Talking House Transmitters go to 1700kHz. I want to push a used one experimentally up 10kHz to 1710. Is there any way to modify the circuit to do that?
I am going way out on a limb here since I don't know the circuitry used in the TH and I could be very wrong but this is my guess based on other synthesized devices I have used. Most likely the operating frequencies are programmed in a microcontroller and are not modifiable beyond what the program allows. Possibly, the controller sends data to a phase locked loop IC which accepts a digital input. If so, then the digital lines could be modified to change the actual frequency though the controller and display will still work as if a programmed frequency is used and the wrong frequency will be displayed.
The other approach, and probably easier, is to change the crystal to one which will move 1700 up to 1710. This can be calculated as (171/170)X(original crystal frequency) but this will skew all the other frequencies off their nominal values.
Bear in mind that any mods. to a certified device voids the certification.
If it were mine I wouldn't bother unless there is a clear advantage to do so.
Neil
It uses a motorized push/pull ferrite core coil to change frequencies. If you open the cabinet, you can watch it move. It's a patented method. It's quite accurate, but I assume the circuit makes a final digital adjustment to get it right on. I was thinking maybe I could push the slug a little further to get it to 1710 kHz ... but maybe not.
The motorized slugs you refer to are for the antenna tuning circuit, not setting the transmitter frequency. One coil/slug is for the high end of the AM band, the other for the low end of the AM band. The correct coil is selected depending upon the frequency selected.
The tuning control circuit monitors the antenna voltage while moving the ferrite slugs back and forth until a peak voltage is reached. That's the point at which the antenna is tuned for max output.
I have had a few Talking Houses that would not transmit above 1680 kHz even though the display showed otherwise. The problem was corrected by slight adjustment to the slug of the small metal can next to the above mentioned tuning coils.
A 10 to 150 pf tuning capacitor was added to the PLL reference oscillator crystal to bring the actual frequency to that of the displayed frequency. Makes a big difference at night when other stations are on my frequency by reducing the annoying heterodyne beat note (squeal) to where it is barely noticable.
OK I understand now, but mine doesn't tune the antenna as I'm using the external/outdoor antenna feature. IOW, the antenna is hand-tuned to the frequency set by the TX. It doesn't move back and forth in my unit ... just once to the frequency chosen. Then I hand-tuned the external ATU. Because the sea is such a good ground, it does quite well much of the time, depending mostly upon atmospheric conditions. It stays like that all the time.
On 1650, I get beat up between an hour before sunset until about 10:00pm PDT, although it's certainly listenable without much fatigue ... then it gets better through the night. I carry phone and driving for a taxi service until 2:00am, when I can finally get to sleep.
I play mostly local music and stories (we have a lot of great musicians in the San Juan islands), but I run Jazz Classics generally from 10:00pm-midnight or 1:00am, then I turn on NOAA Weather for Puget Sound for the night.
I want more coverage to the South end of town with another TX, and 1710 is totally empty except it does pick up some kind of erratic popping static stretches from time to time ... like someone operating an electric welder. I could try 1630, but it often gets a beat and a ring in it, hard to get rid of. I tried 1570, and it was testing out OK for a few days, then suddenly someone started broadcasting in late afternoon. Haven't heard a call sign yet.