since nothing is selling anytime soon i have absolutely zero chance of getting any of that C-C stuff from tommy j. and will need to just drop 100.00 into fixing the Cunningham but i want to add PLL capability to it. any one have any ideas how i can add pll to the Cunningham?
i can't afford crystals each time i need to change frequency. will cost a little under 100 to recap and new tubes and tip 100. figure one month ill fix then next months pay i'll add pll then next month modify tuner.
then i need to modify the Cunningham universal tuner to work as a carrier current tuner.
Send me a copy of the schematic and I will be glad to see what's what.
RFB
it's posted here already. this is the one i have...
http://part15.us/node/1938
it was originally alan's (RadioBoy / WJJD)
Excellent. Let's see what we have here!
RFB
iirc the UT is in their as well. it appears to be just a L match
Sure does. The only issue I see with using the UT as a CC coupler is the rejection of AC energy feedback from the lines into the final, which will cause hum.
What should be done is to modify that tank so that it functions as a tunable high pass filter, blocking out the low AC 60hz energy frequency.
On to the oscillator.
I see no reason why that cannot be converted over for a PLL.
Take a look at how that oscillator is configured. It is very similar to the tunable oscillator in the KT 195 unit. Sure the tubes are different and the tank circuit configuration is between cathode and grid and in the Cunningham between plate and grid, the tank circuit in the KT with a little modifying should work on the Cunningham, and turn that into a tunable oscillator.
Now with the tunable tank on the oscillator in the Cunningham, we insert a varactor in parallel with the coil and capacitive coupled at the varactor cathode. Then from that point through a padding resistor to the PLL control voltage lead. Sampling for the PLL can be done by loosely coupling off the oscillator plate with a cap or even just a pickup coil and feed the PLL sampling loop. Part values would have to be experimented with and a pre tuning procedure developed so that the tank can be locked across the entire MW band. Meaning the tunable inductor coil in the oscillator tank will have to be adjusted first prior to establish a working range for the tank which will be dependent upon the part of the band being tuned to.
I think its highly doable. There probably are a dozen ideas and suggestions out there that can be considered as well. In all I do not see why this cant be done.
Corrections added...grid! Been awhile since engineering with tubes! Gotta love the glow!
RFB
Another way to attack this would be to simply bypass that oscillator section completely and use something like a Ramsey AM 25 as an exciter feeding the final stage. Since modulation takes place on the final, the Ramsey would only be used for generating a PLL locked carrier. Or use an AMT3K or 5K or any other low power AM TX with PLL or DDS.
RFB
how about an aspisys?
"how about an aspisys?"
Absolutely! Why not! In fact, that ASPiSYS unit is not only a good choice for a DDS generated carrier, it can turn that Cunningham into a C-QUAM transmitter as well.
That ASMAX unit can split the path of the L+R and L-R. There is a jumper which removes the L+R audio path from the carrier where the L-R is. That L+R exits the board on the back where the uninstalled connector goes. That feeds the standard audio input of the Cunningham.
The L-R plus carrier exits out of the normal RF output jack and that would feed either the buffer stage of the Cunningham, or directly to the final tube if the ASMAX has enough drive level.
Your audio will feed into the ASMAX unit. As the ASMAX unit is sending L+R via the other path and into the Cunningham audio input, the "mono" path is completed! And of course, the L-R plus carrier comes from the RF out and drives the buffer or final. Wala, C-QUAM DDS Cunningham!
The "big boy" C-QUAM transmitters which were converted over did this exact method to produce C-QUAM. A C-QUAM exciter, which produced the carrier plus L-R feeds the power amp cabinet with the RF. The exciter also outputs a L+R audio signal, which was connected to the audio input of the amplifier cabinet.
The ASMAX does have an adjustment for both the mono path and the stereo path so you can adjust each for proper mixing through the transmitter (Cunningham).
RFB
Here is a quick block diagram I made on how the units would be connected together.
For the RF input from the ASMAX exciter containing the DDS generated carrier and the L-R audio, disconnect C6 lead in the Cunningham from the oscillator section and connect that lead to the RF signal from the ASMAX. See HERE for the bypassing point and injection point.
Also see HERE for noting the oscillator plate voltage potential that will be on this connection point at all times. Use a coax fitting with its center pin left open and the ground of the fitting will cover and protect you from the voltage present at this point.
Use both the ASMAX adjustments and the Cunningham audio level adjustment to obtain the proper balancing between the two pathways.
Begin with the ASMAX set for minimum RF output and slowly turn up the RF output pot and watch the TX's meter as you adjust.
RFB
While going over the road map of that Cunningham TX and remembering your conversation with another member about your unit's issues and smoking cap, and if I remember correctly, you said the "black cap" let it's smoke out, and referencing to the photographs you posted showing that black cap underneath, that is part of the bias supply network.
Just taking a wild guess without actually having the unit on my bench probing around, have you checked those rectifiers D1 and D2 and the variable resistor and other fixed resistors along that bias voltage path?
That path controls the final tube's bias which in turn controls the RF power output. If the path is open due to one of those fixed resistors or variable is open, that voltage produced by the rectifiers D1 and D2 will ramp up beyond the filter capacitor's rating and POOF! Or if there is a short along that path..same thing..POOF!
Starting at the final, pin 5 of the tube is the bias grid. Pull the tube and measure from pin 5 to the cathode pin 3 for any resistance or short. If all the fixed resistors and variable check out, as well as the rectifiers check ok, then the problem is going to be either that final tube, or a shorted coupling cap C8 between the final's grid and the buffer tube's plate sending plate voltage right back into the negative bias path. Those resistors in the bias path are preventing a total short if this is the case and would probably also go POOF if the unit was not shut off right away or fuse pop.
Just my thoughts on that.
RFB
will check that it should be noted i have one of the first generation asmax not the newer ones
Ok looking at the photos of your unit, it does have the empty space where a connector is installed. That is the L+R path output. Just above that and in front of and IC should be the jumper right next to a couple of electrolytic caps to select between split mode and combined mode for the
L+R and L-R paths. The connector point is between the RF drive pot and the left channel audio adjustment pot.
HERE is a blow up of the inside photo with two colored circles. The jumper is circled in RED, and the connector location is circled in WHITE.
RFB
i do own a BE AX-10 (Broadcast Electronics CQuAM Exciter) as well and a set of motorola monitors (1400 i think) with the reference oscillator board unlock problem.
"select between split mode and combined mode for the
L+R and L-R paths."
the guy who designed that transmitter certainly designed it well.
wonder if the new generation has the same setup to spit the L-R an L+R