Apollo PM Test Receiver First Light

I am falling further and further behind in my videos, but our current latest advance was to power up the PM test receiver. Previously we had done receive tests using an HP mixer to bring the microwave signal back down to an IF of 20 MHz, then I used my HF ham radio as an IF section and got the FM baseband out of it. Good enough to confirm we could hear a tone from the PM and FM downlinks of the transponder, but the demodulation is not correct (much too narrow for the Apollo FM and no real PM). So the next step was to try to bring up the test PM ground receiver.

After making a power cable and repairing the damaged fuse holder, it powered up OK, the power supplies were right on (I think the +15V was like 15.00V). We got a few lights but not many. But after we changed the GE330 bulbs thanks to Mike SLA printed bulb extractor (!), most responded to button presses.

We tried to feed it a signal at the CM downlink frequency, but nothing happened. Nothing at all, no indication of any signal received. We proceded to debug the receive chain with the RF spectrum analyzer and did not have to go very far. Looking at the input mixer, it got the received signal alright, and we saw the local oscillator. But it was way off, 100 MHz away, at the edge of the screen. Should have been 50 MHz away.

Frequency measurement confirmed that the VCO was off, at 2196.9 MHz

Then I went, aha! This receiver must have been made for the LM, not the CM! So I checked the LM downlink frequency. But it's only 5MHz different from the CM, not 50MHz! So this receiver was re-tuned to something else after the Apollo program. The last date of calibration of the receiver is 1977. Used for Skylab maybe? An exploration satellite? Early shuttle experiments?

Anyhow, I went ahead and fed it 2246.907 MHz, hoping that it would receive something in the neighborhood of that. And it did! After some hesitation, we got the "locked" green light, and the received signal indication moved towards the little green area.

It turns out this receiver is not sensitive at all. It was meant to be connected directly to the transponder after an attenuator. It has its own variable 10-30 dB attenuator right upfront too. We had to feed it -20 dBm to get into the green. 

Now we have to find out how to get it configured back to the Apollo frequencies. If it's just changing the VCO crystal, that might not be too hard, but if it requires internal tuning, that is going to be way more difficult. There are many variable L and C adjustments in the VCO, maybe 10 poking through the cover.

If we can do that, then we have a receiver with the whole PM demodulation section, including the Voice signal extraction, on the bottom left gold connectors.

More exciting work in perspective.

Marc