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Motorola Kazoo XPR4550

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Golay

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Apr 28, 2016
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I really had something weird happen to me last week.

I'm in a digital QSO, on a XPR4550 on a power supply. All of a sudden the radio starts squealing while the other gentleman is talking. Loud enough to completely cover up what the other ham was saying. I reach for the volume, and as I do, the tone of the squeal changes. I could literally run the musical scale by where my hand was on or near the radio. Thinking "Oh great", I turned the radio off. Turned it back on, the squeal was gone.
Have used the radio half a dozen times since, hasn't done it again. Internal speaker. The mike wasn't keyed, was about a foot from the radio, and I wasn't holding the mike. Not getting a shock off anything when I touched the radio.

The only thing different is this:
About a month ago, the power supply quit.. It's the usual Antron power supply that Motorola sells for mobile radios, with the shroud on the top of it for the radio to fit into. Opened it up, the fuse was vaporized, the MOV was cracked. Did some googling. Bought and soldered in a new MOV, replaced the fuse, and the power supply has been fine. Reading 13.8VDC with no AC measured. Probably don't mean anything, but it's the only variable I can think of.

Any ideas what would of caused the radio to squeal?
 

RFI-EMI-GUY

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Dec 22, 2013
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Mechanical feedback between speaker and VCO. Though in a digital mode I would think you would simply have excessive BER and audio dropout. The other possibility is that a decoupling capacitor in the audio stages has blown and you are getting high frequency motorboating . This radio sounds like it took a power or lightning surge given the power supply damage. That said, the oscillation might be in the power supply. Are you using a scope on the power supply?

Sent from my SM-T350 using Tapatalk
 

Golay

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Joined
Apr 28, 2016
Messages
388
Location
Nankin Township, Michigan
Mechanical feedback between speaker and VCO. Though in a digital mode I would think you would simply have excessive BER and audio dropout. The other possibility is that a decoupling capacitor in the audio stages has blown and you are getting high frequency motorboating . This radio sounds like it took a power or lightning surge given the power supply damage. That said, the oscillation might be in the power supply. Are you using a scope on the power supply?

Sent from my SM-T350 using Tapatalk
No. I don't have a scope. Entirely possible that the radio took some sort of spike when the MOV went.
Like I said, there is no ripple on the DC output voltage. I do know people with scopes. Where and what can I look for in the power supply? It's an Astron Model SL-11R-RA. When the MOV and fuse went, I downloaded this schematic, The VR1 on the left is the MOV I replaced. Sorry the picture is a bit fuzzy, about the best I could do. I haven't done any of the voltage checks in the A1 table. I will take it back apart this weekend, and see what I get.
 

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RFI-EMI-GUY

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Dec 22, 2013
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I would try another power supply to see if the radio behaves same way. You will need an o'scope to figure this out, though DC readings may indicate a bad pass transistor under load (Q101, Q102). MOV's failing shorted points to a traumatic surge.
 

thassler

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Sep 29, 2011
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Location
Tennessee
If the firmware level is > 1.12.13, there's a bug that can cause that symptom. When it happens, the only cure is to power cycle the radio.
 

RFI-EMI-GUY

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Dec 22, 2013
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If the firmware level is > 1.12.13, there's a bug that can cause that symptom. When it happens, the only cure is to power cycle the radio.
Well there we go, we are worrying about erratic electron flow when in fact it is probably a typo!
 
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