Day 26 IFfy Amp

Having an oscillating BFO provided motivation for building the IF amp.

Looking at the schematic, I thought this would be a two hour tour and I’d be ready to move on.

But not quite. The JMD uses some IF coils that in the good old days were made from Miller coils installed in off the shelf Miller cans. I found the original Miller coils, but no dice on the cans. So I had to:

1) salvage cans from my most unneeded junkbox IF transformers

2) drill a hole in the top of each can to fit the Miller coil

3)modify the coil a bit so that its terminals would fit in the can ; and

4) wire some leads on the coils, which were too short for the cans, so I could wire the coils to the below chassis locations.

As before, I’ll let the picture speak for itself. The same process is required for the BFO, oscillator and mixer coils.

The junkbox Heathkit SSB filter was mounted to the chassis with little difficulty, but the impedance matching exceeded my grasp a bit. After reviewing old Heathkit schematics, I decided to do what Heathkit did and terminate the filter with 2000 ohm resistors.

Mostly things went ok. I injected a signal into the filter output terminal and measured the voltage out of the IF amp. It looked like about 20 to 25 dB gain. When I injected the signal into the filter input, I lost about 10 dB, leaving a net IF amp gain of about 15 dB. I’d like a bit more, but the filter loss is consistent with the filter loss I measured when testing the filter in a 2K ohm test jig.

But L7, a simple parallel resonant circuit at the IF frequency of 3395 kHz, refused to peak. I measured the inductance of the coil and capacitor, and both looked good. I disconnected the coil and saw no effect. As I’ve seen this IF set up without the second tuned circuit, I’ve left it disconnected for now, as the stage seems to be working well enough. On to the mixer.

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