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2950DX with no TX output

W

W3FAW

Member
Oct 19, 2012
20
2
13
It started out with bad TX audio. I checked the TX bias on the mosfet driver and finals. I found that the driver was fine at 3.7 VDC and one final was 2.4VDC and could not be adjusted higher and the other was less than 1VDC and could not be adjusted at all. With a multimeter, I checked the final with 1 volt bias and found that it was shorted. While the other final did not show a hard defect, I decided to replace both on general principles.

I replaced the IRF520's with IRF520N's which is what the vendor sent me. I was real excited when the bias voltages dialed right in: 3.7, 3.5, 3.5VDC. Then I tried to put it on the air but there is no output in any mode. Yes, the shorting board was in place and yes the mic gain was up. I removed the finals and bench tested them and they checked out fine. I also checked the zener's and they looked good. I re-installed the finals making sure that my solder connections were good but still no output.

So there you have it. I'm hoping it's just something I have overlooked but at this point I need help.

I'll appreciate any assistance. Thanks. Jim
 

Well, I have signal at the gate of the driver, but after that I'm not sure what I have. I can tell you what I have, but I'm not sure what it means.

The rf signal at the gate of the driver is 300mV. The signal at the middle pin (drain) is 75mV, and the signal at the third pin (source) is very small. The signal arriving at the gates of the finals is 75mV.

I would have thought that the signal on the drain of the driver should be higher, unless the mosfet is blown. Don't ask what the signals are on the drains of the finals are, I took them out again thinking they were pulling down the output of the driver. Regardless of the exact value, there is no output. All of the bias voltages remain correct.

As far as I can tell by a multimeter bench test Testing a MosFet
the finals are good.

Yes, there is VCO.

Does any of this gibberish make sense?

BTW: I know you are pulling my leg about your being a beginner troubleshooter:)
 
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My 2970n2 had a similar problem. After it tx'ed in to a high swr antenna it gradually lost more power until it only did 40watts ssb which was garbled sounding. Am/fm power was down also but the audio was okay.The rf meter on the radio woundn't swing fully either. A tech resoldered some joints around the finals and realigned it and brought it back to life for me. Let me know if you get it fixed and what you did to fix it.
 
Robb, One other strange thing is that I cannot see a signal at TP17. It is not shown on the schematic that I can find, but it is somewhere in the transmit section after the mixer but before the driver. It's probably just before the driver. It just doesn't show up on the scope. If the signal really wasn't there, it wouldn't be getting to the gate of the driver. Just additional information. Thanks.
 
W3FAW said:
Robb, One other strange thing is that I cannot see a signal at TP17. It is not shown on the schematic that I can find, but it is somewhere in the transmit section after the mixer but before the driver. It's probably just before the driver. It just doesn't show up on the scope. If the signal really wasn't there, it wouldn't be getting to the gate of the driver. Just additional information. Thanks.
Click to expand...

I think you meant L17; not TP17.
TP3 is one place to start and it is at L65; use a freq counter.
Ranger RCI-2950 Service Manual
http://www.cbtricks.com/radios/rci/rci_2950/rci2950_adj_pll.htm
Then test D45 on TX for output. That will tell you if the VCO/PLL is getting the signal out to the TX mixer IC.
 
I'm sorry. I'm mixing you up. In this thread I'm working with a 2950DX. That is way different from a straight 2950.
 
Sorry Robb. 2950dx was a complete redesign. Here are the best images I can find for both:

2950
rci2950_ept295013z_main_pcb_layout.gif


2950dx
rci2950dx_component_layout.jpg
 
OK; the board was re-designed using SMD/parts. But you can still see TP3 there. Or just go to D45 and see if there is some output freq when TX/keyed.
Either way, you will know if you are getting a signal.
 
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Robb,
Old TP3 is now TP7 - No Signal
Old D45 is now TP5 - Good signal

I'm discouraged by the lack of signal on TP7 and TP17. TP17 is not on the schematic, but it is somewhere at the end of the exciter stage. That looks like real trouble. I can't work on that surface mount stuff.

I'm going to give it a rest for a few days so I can develop a new strategy.

Thanks for your help.

Edit: I think I must have blown some mosfets with static electricity. I just learned that solder suckers are dangerous because they generate static electricity. I have been using a solder sucker throughout this project.
 
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The VCO output is still at TP3
Check it:
http://www.cbtricks.com/radios/rci/rci_2950dx/graphics/rci_2950dx_70dx_sm.pdf

It's possible that the MOSFETS are toast due to static. Bought some online and they were bad when they got here. If it were me working on that radio, I would ask myself if I did the install right too. Did I put the zeners in backwards - stuff like that. I make more mistakes when I am in a rush than when I take my time. It is easy to make a simple mistake as well.
 
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