A Partial Schematic
All the time you thought I was talking about Ham Fests, Slide Rules and Parts, you likely didn't suspect I was really working on Project X.
Yes, it is receiving and even transmitting (at low levels as the Power Supply is still in work). But that only exposes more mysteries. The Receiver part is in need of a full alignment as some bands work far better than others. The Transmit part will be the head scratcher!
Above is a partial schematic of the Carrier Oscillator and Balanced Modulator. As I describe the problem keep in mind this is also the Product Detector on Receive. This is a very common approach in these circuits of this vintage and even in current rigs (The SBE-33 and SBE-34 and the P3ST).
The 1st positive note is that it is a diode ring, as opposed to the two diodes often seen like in Bitx radios. It has two balance controls, the 1st being the carrier amplitude via the 500 Ohm pot which also is the injection point of the carrier signal. The second balance is for phase and the 15pF and the 5-18pF trimmer form the basis for that part of the balance. Adjustment of these controls does indeed reduce the carrier level as measured on the secondary of L1.
Typically, when all is nulled the level of carrier (i.e. no modulation) is very small and frequently is 50 dB down from the sideband signals also generated out of the Balanced Modulator... Duh, why do you think it is called a BM.
So, what would cause a significant level of carrier (like 2 Volts PTP versus signals which should be millivolts). Notionally you would start with the signal level of the BFO. If its level is way too high that is a possibility. I tried detuning L2 to see if that would reduce the carrier level but that did not prove fruitful.
When you get carrier balance the level drops into the mud. You can see it on a scope, but you can also hear it on a nearby receiver. No question, you will hear the null like it is a whisper versus a booming sound without balance.
The BFO has no provision for adjusting the carrier frequency as a condition could exist where the BFO is smack dab in the middle of the crystal filter pass band and that would pass right through the filter. But this is likely the lowest of the probable causes. Receive sounds OK as that would be another indicator of this possibility.
Another possibility is one or more bad diodes in the diode ring. I have ordered up a batch of 1N5711 Schottky diodes and will replace all of them. Even if it is not a diode having all Schottky didoes in the ring is a good change. It also is scratched off the list should the condition still prevail after the change.
I can look at the transmitted signal at the collector of the final driver transistor and with no modulation you see a "fat band" of carrier output versus what should be a very small trace. With audio applied you see the Christmas tree pattern over the carrier with some nice peaks.
It is clear this is not a one and done project but in fact a long-term opportunity to use my slide rule and tribal knowledge.
Them that know can make things go.
73's
Pete N6QW