[GreenKeys] Loop supplies/ST-6?

[John]

My plan for my ST-6 is to remove the current HV transistor and replace 
with the same device (AQY280EH?) used as the loop driver in the Volpe 
board.  Then the output can be patched anywhere like a keyboard.  Bear 
in mind my ST-6 is a home brew on perf board so I have no qualms about 
such a modification.

Actually you could use the AQY280EH on an outboard assy., put a diode 
bridge and resistor arrangement to knock the 60 ma down to something 
around 2ma for the input and then use the output anywhere you'd use a 
keyboard.  I think you'd also want some sort of snubber on the output of 
the AQY280 perhaps even the usual 470 ohm in series with the 100 nF 
cap.  That would eliminate any need to modify the ST-6 or any other TU 
that has the internal loop supply.

John, W9DDD

On Apr 8, 2024, at 6:44 PM, W2HX <w2hx at w2hx.com <mailto:w2hx at w2hx.com>> 
wrote:

> Hi all
>
> For many years, I’ve used my ST-6 as my TU with its built-in loop 
> supply. These days as I am using a different TU for the 850 shift 
> stuff. I still use my ST-6 but in STDBY (mark hold) mode just to have 
> it supply loop current to the equipment as my CV-2460 expects and 
> external loop supply (but not use the ST-6 as a TU for 850 Hz).
>
> The number of machines on the loop has grown and therefore has been 
> reducing the current that is able to be supplied by the ST-6 as I 
> don’t think it was meant to handle more than a few devices. I have 
> maxed out the rheostat in the ST6 (or minimized it as is the case). So 
> I acquired a navy loop supply, designation PP-3495A (attached) that 
> can supply 120VDC at a full 1A output.
>
> I was wondering if anyone has modified an ST-6 to disable its internal 
> loop supply and allow it to be used with an external supply? This way 
> I can use one loop supply regardless of which TU/CV I am using to 
> drive the loop (of course, the unused TU/CV would have to be in 
> bypass/standby/mark-hold mode).
>
> Thanks
>
> 73 Eugene W2HX
> My Youtube Channel: https://www.youtube.com/@w2hx/videos
>
> <PP-3495A--UG.JPG>
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