Drake L-75 Hum on Transmit problem

Discussion in 'Amateur Radio Amplifiers' started by K5JCJ, Jun 26, 2019.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Did you actually check how much the HV changes from "key up" to "key down" at full power as recommended in post #6? That takes ten seconds and can be very revealing about how well the capacitors are working.

    The only other thing that comes to mind which might induce hum is a ground loop created by ground differential between the amp and your rig; in such a case, the rig itself would be creating the hum, but possibly only when connected to the amplifier, and different rigs respond in different ways.

    Since you have more than one rig there, have you listened to your own transmitted signal using the other rig with no antenna connected (to help prevent overload) and headphones? What's it sound like?
  2. N8FVJ

    N8FVJ Ham Member QRZ Page

    If the capacitors are only 100uF, you need at least 220uF caps for the high voltage. 12.5uF without a choke will cause hum and the Drake does not have a choke. 220uf gives 27.5uF and that is the minimum capacitance required.

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