Hi Dino -
The FSK tab was present in all the 4 Series PTOs, with the exception of
the TR-4CwRIT where that same tab was rewired to serve as the RIT port.
The early 4 Series PTOs, with the Bipolar oscillator, had a very simple
pair of capacitors that were connected across the oscillator voltage
divider and the Buffer input. An external capacitor of ~ 259 pF was
connected from the terminal through a diode switch to ground. When the
diode was reverse biased, the PTO was "on frequency" and when the diode
was forward biased the PTO was shifted ~850 Hz. Less capacity was
required for 170 Hz shift when that was developed.
The later 4 Series PTOs with the FET oscillator, required a second tap
on the PTO linear inductor along with the capacitor across the Buffer
input to effect the shift at the higher impedance of the FET
oscillator. So to answer your question, I suspect it would be difficult
(adding the tap to the coil, linearity effects?,) to modify the RIT
and/or 7 Series PTOs for FSK.
Effective with the TR-4CwRIT, the RTTY shift terminal was re-tasked to
be the RIT terminal, and an internal varactor circuit was added to give
the large shifts required for the RIT function.
According to Bill Frost, there was some concern at the time about losing
the FSK capability, but it was determined that by that time most
everyone was using AFSK and the FSK function was no longer needed.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Dino Papas wrote:
When I first got my twins in 1979 I wanted to use FSK....the manual
described the FSK tab on the PTO but it appears that in the very late
models towards the end of the run Drake deleted the tab. Not a big
deal I just ran AFSK for many years.
With all the talk about the RIT tab on the TR7 PTOs I wonder if it
would be possible to mod these late PTOs to add the FSK capability?
This isn't a big deal either but I've always wondered and someday may
put together my old RTTY TU and it would be nice to at least have the
capability.
Anyone ever done it???
Dino KL0S
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