[GreenKeys] ST6 questions

[Bill Henry]

HI Eugene,

The shifts are very good.  No problems there for receive.  Also, as Don 
(WB5HAK) says, if you transmit and receive on separate devices, there's no 
problem when getting on the air.  The miss-match problem occurs when you 
and the other guy both run transceivers and the receive filter center 
frequencies are noticeably different from your transmitter tones.  Even 
then, you can use "RIT" (Receive Incremental Tuning) to correct for any 
offset the other station may create.  The rule is that once a QSO is in 
progress, avoid changing your transmit frequency.  Adjust your receive 
frequency if you must but leave the TX dial alone!

The big resistors are 20W power resistors.  You have 2750 ohm resistors 
with 60 mA of loop current.  I^2*R works out to 9.9 Watts of heat on my 
slide rule.  That's pretty hot!  I am not sure why one resistor may be 
larger than the other as the part numbers indicate to me that both are 
rated at 20W.  Yes, you can do as you suggest and run a long bolt through 
the middle of the power resistors and faster it to the rear cabinet 
wall.  Ohmite, IRC, and Clarostat used to sell "kits" just for that 
purpose.  They provided a long 6-32 screw (2"?), nut, special flange 
washers, and insulating washers.  You can probably find these in a junk box 
at a hamfest.  The 25W loop pot on the ST-6 was another "casualty" of 
component availability. When we started the ST-6, we found a supplier who 
had what seemed to be an "infinite supply" of these things.  Ha!  Then we 
started selling ST-6's and the supply was proven to be "finite" - hi!

The units without serial numbers on the cabinets are no doubt kits.  We had 
several ham customers who made a pretty good side business building and 
reselling kits.  And then every so often I see a mint, unopened ST-6 kit at 
a hamfest.

The ST-8000 was my "engineer's dream" back in the late 1980's.  It has 
everything I ever wanted to put in a TU. And the production people would 
tell you it has two or three of every part in our store room (not quite, 
but it seemed like it) - something like 1400 parts, I think.  It took 28 
hours to build each ST-8000 and another 6 hours to test it.  Contrast that 
with 160 parts and 35 minutes for the DSP-4100 and you can see why nobody 
can afford to make monster gadgets like the ST-8000 any more.  The ST-8000A 
is a MIL modem based on the ST-8000.  It has fewer but MUCH more expensive 
MIL-rated parts.

73,
Bill Henry, K9GWT


At 01:23 AM 7/24/2006 +0000, Eugene Hertz wrote:
>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.
> >