Oscillating Knight R100A

[N0WVA]

Well, Ive tried about all I know to figure this thing out. Changed out tubes, bypassed bypass capacitors, grounded things better, especially the ground lug in the middle of the front end circuit board.

 Still cant keep the whole mixer/rf assembly from taking off and oscillating on its own around 4mhz when I turn the antenna trimmer. This is the only band I am having trouble with.

I can use the receiver as is, but it is difficult on 75 meters where it wants to oscillate. Any tips for this problem? Thanks. Doug

[DMOD]

Some questions for you:

Are all of the voltages according to the schematic plus 10V?

What mode are you in when it does this? Make sure you have the Q multiplier off when tuning around.

Did you replace the by-pass capacitors rather than bridging them?

Is it motor boating or oscillating? If it is motor boating, it is a problem on the IF board. Make sure you have good 0.1 uF's at the IF amp cathodes.

Is the OB2 OSC regulator functioning with 105V on its anode?

Is there a shield on the 6BH8?

Have you checked all of the PC runs with a magnifying lens to make sure they connect according to the schematic? I have found PC runs burned in half.

As with any boat anchor, tightening all screws helps with grounding, but with a PC board based system, never rely on the PC surround-ground to be in contact with the chassis. The IF PC board uses IF transformer shields to continue grounds. I highly recommend a single point grounding system. Starting at the chassis ground lug near the first IF tube, insulate solder wick and run it from that lug to the PC board at the first IF amplifier ground pad, then on to the second and third IF transformers, then the filter capacitor ground, and finally to the RF circuit board ground.

Phil - AC0OB