a cut-off bias as designed).Even if the tube was shorted plate to cathode, the current would only amount to 9 ma , not mention it would make that 51K resistor a crispy critter.However, the tubes must be installed for this loading effect to take place. 
These are the older types with the cloth wiring, so I am wondering if there is some bizarre arcing between wires or perhaps some carbonized fibers from soldering causing this.

These are scr-274 types, so I can monitor pin 2 on the rear connector for tube bias. The offending transmitter has about 600 mv when there is no transmit on either one. It sinks down to 250 mv when the other transmitter is keyed, probably due to supply sag more than anything else. It has - 41v in transmit. 
Being unfamiliar with the "normal state" of these,  I also made  the observation that the blue tube gas lights up in the de-selected transmitter to a lesser degree than the selected finals. It this just ionization due to presence of HV? Does this happen with a properly working transmitter ? I think I did notice it on the one that did not load the supply as well...
I am loading these with a home-brew 5-j400 ( approx) load via the antenna switch. 
Also tried using a 4:1 un-un autotransformer to match a standard 50 ohm meters and dummy load. Got about 18w with loading coil at 0 and coupling maxed. ( using no antenna switch which seemed to add reactance in this configuration)
A 9:1 un-un may be better because these scr-274s are designed to load into very lo-z capacitive antennas.I am certain that I am getting max output with the 5 ohm load setup where the loading coil is 40-60 percent on each transmitter and the coupling is about 70 percent. I got about 2.5 times the deflection that I had with the un-un setup.


 


     On Sunday, June 28, 2015 2:06 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:
   

 Heater-cathode short in one or both 1625's?

73 de Neil ZL1ANM

Sent from my Raspberry Pi computer

On 6/28/15, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> Both transmitters work well, but one seemed to draw too much current (not
> due  a resonance problem). Upon further investigation , I discovered that
> the "deselected" transmitter 1625's were conducting somehow while it's
> supposed to be inert. I proved this by leaving the offending transmitter
> docked with the finals removed. the problem went away. Tried a new set of
> 1625s but they also conducted while in standby. The select relay is OK (
> both contacts open in standby). The other transmitter does not do this.
> There isn't much there to go wrong, other than what I have already explored.
>
> Any suggestions?
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