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Author Topic: AN762 Physical Vibration From PCB ???  (Read 4286 times)
KO4NX
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Posts: 179




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« on: November 19, 2012, 11:17:11 AM »

I have built the AN762 180W version (2X MRF421). After doing some initial bench checks, I am observing a phenomena that I have never seen in a Solid State Amplifier before. There seems to be a physical vibration emanating from the amplifier board itself when transmitting. This phenomena seems to occur around 60 Watts and continues all the way up to 100 Watts.

The vibration/noise seems to be at it's worse in SSB mode, but clicking can also be heard in CW. The only way to alleviate this noise is to increase the BIAS ADJ potentiometer to reflect a BIAS current of around 900mA in standby. Has anyone here observed, heard of, or expierenced anything like this?  I would like to have an idea of what I am dealing with before I do any futher troubleshooting.

73

Rich, AJ3G
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M0HCN
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« Reply #1 on: November 19, 2012, 11:41:09 AM »

You got some DC current where there should be none....

Check the windings on T2, if one of the bifilar pair is open circuit then DC current will flow in the output transformer and everything will get hammered into saturation, which will cause acoustic noises, distortion and even more heat then you would expect.

I don't really like the DC continuity between the collectors via in T3, it is asking for this sort of thing, plus it makes fault finding it harder then it needs to be.

Can you temporarily lift one end of T3 primary then with no drive and a current limited supply power up and ensure that both collectors are at the supply rail, and that both devices can be biased up correctly?

Something is causing low frequency current in the magnetics, the question is what?
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KO4NX
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« Reply #2 on: November 19, 2012, 12:22:19 PM »

Thanks for the information on what might be the cause of this issue.

I will check T2 tonight to insure it is in fact not open circuit in one of the windings. If it is I would be shocked, but all the same I will double check for continuity on both the top and bottom side of the board.

To answer your question about T3, I think it may be possible to lift one lead from the primary (although it may be hard to accomplish). Just to be sure I undertand correctly, you would want me to lift one lead, check to make sure I can set the bias of the opposite transistor to approx 150ma and then check to make sure the voltage on the collector is at the 13V8 Rail?  If it is, then duplicate the same test on the opposite FET..

Lets assume for a moment this test turns up a favorable result, where would you start looking next?



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M0HCN
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« Reply #3 on: November 19, 2012, 12:57:30 PM »

If we have DC continuity through the bifilar and to both transistors, then I would have to start thinking about the input network, but my bet is that there is something rotten in the output or DC injection arrangements.

You did make sure T2 has the same number of turns on the two DC injection windings (That would do it)?

Do make sure you use a current limited supply for the above test, strange things can happen in power amps when you have the output transformer unhooked, and oscillation is always possible.

The objective here is to establish that there is DC continuity from the supply to the transistors that does NOT involve the output transformer, and that both transistors bias up to a reasonable current at about the same bias setting.

Regards, Dan.
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WX7G
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« Reply #4 on: November 19, 2012, 02:10:38 PM »

I bet the MC1723G regulator is oscillating.
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KO4NX
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« Reply #5 on: November 19, 2012, 02:28:19 PM »

I think I may have stumbled across the issue. It appears as though the secondary of T1 is open on one side. From the looks of it, it burned open, but not in a major way. There is the obvoius scortching marks, but it is not very visible until looked at under a magnifying glass. I hope this is the result of a cold solder joing causing the heating, and not something more!

73
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KO4NX
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« Reply #6 on: November 19, 2012, 03:18:45 PM »

T1 was not as bad as I thought, but replaced the lead anyways. The material they use to make the primary is very thin PCB, and it is possible the burning I saw was in fact from my own soldering job.

The good news is, I found out where the vibration was coming from, but this is where it gets strange to me.  The vibration is coming from the snap on ferrite chokes I placed on the DC input leads. It appears as though they are vibrating around inside the plastic housing which holds them in place!

I am not sure if this is an issue, but I have to say this is a new one on me!

73
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WX7G
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« Reply #7 on: November 19, 2012, 03:34:59 PM »

Using an oscilloscope and a X10 probe I would look at the output of the bias regulator MC1723. You probably find that it is oscillating and drawing current in pulses.

And snap-on ferrite chokes on the input leads should not be placed individually on each lead. This adds differential-mode inductance and can cause the the bias supply to become unstable. The ferrite chokes should be placed in a common-mode configuration (plus and minus power thru one ferrite).
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KO4NX
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« Reply #8 on: November 19, 2012, 04:38:45 PM »

I just checked the output of U1, and it looks good. If it is oscillating, then I cannot see it with my DSO. I did follow your recommendation and placed both leads into one choke. The Vibration has disappeared, and I am able to keep the bias around 300mA without issue. Furthermore, you can tell it is running a more efficient, as a 100W Key down for 1 Minute into a 50 Ohm Load does not yield the same temperature increase as it once did when the amplifier was set for 900mA!

If U1 is oscillating, and I cannot see it with the scope, then I am sure it will show up in IMD testing later on. I will keep my fingers crossed that I do not have address that issue, as it does not sound like fun to fix!

I would like thank the both of you for the help. I think I am in good shape now, and can move on to the testing the Driver, and LPF assembly!

73

Rich, AJ3G
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WX7G
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« Reply #9 on: November 19, 2012, 07:28:50 PM »

If the output cap has low ESR and high C there won't be much voltage variation to see during an oscillation. But if you can view the current into the cap you might see variations.

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KO4NX
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« Reply #10 on: November 20, 2012, 04:53:56 AM »

In the near future I will be cutting some of the tracks off the board to install a MOSFET to switch the BIAS supply when Transceiver is placed in transmit. I think when I install this modification, I can certainly observe the current through the capacitor.

FYI the capcitor is a 500uF electrolytic, thus it does qualify as a low ESR high value capacitor.

73
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WX7G
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« Reply #11 on: November 20, 2012, 07:50:18 AM »

Current due to oscillation can also be measured at the Vin to the amp. A clamp-on current probe would be ideal and a 0.01 ohm viewing resistor with the 'scope floating will also work.

But the oscillation needs to be taken care of. The 1723 will only oscillate if something is not right in the feedback loop or output. Are the regulator components the correct values?
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KO4NX
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« Reply #12 on: November 20, 2012, 08:30:41 AM »

To be honest, I am not convinced the regulator circuit is oscillating. Once I solved the physical vibration issue by installing both DC leads into one choke, the amplifier seems to be very well behaved.

Is there something else I missed, which makes you believe there is still a 1723 oscillation problem? If so, then I will be glad to investigate further to solve the issue.


73 
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WX7G
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« Reply #13 on: November 20, 2012, 08:32:39 AM »

It does look like the oscillation was due simply to the increased source inductance.

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N6AJR
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« Reply #14 on: November 24, 2012, 11:42:25 AM »

I think AC or any varying voltage or current has the potential to vibrate.  loose screws in a transformer frame will buzz .

 several of my electrician friends say they see where the 10/240 volt leads come into a house panel will vibrate the screws  loose on the point where they hook to the panel.  they say the ac causes the screws to vibrate out eventually.

This may actually make sense and the current in the wires has to produce a small magnetic field at the junction.  any how,  I can see the ferrite beads vibrating as they too work in an induced field.
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