Oscillation with Hycas AGC Amplifier


Richie Chambless
 

Hi,

I've recently built a receiver using the Hycas amplifier and I am have some oscillation issues. First, here is the basic layout of my test receiver:

Antenna > 40 Meter BPF > SBL-1 > TIA Amp (copied from uBitx) > 12 Mhz SSB filter > Hycas amp (L match retuned for 12 Mhz) > SBL-1 > LM386 AF Amp with 2N3904 preamp

When I power up the Hycas amp and AF section (with no TIA, LO, or BFO running), I get a loud motorboating sound. When I jumper across the AGC off pins, the motorboating stops. When I turn the rest of the receiver on, I get a high pitched oscillation with a frequency that varies with turning C2. I can null out the squeal and here some stations, but the noise floor (hiss) is very high. But, when I jumper the Mute pins, the gain is reduced and I am able to hear stations at a comfortable level. Now, I know this module wasn't designed to be used on mute, so I know I have something wrong. Any suggestions on where to start? Is there any particular thing I need to do, such as decoupling caps or shielding? I appreciate any help you can give.

Thank you,

Rich

KM4TLR

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