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Author Topic: Another Viking Ranger mystery  (Read 2076 times)
N6AF
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Posts: 86




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« Reply #15 on: October 30, 2018, 02:01:27 PM »

I'm trying to keep the Ranger as stock as possible. The original design closes the loop around the driver and final modulator stages.  The new transformer has the desired 10K to 90k impedance ratio but likely a different transfer efficiency than the original one. So the closed loop response is now out in left field or out of the park. The best I can do is tweak the feedback path gain for stability, which seems to have worked although I think the hum attenuation isn't as good as it was originally. It sounds very good on the remote W6DRZ SDR receiver over the hill from my QTH. We'll get some real critique on 3870 tonight. Fun stuff.

But I am considering the mod that uses the driver 12AU7 in a direct, transformerless approach. There are several write-ups about that.

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KE0ZU
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Posts: 469


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« Reply #16 on: October 30, 2018, 03:53:19 PM »

Have you grounded the cold end of R27 to see if the feedback is involved or not?   

Have you replaced C60? That reduces the "highs" in the modulator.

What about C59B, make sure the "+" goes to ground since the bias is a negative voltage.

Next I'd pull the modulators and see if the preamp still oscillates.   
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Regards, Mike
https://mikeharrison.smugmug.com/
Pics and bold print are usually links.
N2DTS
Member

Posts: 967




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« Reply #17 on: November 01, 2018, 05:09:36 AM »

Changing the transformer likely added more phase shift or a wider frequency response (or both) and caused the problem.
Its odd the original transformer failed....driver transformers usually are not a weak point.
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KM1H
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Posts: 5559




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« Reply #18 on: November 01, 2018, 09:36:25 AM »

Quote
If it was mine I would be checking all the ground terminal to chassis resistance readings ... anything over ZERO needs some work.


Yep, a common problem with kit and even some factory built products.

Also pull the 6146 and give it a try plus measuring/replacing its parasitic suppressor resistor.

There is also a difference between early and late versions at the 1614 cathode, The late has only the metering shunt resistor to ground and the 15uF electrolytic is connected to the Xfmr CT to ground only while the early model has a 10 uF from cathode to ground.

Both versions are here:
http://bama.edebris.com/manuals/johnson/ranger/

Did you replace or test the 1614's? In some cheap testers use the 6L6 settings but 12V filaments.
Also only a quality tester will find problems with those and the 12AU7/12AX7. Ive used a TV-7C/U and Hickok 752A for many decades which have found problems others ignored.

Carl
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W3RSW
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Posts: 608




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« Reply #19 on: November 11, 2018, 09:31:17 AM »

Reverse the feedback leads if you haven’t already tried that regardless of how right you felt your installation was originally. 
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Rick, W3RSW
KM1H
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Posts: 5559




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« Reply #20 on: November 11, 2018, 03:02:10 PM »

If it never worked properly and was a kit try reversing the plate leads to change the phase. Real common with homebrew.

Carl
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WA1RNE
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Posts: 1010




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« Reply #21 on: November 28, 2018, 07:21:38 PM »

 The new transformer's response characteristics are likely very different than the original which could have changed the closed loop stability of the audio chain.

 The major components in the loop consists of the driver V8, transformer T3 and the push pull 1614's and T2.

 Without plotting the open loop response of the driver and output circuit, you could try something simple like increasing C60 from 0.005 to 0.01uF to tail off the transformers HF response a little.

...WA1RNE
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WA1RNE
Member

Posts: 1010




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« Reply #22 on: November 30, 2018, 10:13:12 AM »

Incidentally, N6AF indicated in his first post that once the pots were installed in the correct locations, the modulator operated correctly.

So I suspect the feedback is not reversed.
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