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Ranger 2950 BIAS settings

W

W3FAW

Member
Oct 19, 2012
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Hello, Does anyone here know how to set the bias on an ordinary 2950? The driver bias is supposed to be measured between TP3 and TP2 It is supposed to be adjusted by VR11 to 50-60mA. On my rig the driver bias current is around 270mA and VR11 has little or no effect on it.

The final bias is measured between TP1 and TP3. It is supposed to read between 100 to 120mA. Mine reads about 10mA.

I got these values from cbtricks.com Ranger RCI-2950 Service Manual I'm using these procedures because the factory manual doesn't mention bias settings in the alignment section of the manual.

The rig basically runs well, but I'm checking bias settings because I do get an occasional report of rough audio. I can't read the markings on the transistors so I don't know if they are factory originals.

Thanks for any and all help.
 

Do you have it in SSB mode/0 gain on mic?
Makes a huge difference . . . lol . . .
If you did that and still won't adjust; then I would check the bias diode/MV1Y' next to it.
Also make sure that VR is working; sometimes that is the problem too . . .
 
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Hi Robb, thanks for the reply.

Yes it is in SSB mode with zero mic gain.

The diode measures good and VR11 is functional in that it can adjust the current from 220 to 270mA.

I can't understand how the radio can still work with values that are so far off from spec.

Could it be that someone has installed mosfets, which are biased by voltage? The quarters are cramped in that section but I was able to measure the bias (gate?) voltage on one of the finals and it showed 0.73VDC which is not a typical mosfet bias voltage. If they were mosfets they would be around 3.5VDC.

Oh well, we'll just keep trying to figure it out.

Regards.
 
Hi Robb, thanks for all the ideas.

I believe in making the "easy" measurements first so I got past the obstacles and measured the base voltage of the driver and two finals. They were well within range of the transistor voltage chart. The finals measured 0.769VDC Which is 0.1 volt above spec and the driver measured .73VDC. At least that puts my mind at ease about the possibility that someone put mosfets in there.

Regards.
 
Well, I was able to get the final bias current set correctly at 110mA. I found that VR7 (SSB Carrier Balance) was maladjusted. It was set in such a way that it sent carrier signal to the finals during the test. Once I adjusted it correctly, I was able to use VR11 to put it on target. The driver bias current however is still wrong at 230mA.

Well at least 50% of the problem is gone.
 
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W3FAW said:
Well, I was able to get the final bias current set correctly at 110mA. I found that VR7 (SSB Carrier Balance) was maladjusted. It was set in such a way that it sent carrier signal to the finals during the test. Once I adjusted it correctly, I was able to use VR11 to put it on target. The driver bias current however is still wrong at 230mA.

Well at least 50% of the problem is gone.
Click to expand...

Glad you sorted some of the problem out. But this also makes it clear to me - by process of elimination - that the problem that still exists is in the driver bias circuit. Sooooo, I would take out and test each part in that circuit for possible loss of value. As a newbie to troubleshooting, that would be my next step.

I would see if the bias resistor has changed value - for one. Check the electrolytic caps in that circuit too. I have read that it is possible that the bias diode may check out fine; but in circuit it may not work as it is supposed to. I would take the MV1Y out of circuit temporarily and try a 1N4148 or 1N914 instead and re-check the VR for a better mA range. . . .
 
Hi Robb, for a beginner at troubleshooting, you have good ideas.

Well, I removed the driver transistor and the MV1Y and found everything to be in order. I didn't remove any other components but I tested for shorts on everything in sight. I couldn't spend much time on it. I've got too many other projects.

It's back together and working the same as before. I have to do some on air checks but I believe now that the SSB Carrier Balance and the final bias are correct, it should sound pretty clean. I'll let you know when I get more time to work on it.

The thing that is mysterious is that the base bias voltage on the driver is correct even though the current is so high.

See you later. Thanks for your help.

Edit: I just got a signal report from a friend and he said the audio sounds real clean. He is someone who heard it before.
 
Last edited:
Think I might have found your problem . . .

I also have a RCI-2950. So, I opened it up and checked the driver bias against the info on CB Tricks. Seems that they got their info wrong. TP1 & TP3 are for the driver; not TP2 & TP3 - as it states. This would also explain why your radio is getting fair TX reports.

The + lead of your meter goes to TP3 and the - lead goes to TP1.
LSB mode/mic gain 0.
Now; retest and adjust.
Worked for me . . .
 
That's great Robb, thanks. I tried it and it works. Now, that begs the question of how to measure the final bias current.
 
W3FAW said:
That's great Robb, thanks. I tried it and it works. Now, that begs the question of how to measure the final bias current.
Click to expand...

TP2 = "-" test lead
TP3 = "+" test lead

I got 142mA when I tested . . . a bit high . . . but no adjustment available . . . I'm guessing that one or both of those 18 ohm bias resistors are out of spec when under load and heating up. . .
 
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Good for you. But my final bias current is 240mA. Looks like I'll be taking out the finals tomorrow:) I'm betting on one of those MV1Y's. Good Night!
 

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