What should be clear is still veiled!
So OK I have a lot of transceivers that I have collected with most being acquired on the cheap. So while there may be many, the invested dollars are relatively low. Periodically I will drag one out of the storage bin and try to fix the unit. This is done more so as a technical challenge; but often I drive myself nuts in the process.
Here is a recent example. Several years ago I bought a Ten Tec Triton IV Model 544 which is the digital version of the Model 540. Several years ago I actually converted a 540 to a 544 and you can see that on my QRZ page. It was also a subject of an article that appeared in QRP Quarterly. Thus I have a bit of background with this very FB Ten Tec rig.
The Model 544 I have supposedly was hit by lightning and was inoperative. Well that was true; but not a total disaster. (That is why I got it so cheap.) Replacing two back to back diodes that were shorted ahead of the reed relay that disconnects the receiver during transmit cured one of the problems and a new set of finals cured the other major problem. The rig is now operational.
BUT and the Big BUT the display does not stay fixed on frequency. You get that annoying drift of a few hundred cycles and the ever flickering last digit over time. Guys it is still an Analog PTO with a digital readout. That had to get fixed.
Most if not all Ten Tec rigs have an accessory socket where you can add an external VFO and the Model 544 has that functionality. This is where you laugh a bit … on the back panel is an octal tube socket fitted with a jumper plug. [Octal tube sockets and solid state rigs -LOL]
The stock configuration brings the LO signal to Pin 8 where it is physically connected to pins 6 and 7. The theory is that with these three pins connected, the transmitter and receiver are on the same frequency. With an external VFO connected to pins 6, 7 and 8, then a switching arrangement in the remote VFO enables:
- Transceiver operation on the remote VFO
- Transceiver operation on the Model 544
- Receive on the Model 544 and transmit on the remote VFO
- Transmit on the Model 544 and receive on the remote VFO
Thus my plan was to essentially uncouple the internal LO which has various ranges depending on the band. The basic PTO operates in the range of 5 to 5.5 MHz and that is used directly on 20 Meters. For the other bands the PTO is mixed with various crystal frequencies to produce LO signals in various ranges from 5 to 19 MHz for injection into receiver and transmitter mixer stages.
I have a band switched Arduino/Si5351 that I simply programmed to match the ranges of the LO for each band. You can even do this with an AD9850 as you only need the LO as you retain the BFO built into the Model 544.
Now you don't even need a display on the Arduino/Si5351. By injecting the proper LO frequency and having the band switch on the Model 544 set to the right band, the internal Model 544 displays the frequencies just like when the PTO was in the loop.
I was joyous as the only modification to the Model 544 was to unsolder the Jumper wire connected to Pin 8 on the external plug. There is no messing with the radio innards.
In fact if you can locate a non-functional 8 pin tube just remove the tube itself and use the base as a new plug. If you go this route then you can do some amazing things like separate the receive and transmit frequencies. My LO sketch includes two VFO's so now you have even greater capabilities.
I made all of the connections (one) and my plan worked. I could tune all of the bands and better yet Ten Tec had an auxiliary band switch to cover the four ranges on 10 Meters. I can do it with just one 10 meter range.
You do the tuning on the remote VFO but the Model 544 does all of the displaying. What is also nice is that I can manually change the step tuning rate which is nice when you want to QSY across the whole band. The one thing I have not resolved is the offset tuning --not a major issue just something not done as yet.
So I am listening on the various bands and I do happen to have a display on my remote VFO assembly. The Arduino/Color TFT is reading 16200800 and the Model 544 is reading 7.200.8 (with the 8 being a Green LED). Then it happened the last LED on the Model 544 starts to 'flicker up/down" but my Arduino/Color TFT which has a greater resolution is rock solid.
So what conclusions can you draw from this?
- Something else aside from the PTO and the Crystal mixing circuit for the various bands is at work.
- There is a crystal clock in the internal Model 544 LED display board. It may have some issues and therefore is causing a shift in the time base.
- The BFO frequency is summed with the LO in the display circuitry so that the BFO is added or subtracted from the LO depending on the band. The IF is at 9 MHz and thus the LO runs in the 12.5 to 13.0 MHz range for 80 meters. (Subtraction LO-IF.) Whereas the LO runs in the 12.0 to 12.5 MHz Range for 15 Meters (LO + IF). Possibly there are some issues with the circuit that mixes the LO and BFO for the display.
- In an earlier Model 544 that I owned there was a problem with the ua723 voltage regulator that had to be replaced and that cured a frequency jumping problem. I did initially replace the 723 regulator as it did have a frequency jumping problem; but that did not resolve the flicker.
- So the concept of a remote VFO is doable with a minimum (and reversible) modification to the Model 544 and this should work for other rigs like the Drake TR-4 or Yaesu FT101 both of which can have remote VFO's. I did something similar to this with an Atlas 210X --so lots of possibilities for updating the older boat anchors.
- If you want the code send me an email to firstname.lastname@example.org.
- If you have any ideas about the flickering send me an email to the above address.
- Pete N6QW