[GreenKeys] Tempest Model 28?

[Bob McConnell]

Roy Morgan wrote:
> I see that Tom has made up some little interface boards with an opto  
> isolator to run the ST-8000 (or other low level logic output T/U) with  
> a normal voltage loop supply.  I plan to get a couple.
> 
> BUT, I have in storage a Model 28 that I think is set up for low  
> voltage/tempest operation.  I have not seen it for some time, but it  
> has extra "stuff" in the pedestal behind the sloped knee panel.   No  
> doubt someone has run into these in military service.  I presume it  
> would work directly with the Tempest Dovetron or ST-8000.
> 
> What's it called, and where would I get technical info to hook it up?

Well, this is stretching the memory quite a bit, but I will try to 
recall how it all worked.

Internally, the M28 still used 60 mA loops to the selector magnets. 
Tempest introduced an external circuit that was +/- 7 volts at less than 
1 mA. The keyboard and TD contacts were also wired into this low voltage 
circuit and required gold plated contacts so they wouldn't build up 
corrosion when they didn't have enough voltage to burn through it. A 
miniature mercury whetted relay was sometimes used to switch between the 
two. There was a numeric designation for that circuit, but 1553 is all I 
can think of at the moment.

So look for an interface between the low level and high level signals. 
Gold contacts on the keyboard and TD will be the definitive indicator. 
If those contacts are not gold and in the high level loop with the 
selector, it is not a tempest unit.

Where it got interesting was when some inexperienced tech would 
"accidentally" plug a 60 mA loop into the low level patch panel jacks. I 
replaced interface relays about twice a month at Wheeler the last six 
months I was at the repair shop in Pearl Harbor (through April 19, '74). 
The contacts were welded together by the higher voltage.

Bob McConnell Ex-RM2, USN - NFIT (8/70-4/74), NLM (4/74-4/77)
N2SPP