[GreenKeys] ST6 questions

[Eugene Hertz]

Bill, et. al.

Finally found some time this eve to play around some more.  Using the very intuitive (although I never thought of it) tuning procedure, it seems that my shifts are all in very decent shape, considering I am no longer looking at the actual frequency but just the shift. Here is how things fair (ST6 #1) for the following shifts

170 - off by 7Hz
428 - off by 3.5Hz
850 - off by 11.5 Hz

So all is pretty darn good! I *might* choose to try adding a cap somewhere (have to double check the manual) for the 850 shift to bring it down a bit, but probably good enuf for government work.

I am curious, however, about the comments regarding transceiving.  I do expect to have some RTTY QSOs at some point.  I have yet to measure the output frequencies of my XTK100 boards, but I would guess they will be pretty close to "right."  So does this imply that if I am to QSO that I should try to bring the receive frequencies to their right values?  OR, because I actually have separate transmitter and receiver. Does this obviate the need to such fine tuning because I can separately tune Tx and Rx?

Question #2. The big loop resistor. I have to different ST6's both with this giant fixed resistor. One of the TU's resistor is even bigger than the other one! I've been playing with the "smaller resistor" TU and notice this gets so hot its quite concerning.  Assuming they are both properly rated for power,  do I understand correctly that no matter the size of the resistor (wattage) it will generate the same heat? Its just a matter to make sure that the resistor won't self destruct with that much heat - meaning, make sure its properly sized? 

If thats the case, then there is nothing I can really do to reduce this heat. I was wondering if there could be any benefit to fitting a long 6-32 screw with metal washers on either side and run this through the hole in the center of the resistor.  Something like a heat sink, perhaps with a chunk of metal attached somehow. Anyone do anything like that? I let this run for a few hours and no melting of the resistor, so I am assuming it is properly rated. 

I see no wattage rating, but labeling says "CLARO VPR20H 2.75K"  The other one that is much bigger says "CLAROSTAT PR20K 2.75K" 

The other two ST6's I have use the rheostat.

Thanks and thanks for all the hints and support.  (Soon onto my XTK100 and non-functioning autostart boards)

Eugene


>-----Original Message-----
>From: Bill Henry [mailto:ghenry at halcomm.com]
>Sent: Monday, July 17, 2006 10:00 AM
>To: 'Eugene Hertz', greenkeys at mailman.qth.net
>Subject: Re: [GreenKeys] ST6 questions
>
>Hi Eugene and greenkeys guys, 
> 
>Regarding the removing of turns to tune an 88 mHy toroid, we found: 
> at 2125 or 2295, the frequency increases 3 Hz for every turn removed. 
> at 2975 Hz, the frequency increases 5 Hz for every turn removed. 
> 
>Be careful when removing turns - take off "almost enough" and re-check! 
> 
>To lower the frequency, adding 100 pF in parallel (silvered mica, 
>polystyrene, or mylar - NOT disc ceramic): 
> at 2125 or 2295 Hz, the frequency lowers by 2 Hz 
> at 2975 Hz, the frequency lowers by 5 Hz. 
> 
>When measuring the "center frequency" of these filters, do NOT try to find 
>the peak. The filters are too broad. Rather, look at the tuning meter 
>and look for side frequencies equally down on each side of the peak and 
>computer the center. For example, if the tuning meter peaks at 0.7, 
>measure the frequencies above and below resonance when the meter reads 
>0.6. Add them, divide by two, and that's your true center frequency. 
> 
>I have noticed that over the years the tuned circuit frequency of an 88 mHy 
>toroid filter seems to move up. I have no idea why but it has been 
>consistent. Maybe the toroids "age"? I have never caught an orange drop 
>or silvered mica capacitor aging but I suppose it's possible. 
> 
>In regard to absolute calibration, what others have said is true. On 
>receive, only the shift is critical and the filters are broad enough that 
>anything from 160 to 200 Hz M-S difference works well for "170 shift". We 
>always tried to set set them "dead-on" but as I said something seemed to 
>drift with time. 
> 
>HOWEVER, if you are going to use a transceiver, it is very important that 
>your transmit tones exactly match your demodulator receive 
>filters. Otherwise, you won't be sending and receiving on the same radio 
>frequency and your QSO will probably "walk up and down the band" as the 
>other guy tries to stay tuned to your frequency. 
>