[GreenKeys] Dovetron MPC 1000R Hum

[Harold Hallikainen]

On Mon, December 4, 2023 4:29 am, Cecil Lale via GreenKeys wrote:
> I hooked up a audio amp to the Afsk out port and I can hear the hum.
> That's the only thing plugged into the dovetron. Can a bad Voltage
> regulator let some ac ripple through? Its pretty loud.ThanksCecil
> laleKF6ZOK

If possible, I'd look around with a scope. A typical voltage regulator
like the LM7800 series has a dropout voltage of about 2 volts. So, if the
input to an LM7812 has ripple such that the input goes below 14 volts, the
output will have ripple. Regulators have a "ripple rejection"
specification. For the LM7812, the minimum is 55 dB with the typical being
71 dB ( https://www.mouser.com/datasheet/2/149/LM7812-461970.pdf ). So, if
there is 1 volt of ripple going in, there will be less than 1 mV of ripple
coming out, provided the input is above the dropout voltage.

Power supply ripple is usually 120 Hz instead of 60 Hz since most power
supplies (though not all) use full wave rectification. I'd probably use a
scope to look at the regulated power supply rails. If there is no hum, the
problem is elsewhere. If there is hum, I'd look at the input to the
regulators. Is the ripple dropping below the dropout voltage? If a full
wave rectifier is used, is the ripple frequency 120 Hz? If not, perhaps a
diode has failed or a transformer winding has failed (where the full wave
rectifier uses a transformer center tap).

Good luck!

Harold
https://w6iwi.org