2018-10-08 Air Band Radio Kit HMOOABRA_4.3 Part 5

from FB group Building Transistor Radios comment 2018-10-08

Bob Johansen
Receive frequency? I can’t receive a signal, so the receive frequency is not known. This receiver has no tuned circuit in the RF side of the NE612, other than the bandpass filter made up of the coils on the PCB and their capacitors. The bandpass is supposed to be 118 to 136 MHz, but as I said before, the dipmeter shows the coils dipping above the 2M band, somewhere near 200 MHz. However that may be caused by interactions between the filter elements.

I have measured the LO oscillating over the full range of the tuning pot, up to 158 MHz. But this is what seems odd. With squelch at minimum I put the DMM on the tuning pot wiper and get the tuning voltage from 1.85 at min to 8V at max. At 1.85V I hear hiss, and it peaks at 3.5V. But above 4V it falls off to silence.

Bob Johansen
Yeah the funny part is I can’t hear any FM stations either through the front end filter or bypassing the filter. I have heard an occasional change in the noise level, like one would get if a very weak signal was buried in the noise. But then I’ve heard a lowering in the noise level when I touched the tip of metal screwdriver to some parts near the LO, and the change stays after I remove the tip. I also noticed the same effect when I touched the brass slug with the tip of the screwdriver but not with a nylon tuning wand. I thought that it might be caused by wear on the coil by the slug, but when I removed the slug to see, it wasn’t touching the coil.

I can get noise and hum when I touch the tip to Out A, pin 4, so the 10.7 MHz side is sensitive. It seems that the NE612 is where the signal isn’t getting through. So I ordered some NE612 chips on eBay, they are stocked in the US even though the seller is in China. The chips should be arriving in a week. I may remove the chip and install the socket and try the SA602 from the Vectronics receiver just to see if it’s a bad chip.

BTW I measured the supply current, it was about 50 mA dead silent, and up to 70 mA when very noisy. The 78L08 gets quite warm.


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