Finally Access to the VFO Board!
In looking at the schematic and measuring voltage at the emitter we had power to the circuit but no oscillation. The 1st thought was that the Germanium transistor was bad... I pulled it only to find it tested good. Plugging it into a socket on a working homebrew rig (Plessey Circuit) it was good!
Now that I had access to the components I could make other voltage measurements. I noted there was voltage on the collector BUT none at the junction of RFC and 5.6K and the .01 Cap. An ohmmeter measurement across the RFC (shocking) it was open. Ah hah, found it!
Not so fast McGee! True it was open, further checks once removed from the circuit affirmed that. But there was no visible sign of an open, it was not crisped nor any leads broken off. It was just plain open.
Seeing the choke was inaccessible, there was no probing of or any accidental damage to the choke before it died. It just stopped working. The transistor is good so no huge current surge that would have smoked the choke. I must not rule out the Malocchi!
The replacement choke is a 1000 microhenry (AKA 1 Millihenry) and unfortunately, you have to reassemble everything to test if the repair worked.
The rig designer never thought DFMA or that any rig maintenance would ever be required. I did get a chance to use my Krud Kleaner on the chassis and the Project X front panel. It worked well.
Hopefully I will get it tested today and see if I really did find the problem. A real help to me was I started off by writing down what had to be done and the sequence of steps. I also followed the script as I did the task! RTFM!
Tasks and Sequence!
I stopped last night just short of powering on the rig as I was tired and was concerned about getting this far and making a boo, boo. That also gave me time to rethink anything that I had not covered in the work tasks and the fix. I will start by retracing what I did and verify, no shorts or solder bridges.
At this stage, I am glad I decided to fix the VFO as I learned a few things along the way. If the choke replacement is not the cure, then that still is good info. We know the device is good so that reduces the set of what is the problem.
Someone was diddling with the dial drive mechanism which I fixed. The main tuning knob consists of a National Velvet Vernier reduction drive that is panel mounted. At the end of the vernier shaft are two discs one is fixed and the other spring loaded like a pressure plate. A large diameter disc is fitted to the shaft of the VFO tuning capacitor and this larger disc rides between the two discs that are spring loaded. In effect the ratio of the two smaller disc radius and the radius of the disc on the VFO capacitor is s further reduction.
My seat of the pants calculation suggests with the two drives it is like a 50:1 reduction. Translated 50 turns of the tuning knob to complete tuning across the band. The Swan Monobanders and the SBE-33 used this small/large disc to tune the across the band. But these rigs did not have the added vernier drive.
When disassembling the VFO cap I found that the large disc was not riding between the two smaller discs but on the outside of the first disc. Much smoother tuning now! I had to use a long screwdriver to pry open the two discs as I slid the VFO cap disc into position. I spotted this even before I did any disassembly as I noted at times some backlash in tuning.
Them that know can make things go.
It's been 6 months now so how would you answer the question are you better off today? The super rich are counting their crypto so we have their answer. Are your grocery prices better and is your bank account growing?
73's
Pete N6QW