LNA Problem - Noise Floor High When Cold..

[VK1JA]

Morning All,

I have a problem with my 70cm LNA after I 'repaired' it that I was hoping to get some guidance on fixing..

The LNA is a VHF Design 70cm LNA that was working very well until I had a opps moment and txed through it while it was in rx mode.

https://vhfdesign.com/lnas/lna-qro-with-...bands.html

I replaced the SAW filter and the active device and it's back working again, BUT when the LNA (mast mounted) is cold it has a very high noise floor which it didn't have before I did the opps moment..  When first powered up the S meter will read S9+ noise, then after about 3-4mins of being powered up the noise floor will gradually reduce until after about 20mins it settles on the normal S1-2.

It seems to be temp related because if I turn it on in the early morning when the abient temps outside are 12 degrees, it'll take longer for the noise floor to go down to normal levels compared to when it's powered up in the afternoon in 27 degree abient temps.

The circuit seems quite simple.  I was thinking that a reflow of the SAW filter may be a good idea as the solder pads are on the underside of the filter and were tricky to solder.

Has anyone had something simular or have any ideas? 

Jayson
VK1JA

[ZL1RS]

Sounds like it might be oscillating. Had similar here on 2m ... a re-tune "fixed" it.

GL, Bob ZL1RS

[VK5TM]

It's possible you may have fried something in the bias circuit as well or if you're lucky just one of the caps in the input circuit to the FET.

If you can fire it up on the bench, use freezer spray to target those components individually to try and pinpoint the faulty one.

[VK1JA]

Thanks for the tips. I hadn't thought about the obvious test with the freezer spray. I'll grab some from Jaycar my next trip there, which shouldn't be too far away!