Galaxy 959 crackly rx/tx no AM audio

[Qhickey]

Hi guys,

First time poster long time reader. There are some very smart repair guys in here with a ton of experience and was wondering if I could pick a brain or two. I took a friends 959 in to line it up on LSB. It went ok but than I noticed that I was getting a crazy snap crackle pop noise on AM on a dummy load, as well as when I tx in AM, I get a dead key but no modulation. And it I get crackles there too. It does tx on ssb but I was told I have the noise come in with my audio there as well. I changed out the audio IC8 but no luck. I started checking voltages on transistors but nothing seems too far astray. I did notice a few wonky voltages in IC1 but not sure if it has to do with my noise or not.

The other problem is, my AM dead key is now 25 watts, turning back vr14 and vr16 do nothing! I am totally lost with this.

Can anyone give some insight or point me in the right direction. Thanks.

 

[nomadradio]

First problem is that there is more than one DX959. The one made before 2007 has a bipolar final and driver transistor. These are parts designed from the ground up to be used as RF amplifiers in a radio this size.

All production since then uses a substitute called the IRF520. A handful of circuits are different, most are the same. The ones that are different are in the transmitter's power stages, the final and driver.

Pretty sure that Q54 is shorted internally. When this happens, the AM carrier power skyrockets, there is no AM transmit audio and SSB transmit still works. Q54 is the fat 2SB754 transistor bolted to the side rail just to the front from the power socket. Q55 is a not-so-wide power transistor standing up against the front of Q54. This one drives Q54, and will frequently be damaged when Q54 fails. We adopted the policy of always replacing Q55 when Q54 has broken down.

Do you have a way to check transistors? If you don't this can be a big handicap.

The noises you're hearing in the receiver audio probably are not related to Q54, might be a separate problem. Failed solder connections on the circuit board are incredibly common for this class of radio. If pressing against either side of the circuit board prompts noises from the speaker, suspect this to be part of the picture.

Oh, and don't key it in AM any longer than you have to. A 25-Watt carrier will overheat and blow out the final transistor before too long.

73

 

[wavrider]

the VSWR circuit is known failure, auto sensing and it fails resulting in no TX on AM mode but works on SSB
take a look and see if any components have failed in that circuit also, may be source of crackling noise

 

[Qhickey]

Thanks for the quick reply guys. I actually replaced q54 already without test the old one, same results.
I did not however replace q55. I see that the collector of q55 feeds the collector of q54.
So I just re tested the voltages as follows.

Q54, B and emitter seem fine. Collector on AM tx should be 6.4 its not dropping. Its 11.61.
Q55, on AM tx he collector should be 6.23 AM tx. It is almost 10 volts.
If it isnt dropping the apropriate voltage there must be an issue with Q55.

I think before I go buying any more parts Ill buy one of those 40 dollar transistor checkers as I need one anyhow.

Again thank you guys for you quick response, I will definatetly document all of my information here as I move through this repair so it will help others.

 

[Qhickey]

the VSWR circuit is known failure, auto sensing and it fails resulting in no TX on AM mode but works on SSB
take a look and see if any components have failed in that circuit also, may be source of crackling noise

I will definatetly do that for sure. My high swr light doesnt come on and seems to be ok but I will check around that area for sure. Iven spent all last night checking tr voltages and mostly good so far except that Q55. Without an lcr meter I feel like Im blind. I really need one

 

[Qhickey]

Just a quick follow up I pulled Q55 and diode checked it, there is s diode drop from C-B and E-B so its not open, I will look around the area to see why my collector on Q55 is not dropping as much as it should on AM TX and report any findings back here.