{Collins} 516F-2 mechanical hum

Hi Collins folks. I'm in the process of restoring an S/Line (75S-3B, 32S-3, 516F-2). The project has gone very well - I'm almost finished - but the 516F-2 power supply has a mechanical hum that I'd like to get rid of. Here are the details:

- To my ear, it seems to be coming from the choke stack L1/L2. When I pull the 5R4 (V1) it goes away, so it looks like L1 is the guy.

- It starts a second or two after powering up. If I power up after having it on for a little while, the hum starts immediately.

- It goes away under load - i.e. when I transmit.

- I have tightened all the mounting hardware, top and bottom - no effect.

- I have not yet replaced the original capacitors, although I have replacements ready to go in.

Any advice would be greatly appreciated before I start testing and replacing parts. I would have searched the archives first but the link to them on the CCA site seems to be broken (http://mailman.listserve.com/archives/collins/).

Chris, W2PA

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