Self-oscillation on RX - possible cause and fix

[Ohwenzelph]

Will a 10 volt cap be ok for C25-28?

[msax]

Yes, it should be fine.

[SA4DHT]

That's fine for C22 and C29, but it's too low for QSD capacitors. I'm currently using 220nF for C25-C28, which is the bare edge for removing signals from the neighboring SSB channel. On CW, things are even worse.

Ok, that is an interesting finding. In size 0805 max capacitance for the film capacitors is 220nF.

73 de Dirk

[DL2MAN]

OK, guys....

I received some 0805 470nF Tantalum Caps today. Soldered them in, which was tough and ended up very ugly:

TAN.JPG (133.84 KiB) Viewed 1476 times

The reason is, there´s almost no available space and this is size comparison to 0603 (original caps):

0603.JPG (21.95 KiB) Viewed 1476 times

While there is no longer clicks and scratching from speaker, when you move the rig, touch PCB, or click buttons, it did not help on RX Improvement I feel. Further direct comparison needed for determining the exact effects. But Speaker Howling did not improve at all. So I´m not sure if it is worth it at this moment.

Soldering those 0805 Tantalums would be easier if I used solder paste and Hot Air Soldering.
But I found soem Tantalums in 0603 today.
Not exactly a bargain, but hey: If you want to try.... Drop in replacement....
https://www.mouser.de/ProductDetail/Kyo ... t2xQ%3D%3D

Tantalums are not affected by piezoelectric effect.
In a Nutshell:
Drop in replacement Caps available without "microphonic" effect, not cheap and effect is questionable.

73 Manuel; DL2MAN

[msax]

Manuel, I appreciate you taking the time and making the effort to address the issue I raised. When I switched to poly film capacitors, I was able to eliminate the uncontrolled oscillation of a complete receiver in my case, whether on speaker or headphones, even at low volume. The receiver eventually began to behave as seen in all reported videos of a functioning trusdx. When using the speaker, however, howling remained on the louder setting. So what I and a few others here are dealing with is a one-of-a-kind issue. Perhaps it's just a different capacitor type or manufacturer in our case, or perhaps more sensitive capacitors? Maybe not such a great opamp?
At the very least, it's now usable. Not only was it usable, but I began to like the sound of the trusdx receiver. You guys did an excellent job here.