National NCX3 Speaker Buzzing

[AB2EZ]

Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu

[N0WVA]

Ok, I got the B&K 1474 scope going again, I just had to clean the edge connectors joining the pc boards.

I have a .01 1kv disc in series with my probe, the probe is set on 1X, aliigator clip grounded to the supply, and the scope voltage input is on 5v. With radio on and in receive, power supply open and flipped upside down in front of me, and checking pin 9 on the cinch connector ( 280v line) I get a very clean signal with no noticeable AC until I get down to .1 on the voltage switch and even then its hardly a ripple. So for reference I checked at the junction of L2 and c3, before the 2 330 ohm 10 watt resistors, and then it showed about 4v ac ripple. So a little more ripple before the two 330 ohm resistors and regulator circuitry, which is to be expected, but it looks clean at pin 9 with radio running.

Also I used a separate ground wire  between radio and PS, it did change the hum a little, but its still there.

[N0WVA]

So as a test, try taking a series R-C snubber from the transformer secondary center tap to ground. Try something like a 3K 2 watt (not critical) and a .01 uf 1000 V capacitor...See if it has any effect. If so, perhaps try different values, or another R-C across the entire transformer secondary. The Cap would need to be rated higher there, at least 2 kv.

I cover the diode hash / buzz issue in more detail here in yesterday's post:
http://amfone.net/Amforum/index.php?topic=34793.0

Jim
WD5JKO

Tried this test with a 1.5k/.01  disc. With receiver on, grounding the .01 made no difference in hum level.

[AB2EZ]

When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu

[N0WVA]

When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu

It was a nice clean sine wave, around 4v p2p.

[N0WVA]

Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu

There is some vibration, but the previous owner put a 1/8 headphone plug in the back of the PS, I am using phones to monitor.

[N0WVA]

So its an annoying buzz, and present with the 6aq5 pulled. The buzz starts as soon as the rig is turned on. It is being picked up through two capacitors c37 and c38. Lifting them makes it quiet. But then theres no VOX. Im about at the end of my knowledge here.

As has been previously posted, these capacitors are C97 and C98. They are both supposed to be 0.01uF capacitors. Therefore, the impedance of each should be 132k ohms at 120Hz and 265k ohms at 60Hz.

If the audio output tube is disconnected, either of these capacitors could (as previously posted) conduct current from the 280V B+ through the primary of the audio output transformer to ground (if the capacitor is connected between the top of the primary and ground)

However, given the high impedance of either capacitor at 60Hz and 120Hz, any hum between the 280V B+ line and ground should produce only a small current through the primary of the audio output transformer. The hum should be greatly attenuated because the input impedance of the audio output transformer (with the speaker attached to the secondary of the transformer) should be much less than the impedance of the capacitor.

Nevertheless, N0WVA reports that the hum goes away when both of these capacitors are disconnected at one end (with or without the audio output tube in place).

I suggest the following experiment:

Disconnect both ends of the audio output transformer's primary (i.e. disconnect everything connected to the top side of the primary, and disconnect the 280V B+ from the bottom of the primary). Place one of the 0.01uF capacitors across the primary.

This will result in a 133k ohm load across the primary (at 120Hz), and a corresponding much lower impedance looking into the secondary (where the speaker is connected). I.e. the impedance looking into the secondary of the audio transformer at 120Hz will be 133k ohms divided by the square of the transformer's turns ratio (ignoring the transformer's magnetizing inductance).

If there is a 60Hz voltage from the filament supply being coupled directly into the speaker leads (e.g. due to some problem in the cabling between the power supply/speaker and the transceiver)... and if that is the cause of the hum that is being heard, then the hum should still be present with the primary of the audio output transformer having only a .01uF capacitor across it (nothing else connected to either end). It should disappear (or be greatly reduced) when the .01uF capacitor across the primary is removed.

If there is no hum when the capacitor is across the primary of the audio output transformer (with nothing else connected to the primary) then try placing a short circuit across the primary of the audio output transformer (with nothing else connected to the primary). If there is still no hum, then it is unlikely that the hum is associated with 60Hz (e.g. filament voltage/current) being coupled from the filament supply into the speaker leads inside the cable between the power supply/speaker and the transceiver.

Given N0WVA's reported result that the hum goes away when L2 is replaced with a 1.2k ohm resistor, I would (again) be surprised if the hum is being introduced in the cable between the power supply/speaker and the transceiver.

 Stu

With only the .01 cap across the primary there is hum in the phones/speaker. It goes away when the cap is disconnected.

But just as we are getting this thing nailed down and Im typing this, I just heard the supply bog down and it blew a fuse. Now I get tot troubleshoot two things at once.

[AB2EZ]

When you looked at the voltage across C3 (4 volts ac):

What was the frequency (60Hz repetition rate or 120Hz repetition rate)?

Did it look like a sine wave or a saw tooth wave?

Was 4V the peak-to-peak voltage?

Was the scope input coupling set to DC (sometimes AC coupling will introduce a high pass filter at the input to the scope that will attenuate low frequencies, and produce an inaccurate reading). With the .01uF capacitor present, DC will be blocked, and it will be okay to have the scope set for DC coupling on its input.

Stu

It was a nice clean sine wave, around 4v p2p.

4V p2p across C3 seems about right... given the current that is being drawn from this point by the load (around 100mA). The fact that it looks like a sine wave (rather than a sawtooth wave) suggests that the choke is doing its job. If the choke were shorted, the waveform across 40uF capacitor C3 would look like a saw tooth with a peak to peak amplitude of around 21V. I.e. if the choke were shorted, and since Q=CV, the voltage across C3 would drop roughly linearly by:  0.1A x (1/120)s x (1/0.000040F) every (1/120)s, and then jump back up to full value when the diode bridge conducts.

The ripple from pin 9 to ground should be around 5% of the ripple across C3, if the ripple frequency is 120Hz. I.e. the impedance of the capacitor from pin 9 to ground, at 120Hz, is around -j33 ohms. With the pair of 330 ohm resistors in series, the capacitor from pin 9 to ground works in conjunction with the resistors to form a 33/660 = .05 voltage divider that attenuates the 120 Hz ripple.

Therefore, if you see a 4V peak-to-peak 120Hz sine wave across C3, you should see a .2V peak to peak sine wave from pin 9 to ground.

Stu

[AB2EZ]

Another thought:

Are you sure that the hum is coming from the speaker... and not from a transformer or a choke vibrating?

Stu

There is some vibration, but the previous owner put a 1/8 headphone plug in the back of the PS, I am using phones to monitor.

In the schematic, there is a headphone jack in the transceiver itself. I think you are saying that someone installed an additional headphone jack inside the power supply. Please clarify... is this additional headphone jack across the two speaker wires in the power supply, but isolated from power supply ground; or does it introduce a path inside of the power supply that connects one side of the speaker to power supply ground?

If you connect one side of the speaker to ground inside the power supply... that could be the problem.

Even with a wire (having some amount of resistance) running between transceiver ground and power supply ground... the filament current flowing between the transceiver ground and the power supply ground will cause a 60Hz voltage to exist between the transceiver ground and the power supply ground. There also will be a 60Hz voltage magnetically induced around any closed loop that encloses the power supply's transformer and the transceiver.

One side of the speaker (located in the power supply) is connected (via a wire in the cable) to transceiver ground, at the point where the cable connector is located on the back of the transceiver. The audio output transformer is located in the transceiver. One side of the audio output transformer's secondary winding is also connected to transceiver ground. The other side of the audio transformer's secondary winding is connected to the other side of the speaker (via a wire in the cable). There is no direct connection between either side of the speaker and power supply ground.

If one adds a path to power supply ground from the same side of the speaker that connects (via the cable) to transceiver ground... then you will have 60Hz current flowing through the speaker (provided there is some load across the primary of the audio output transformer). I.e. 60Hz current will flow from power supply ground, to the grounded side of the speaker, through the speaker, across the cable to the audio output transformer's secondary winding, through the audio output transformer's secondary winding, to transceiver ground, and back to the power supply ground via the path through the effective 60Hz voltage source that exists between power supply ground and transceiver ground.

Stu

[AB2EZ]

The wiring of the added headphone jack might explain why replacing L2 with a 1.2k ohm resistor made the hum go away.

If the associated wiring was not done using a balanced - twisted pair approach, then there might be 60Hz being induced into the added headphone jack wiring by the time varying magnetic field of L2.

It would probably be best to remove this added headphone jack, and to return the wiring between the speaker and the multi-pin connector on the back of the power supply to stock form (using a twisted pair of wires).

Stu

[N0WVA]

The wiring of the added headphone jack might explain why replacing L2 with a 1.2k ohm resistor made the hum go away.

If the associated wiring was not done using a balanced - twisted pair approach, then there might be 60Hz being induced into the added headphone jack wiring by the time varying magnetic field of L2.

It would probably be best to remove this added headphone jack, and to return the wiring between the speaker and the multi-pin connector on the back of the power supply to stock form (using a twisted pair of wires).

Stu

And there it is.......The previous owner put a 1/8 headphone in the power supply/speaker unit. Disconnecting the wire that is grounded to the jack solved the problem. Well, I pulled the whole mess out of there and put the original choke back in....The only hum is a slight buzz from the choke, not really objectionable compared to the raspy speaker buzz I was getting earlier.

I can believe this run me in such a large circle.

 Thanks to everyone who responded here, if nothing else, I at least learned a few things from all your knowledge.

[KA2DZT]

 Thanks to everyone who responded here, if nothing else, I at least learned a few things from all your knowledge.

Study, because we'll be given you a test on Friday.

Seems the problem proved to be a grounding issue or as in this case, the ground from the added headphone jack.

Fred

[MikeKE0ZUinkcmo]

Fred,

If you really want to keep the head phones, just use an insulated phone jack and things will be fine.    You just want to keep the speaker/phones ground return line separate from the PS chassis ground.   What you were hearing was the AC voltage drop across the PS to radio cable ground wire.   The buzz was from the load of all the filaments in the radio.

If you connect your scopes ground to the radio chassis, and put the probe(no ground) on the power supply chassis, you'll see the AC.

Good detective work Stu.

Love them ground loops.