The AM Forum

Good day,

Came across a small boat anchor CW transmitter where the 5-25 H swinging choke hums and vibrates a bit on the loud side. Key down, under load, hum is almost totally gone. No load, rises pretty significantly. HV looks good and reasonably stable between key up and down. Talking 500-600V at 120 mA.

Can mount choke on rubber bumpers. Any other suggestions? Do swinging chokes age to the point that loose laminations become problematic? Although this one has a wrap around shell, no side bells and appears pretty tight. However, if I press or squeeze, I can really quiet it down.

Tnx!!, Alan  

tighten the case lamination bolts a bit to see iffin that helps

Tnx for the reply John.

The swinging choke has no case bolts. It is similar to the Stancor pix attached. However, it has the bell sides wrapping around the winding so it is totally encased unlike the Stancor shown here. There are holes available for passing bolts but would I not have to apply shoulder washers for these to mitigate the short turn effect created by the bolts? It is clear, squeezing this choke in the right spot with the proper pressure muffles the !@$# hum/vibration. 

it might but I doubt it .... do a before and after measurement with a scope (the equivalent of a shorted turn would be less filtering due to less inductance)

Why not use non-metallic bolts and nuts, such as nylon?

You might sub it out with a fixed choke, as most CW rigs don't need the same filtering as a phone job.

73DG

You can add bolts with fiber washers.  Also try a thin wooden shim between the core and the lamination.

Pop it out of the case. Soak it HV lacquer. Hank to drip so dimensions aren't to excessive. Dry / cure thoroughly. Reassemble.

Thanks for all the replies! Over the weekend, I applied voltage and left the supply unit on for 8-12 hours continually. It appears that a combination of heat and periodic load current drawn,  substantially reduced the hum and vibration. Now between key up (no load) and key down, the difference in noise level is quite small. Perhaps a combination of driving moisture out and a small expansion of the lamentations with heat is a step in the right direction.

Alan