[GreenKeys] ST6 questions

[Don Robert House]

VERY VERY KIND of you to assist Bill.  You are very high on my list  
of "Good Deed Doers!"
All the best to you and yours.

Don
K9TTY


On 17 Jul 2006, at 10:00 AM, Bill Henry wrote:

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.

In regard to L-C passive filters vs active filters, LC filters are a  
lot harder to design and to tune.  But, their dynamic range is very  
very good.  It takes a LOT of signal to overload an LC filter - such  
as in saturating the toroid or arcing-over the capacitor!   
Conversely, active op-amp filters are a lot easier to tune but also  
have a very limited dynamic range.  Desiging good active filters is  
an art.  Some circuits work a lot better than others.  If given a  
choice ALWAYS choose an op-amp circuit that uses more op-amps - like  
2 op-amp sections rather than 1!  Also, there is a LOT to be said for  
power op-amps from +12 and -12V rather than trying to make do with  
one that runs off of 5V only.   Actually, a vacuum tube (valve?)  
running on 100V  is VERY hard to beat for dynamic range (no doubt a  
"secret" of the TTL-II)!  If you over-drive an op-amp filter, it  
becomes a clipper or limiter, probably capturing on the unwanted  
signal.  Forget it's filtering capability.  Digital filters can be  
designed to produce virtually any band-pass you want - BUT - they  
also suffer from dynamic range problems only more so when  
overloaded.  If you over-drive the A/D converter, it's all over -  
forget the signal or interference.

The ST-6000 design was triggered when we couldn't get enough 88 mHy  
toroids for ST-6's - and the labor to tune them was eating us alive.   
As with all new designs, we kept adding features and the ST-6000  
evolved into quite a bit more of a demodulator than just an active  
filter ST-6.  There were quite a few "active filter ST-6's" written  
up in the RTTY Journal in the early 70's - see the CD ROMs.

In regard to the transistors on the autostart board, to the best of  
my recall, we used either an MPS3394 (plastic TO-92) or 2N697 (Metal  
can TO-5) at Q401.  We always used the 2N697 at Q404 and 405 and a  
MPS3703 (plastic) or 2N2905 (metal) at Q402, 403, and 406.

That's what occurs to me this morning!

73,
Bill Henry, K9GWT


At 10:31 PM 7/16/2006 +0000, Eugene Hertz wrote:
> Hello all,
>
> I've been using an ST6 from ebay for several years now.  It has  
> always done good service but the motor control anti-space circuit  
> never worked which was ok by me.  I never had a chance to really  
> look into it in any detail.
>
> Recently I acquired 3 more ST6s on ebay and now have been able to  
> do some comparative anatomy between them all. Very interesting  
> findings. One has obviously been built from a kit, 2 seem to be  
> factory and the fourth is unknown.
>
> So I took the best of the 3 new ones and began comparing to my  
> regular ST6 and I started looking at the frequency response. Here's  
> what I got using my R&S Polarad SPN audio generator. ST6 #1 is one  
> of the new ones, ST6#2 is my regular use TU.  Note that my regular  
> unit does not have 425 shift.
>
> ST6 #1  (new one)
> MARK    SPACE   SHFT
> 2150    2330    180
> 2150    2580    430
> 2150    3010    860
>
> ST6 #2  (regular use)
> 2160    2325    165
> 2140    2975    835
>
> So, how important is it to have the frequencies right? Or is it  
> more important to have the total shift right. In the case of the  
> shift for ST6 #1, they are pretty good varying 5-10 hz from  
> expected. #2 is 5-15hz off a little worse.
>
> Should I try to get the shifts more accurate to 5hz each?
>
> My receivers only have BFO, no product detectors (YET) so I seem to  
> always be able to tune the receiver or BFO to produce the right  
> tones. However, I hope soon to own a product detector (outboard).  
> How will this affect the need to have the tones accurate vs the shift?
>
> Second batch of questions.
>
> Seems out of a total of 4 ST6's only 1 auto start/anti-space board  
> works.  Can anyone tell me if this is a common problem? In one  
> case, there is a transistor that seems to be in backwards compared  
> the one only board that works. Is it possible this never worked?  
> This board looks like it came from the factory.  Could there have  
> been a substitute transistor used that either had the EBC in a  
> different order?
>
> Thanks all!
> Eugene
>
>
>
>
>
>
>
>
>
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