Hum In The power supply - Galaxy Radios DX-2547 40-Channels Base CB Radio

I doubt the issue will be the regulators if you have the correct voltage on them.
Check where the hum is coming from.
What frequency is the hum; 50/60HZ = mains hum = bad ground. 100/120HZ = power supply hum.
Affected by a control somewhere maybe.

If it is a constant low-pitched buzzing it is probably mains hum which can be difficult to eliminate.
There are two sorts of mains hum; conducted and radiated. Mains operated audio equipment suffers from this when the power supply smoothing is ineffective or when a hum loop has been inadvertently created. High gain preamplifiers with non-shorting inputs, high impedance peripherals and long leads make equipment more vulnerable.
Even battery powered equipment with a high gain can suffer from mains hum received like low frequency radio waves radiated from nearby mains operated equipment and supply wiring.

The hum is likely to be the sound of the ac mains electricity, either because the power supply smoothing has become ineffective in the amplifier or one or more of the audio peripherals or because the audio connections to the input of the amplifier have created a hum loop that is picking up the radiated energy from the mains supply and feeding it into the audio input.

Multiple grounding is often the reason for a hum loop. This is where an audio peripheral is not only grounded through the supply cable but is also grounded through the screened audio lead.

If the hum persists when there is no inputs connected the problem is almost certainly power supply related, otherwise it is likely to be a peripheral that has a faulty power supply or the culprit is a hum loop.

Hi-fi officianados have a number of tricks to deal with hum loops and google could lead you to these.

Grounding is an important safety consideration so appliances that are intended to be grounded must continue to be grounded but the loop could be broken by using special audio leads. The usual lead would use the screening as a conductor and so the screen must be connected both to the peripheral and to the amplifier.
A better lead uses an extra core conductor and the screen then becomes just a screen and is grounded only at the amplifier end and is not connected to the peripheral.

I hope this helps.

one of the most common sources of hum is a failing power supply.
I don't know your radio specifics..... however, some things to consider/try : if it can be battery powered, try it with batteries.
If hum disappears, then source is power supply. Power supply
hums are usually 60 or 120 Hertz (cycles per second). The most
common failure component in power supply, causing a hum, are
the filter capacitors. If you do try to replace these, be very careful
and discharge the old caps before you remove them - to avoid a
(potentially dangerous) shock... caps can retain a charge for many
days, sometimes. Another source of hum is if the radio is close to
a flourescent light, or other appliance which may radiate power. If so,
try moving radio. If the hum is high pitched, then more info is needed.
Hope this helps!

AC hum comes from poor grounding of components.

Is the hum there with NO input cables connected? If not, you have bad cabling. Make sure you use only XLR to XLR mic cables and not the junky ones with a 1/4 inch plug that are for Karaoke machines. These devices usually use switching power supplies and do not have the power supply originating hum of older equipment. If the volume control adjusts the hum, then the hum originates previous to the control which is either in the preamps or the external inputs.

I will assume that it is an active (powered) sub. Does it hum with all inputs removed? Then it may be a power supply problem, bad caps in the power supply.... common problem.

Hi:
On the stanby power supply replace or have replaced C899 with a 100uf 35V capacitor, this capacitor goes bad and produces hum in the supply voltage to the AM FM circuits.

Hi,

As you posted/described, internal work had been done at the factory then perhaps the source/cause of the AC hum is external.

If the unit is being used as a base with a regulated power supply, then the power supply has to be checked. This would include the big capacitor, the diodes and the regulator transistors. On certain occasions, loose transformer E core can also cause the hum.

There are also instances that other electrical devices can cause the hum, these can include fluorescent lamps, air purifiers, dehumidifiers and others.

In some cases, it can be loose grounding of the antenna mount or the braided/shield wire solder to the PL-250 connectors.

If it were mobile, then the antenna grounding is also a possibility. If the hum is higher than 60Hz and goes with the engine RPM, then the alternator of the vehicle has to be checked.

Hope this be of help/idea. Pls post back how things turned up or should you need additional information.

Good luck and kind regards.

Good catch Ray. Thank you for the info.