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Author Topic: Another Viking Ranger mystery  (Read 2071 times)
N6AF
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Posts: 86




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« on: October 28, 2018, 10:43:37 AM »

I'll start with a minimum of details.  Hopefully someone's been down this vintage trail before.

My Ranger ran beautifully on AM for many years.  Recently I turned it on and the modulation was dead.  No AM output; just carrier.  Troubleshooting starting at the mic input I eventually found that the primary side of the interstage modulation transformer T3 was open.  I read several reports on line about that failure mode.  I ordered a P-T156 transformer from Antique Electric, which is supposed to be similar to a Stancor A-53C, which I couldn't find on line.  Several people discussed successfully using the P-T156. 

I received the P-T156 yesterday and put it in.  I determined which connections of the secondary winding would provide the proper phasing for the main modulation transformer's feedback loop.  When you have that wrong the rig really howls up a storm.  The modulation started working!, sort of....

But even with it connected correctly, there's still another issue.  There is a variable oscillation somewhere in the modulator lineup.  It is hard to describe in simple terms.  On the scope when looking at pin 7 of V7B, the plate of the second stage of the 12AX7, I see a bizarre looking sort of pulsed modulation envelope.  It looks similar to a keying waveform in the sense that it has about a 100 Hz basic period with a duty cycle of perhaps 20 percent.  In other words you see a periodic sort of "square wave" that is an RF envelope.

Logically I conclude that whatever's causing this is likely related to or was the root cause of the T3 failure.

But wait there's more.  When I vary the Audio gain control (the mic gain control on the Ranger), at lower drive level settings the pulse oscillation transitions to a full on RF looking envelope.  Again this is on the plate of the second stage of V7.  Man oh man, why can't anything be simple?  I thought I had it licked when I troubleshot and changed out T3.

All help greatly appreciated.

73 N6AF Chuck
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AC2EU
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« Reply #1 on: October 28, 2018, 11:54:17 AM »

have you ever re-capped that rig?
Could be filter caps or bypasses failing.
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N6AF
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Posts: 86




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« Reply #2 on: October 28, 2018, 01:32:27 PM »

Someone definitely went through the Ranger in recent times.  All the coupling and bypass caps are of a new type.  In fact, a bit more history on this particular transmitter.  When I picked it up used several years ago, the modulation percentage was very low, as in maybe 5% modulation envelope.  I troubleshot it back then and discovered that when it was originally built as a kit, the builder accidentally installed the only two potentiometers in the design in swapped positions!  The 1 Meg pot is supposed to be the Audio gain pot while the 100K pot is supposed to be in the keying circuit.  So the builder had placed the 100K pot on the output of the first section of the 12AX7, hopelessly loading down the plate, so voila, a mute Ranger!  When I replaced that pot the rig came alive and worked perfectly up until now.  I suspect that someone had a lot of frustration trying to get the AM working since they clearly had gone through the entire rig replacing all the caps, but they missed the pot error so they never gave it its voice!

So yep, all the caps look quite new.

So the updated status today is that I just replaced the interstage transformer T3 and it is definitely modulating again, so I'm close.  But now there's the strange oscillation issue.  It's so bad that when I back down the Audio mic gain close to zero, where the pulsed oscillation then becomes 100% duty cycle, the MOD current indication pegs the needle.  Not good.  Yep I'd bet a 3-500Z that the root cause of the T3 failure is the gremlin that created the oscillation.  Otherwise that's just too coincidental.  So I'm hoping someone's seen and fixed an oscillation issue in the audio chain.  One more hint - if I lift T3 primary leads out of the circuit the oscillation disappears.  Also if I remove the 12AU7 the oscillation stops.  I checked the 12AX7 and 12AU7 on my Hickok 6000A and both look fine.

N6AF Chuck
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N4MQ
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« Reply #3 on: October 28, 2018, 04:06:49 PM »

That might be the cause of the transformer failure. Woody
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KB2WIG
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« Reply #4 on: October 28, 2018, 04:11:21 PM »



For a guess, check out C 69, the .02 across the mod xformer secondary.

How did you stuff  a  3-500Z into your Ranger?   .... .. 

klc
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N6AF
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« Reply #5 on: October 28, 2018, 05:12:00 PM »

Thanks for the input. I'll check that cap.
I don't have a 3-500z in the Ranger. I went all out and managed to mount an 8877 in the cabinet....
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N6AF
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Posts: 86




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« Reply #6 on: October 28, 2018, 06:34:28 PM »

Well I pulled the .02 C69 cap and all it did was make the Ranger about a quarter ounce lighter; not quite the effect I was looking for....
The cap is not shorted and the rig behaved identically bad with or without it so I'd say that fat .02 uF  1600 WV  cap is not the culprit.  But thanks for the suggestion.  I'm about as desperate as the trumpet player on the Titanic at this point.  I'll try anything.

N6AF Chuck  (Vintage ham dealing with a vintage rig)

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AC2EU
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« Reply #7 on: October 28, 2018, 07:03:04 PM »

Well I pulled the .02 C69 cap and all it did was make the Ranger about a quarter ounce lighter; not quite the effect I was looking for....
The cap is not shorted and the rig behaved identically bad with or without it so I'd say that fat .02 uF  1600 WV  cap is not the culprit.  But thanks for the suggestion.  I'm about as desperate as the trumpet player on the Titanic at this point.  I'll try anything.

N6AF Chuck  (Vintage ham dealing with a vintage rig)



"Shorted" is not the only fail condition! Open ( as in not a capacitor any more) could be a problem too.
Oscillation is often caused by bad filter caps and bypasses that have lost value or have ceased being capacitors altogether.
The fact that the same condition exists without the cap proves nothing!
Your answer about the recapping was a bit vague. Were the DC power filters changed out as well as the smaller caps? 
Usually they are not cans , but have been subbed with axials or radials under the chassis in some way or the can was cut open and re-stuffed.( if it was done recently)
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N6AF
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Posts: 86




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« Reply #8 on: October 28, 2018, 09:34:35 PM »

You're correct regarding the C69 quick check done with an Ohmmeter. I have a capacitance tester that I'll try on it.  Most of the coupling and cathode resistor bypass caps were changed out. C69 is a new looking Sprague that replaced the original paper type. And yes all the power supply filter caps are new looking axial lead ones.
But it does appear to have lots of likely original ceramic dip type caps (values from 100 pF to 0.1 uFd) so it could be one of those. I was hoping to troubleshoot it logically rather than just shot gunning it randomly. Plate and cathode DC voltages are correct and grid DC levels are at 0 Volts relative to chassis ground as they should be. The fact that the oscilation's duty cycle varies depending on the Audio mic pot setting is a hint that I haven't made sense out of yet. Strange because the mic / mic cord isn't connected. The feedback must be getting back into the front end of the audio lineup in the first stage of the 12AX7 since that's the only amp stage preceding the Audio mic pot. I'm 90% sure it's one cap somewhere that's gone bad.
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W9ALD
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« Reply #9 on: October 29, 2018, 04:50:55 PM »

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

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N6AF
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Posts: 86




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« Reply #10 on: October 29, 2018, 06:47:21 PM »

Has anyone measured the LV power supply ripple on their Viking Ranger?  I'm referring to the expected ripple on the "300 V" line right after choke LP2 that filters the output of the LV rectifier, V12.  What value has anyone measured?  Please no theoretical replies or the thread will self-destruct when it turns into a power supply design seminar.  Has anyone actually measured the ripple at that point in their Ranger?
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W9ALD
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« Reply #11 on: October 29, 2018, 11:30:05 PM »

The only person that may have ever measured the ripple voltage at that point on purpose is Manley Youngberg.

If you count the grounds in the audio chain from the microphone jack to the 6146 there are about 20 ground connections any one of which could make a good by-pass capacitor look open in the circuit or a capacitor with a series resistor to ground.

If your ground terminals or tube sockets are riveted the only real fix is drilling the rivet out and securing with machine screws and nuts with star washers.

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AC2EU
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« Reply #12 on: October 30, 2018, 07:16:11 AM »

You're correct regarding the C69 quick check done with an Ohmmeter. I have a capacitance tester that I'll try on it.  Most of the coupling and cathode resistor bypass caps were changed out. C69 is a new looking Sprague that replaced the original paper type. And yes all the power supply filter caps are new looking axial lead ones.
But it does appear to have lots of likely original ceramic dip type caps (values from 100 pF to 0.1 uFd) so it could be one of those. I was hoping to troubleshoot it logically rather than just shot gunning it randomly. Plate and cathode DC voltages are correct and grid DC levels are at 0 Volts relative to chassis ground as they should be. The fact that the oscilation's duty cycle varies depending on the Audio mic pot setting is a hint that I haven't made sense out of yet. Strange because the mic / mic cord isn't connected. The feedback must be getting back into the front end of the audio lineup in the first stage of the 12AX7 since that's the only amp stage preceding the Audio mic pot. I'm 90% sure it's one cap somewhere that's gone bad.

You are barking up the the wrong tree with the ceramics. Those are more likely to be shorted if defective.
You have an oscillation. Bad grounds and/or bypass caps are generally the cause. Check all the voltages you want, but that will not help you. The fact that the gain setting affects the oscillation SCREAMS bypass or filter caps or grounds.
Put  a scope on the AF preamp DC plate supply and you will probably  see the waveform there too!
Forget the DC voltages and focus on what most of us have been telling you.
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N6AF
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Posts: 86




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« Reply #13 on: October 30, 2018, 11:56:20 AM »

Just to re-cap (pun intended), the first and only component I replaced initially was T3 after I'd determined its primary winding was open.  That's when the oscillation business started.  The oscillation is not occurring in either half of the 12AX7 circuits, comprising the first and second audio amps.  Pulling the 12AU7 (audio driver circuit) stopped the oscillation.  The 12AU7 cathode bypass cap was in good shape (checked per EU's suggestion; thanks) so I suspected the P-T156 transformer from Antique Electronic Supply wasn't an exact replacement.  It's supposed to be identical to a Stancor A-53C.  At any rate I noticed that the oscillation also stopped when I opened up the audio feedback loop that uses a winding on the modulation transformer T2.  So I decided to vary the level of feedback by varying R27 and found a point where the oscillation is gone and the audio sounds very good.  So I'm close, if not quite there yet.
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N2DTS
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« Reply #14 on: October 30, 2018, 12:31:44 PM »

Global feedback around multi stages is a bad idea. Even worse when transformers are included because of phase shift.
Often you have to limit the high frequencies in the feedback loop.
Its better to put feedback in each stage if needed.
Say around the driver circuit, and lift cathode bypass caps to provide negative feedback.

Feedback from the mod trans secondary to an early stage almost always causes problems.
Use better transformers (stock is often very crappy) and the response goes out to the supersonic and bad things happen.

Go easy with the feedback,
Do it at each stage (if needed),
Make the circuit need less feedback to start with (better design and components).

I have some home brew modulators that sound great and have no feedback at all.
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