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Hi, have you put small capacitors rigt on the leg of the 7805? I alwais put
1 uf tantal right on the 7805 and for that low frequency perhaps 10 uf tantal also.
It could be that 7805 osc. on the low frequency.
73, Oz8pz, stig
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Thanks for the idea Roger.
I run from a battery with an LM7805 bypassed and filtered (3000uF) on the input. I added 4400uF to the output side and the problem is still there (sadly).
I'm wondering now if my bypassing is sufficient for this frequency. The mods page (http://qrp-labs.com/ultimate3/u3mods.html) lists a couple of 630M mods. Toni also changed the bypass capacitors (and C5). I'm wondering if my bypass capacitors on the regulator are sufficient as well..
Thanks for the thoughts Roger, I'm now on a bypass quest!