Frequency counter

[aleeeeaaa]

I have an external frequency counter that i want to power up off from a shortwave receivers power supply. When i made the connections i can now hear a slight hum noise from the radios speaker. Unhooking the counter and it goes away. With what and how could i solve this hum issue?

 

[AnalogKid]

You have given so little information that it is impossible to say. Could be that the secret frequency counter secret power requirements are greater than the secret receiver can provide. Do you know any of the voltages or currents?

ak

 

[aleeeeaaa]

The receiver is a Realistic dx-160 and the frequency counter is the one in the pdf file i bought on line. I ran the counter powered by a 9 volt battery and no hum out of the radio speaker. Everything works fine in this mode. But i want to power it from the radio so i don't have to mess with a battery. I hooked the counters red wire to the power side of the radios fuse and the counters black wire to the radios circuit board ground. Thats when i get the hum out of the radio speaker. The counter works fine either way. I measured the voltage out of the radios fuse and it is 15 volts.

 

[AnalogKid]

If you have the correct manual with the schematic, please post it. Based on the DX-150 manual -

The additional 160 mA load is increasing ripple out of the power supply bulk filter capacitor to the point that it is getting into the audio power amplifier at an audible level. There are no differential stages, current sources, or any other circuit techniques that reduce power supply sensitivity. The radio circuits are powered by a voltage regulator that reduces the ripple seen by those circuits, but the amplifier section is not.

The bulk filter cap is C64, 2000 uF / 25 V. Double it with an additional 2200 uF, or replace it with 4700 uF.

In the audio amp, it looks like C60 is 1 uF / 16 V. Increase to at least 10 uF, preferably 100 uF, whatever will fit.

If these changes do not improve things, then it the ripple is getting into the audio through the audio amp output stage. Nothing you can do there.

ak

 

[Robert132]

You need to come off the 12v pin on the circuit board for power, not the fuse power.

 

[AnalogKid]

You need to come off the 12v pin on the circuit board for power, not the fuse power.

Depends on the construction.

The pass transistor is a 2SD146, which is a 40 V, 1 A part. It crosses to a 2N4912, which is an 80 V, 1 A, TO-66 part, so it doesn't have a huge radiating surface. Using the regulator output will make for less hum, but more heat. My guess is that it more than doubles the regulator power dissipation.

Also, there is a 47 ohm resistor in series with the pass transistor, probably to offload some heat. An additional 160 mA through the resistor equals a 7.5 V voltage drop *increase*. This will almost certainly cause the regulator to drop out for lack of headroom.

A photo of the inside of the receiver would help.

ak