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Author Topic: Valiant load caps question  (Read 12873 times)
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WQ9E
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« Reply #25 on: February 15, 2013, 09:16:34 PM »

Replace the 10uf bypass cap on the audio driver and see if that helps.
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Rodger WQ9E
w8fax
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« Reply #26 on: February 16, 2013, 12:12:52 PM »

OK..I replaced the cathode cap (10 mfd). Had to go get one from the trusty surplus store. It made no change at all. I guess next is to look at voltages on the tubes and transformer? Ohms check out seemed to be OK, some up a little and some down a little. I get modulation but have to have mod control up pretty hi and the audio is very "raspy" for a word. The scope is a weird shape also and as best I can tell only reaches about 50% modulation or so. Increasing the audio swings the mod current way too high, as expected, but does not help the raspyness. Is it possible that the driver or mod transformer is defective?? It seems if one or the other was open or shorted that I would see it on the meter or scope, but dont know myself....also rechecked the bias on the modulator...OK. Swapped tubes again, including the rectifiers in the low voltage section, as these are the only tubes left that have not been replaced. Also tried another set of 6146's in the mod, but no change there either. Al/W8FAX
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WQ9E
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« Reply #27 on: February 16, 2013, 12:22:03 PM »

Check to make sure the clipper control isn't the problem, that raspy sound would happen if the clipper circuit is operating at full clipping.

Using your scope trace through the audio section and see where the problem starts.

Don't forget that the metering resistor (particularly in a kit built Valiant) could be way off so measure the actual cathode current of the modulator tubes with your DMM.
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Rodger WQ9E
w8fax
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« Reply #28 on: February 16, 2013, 08:45:35 PM »

Alright...the score so far. I injected a 1000 hz sig at the mike input and started the checking with my scope. I found one cap that had sig on one side, but weird on the otherside, but still passing, just reshaping somehow??? Cap # C83. I was getting a nice sig out of the 12AX7, but further down the line it went to crap. After I changed that little cap it cleaned up. However, the sig coming out of the mod xfrmr was not good either. What I found was that it would be good at first when turning on B+, but would soon go to crap. The 22 ohm resistor in one of the 6146 mod tubes was heating up and changing value rapidly. I just caught this by chance because when it was cool it checked OK. When I replaced the HV wire to the caps (due to arcing underneath on a term strip) I had pushed some heat shrink over it. It got hot enuff to shrink the shrink. I did not have another 22 ohm resistor, but I did have a 32. I put it in there and guess what. The mod sig output on the scope cleaned right up. Applying voice looked good also. I got a quick check from W9T on 80meters and he said it sounded good. The audio input knob still seems a little high to me, but that is only compared to the other valiant I have. I hope that this should at least get things under control for this rig. I still have to open the VFO and see if the meltdown resistor has been changed. Also...the keying waveform looks good, and the leading edge is not real sharp. I also think I will start a search for door knobs to replace the layered mica in there. I really want to thank everyone who pitched in help and comments on this project. When I first brought it up and saw all the arcing and sparking I never thought it would ever work...but it does. Plus I learned some stuff too......Al/W8FAX
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w8fax
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« Reply #29 on: February 16, 2013, 09:01:35 PM »

One other comment...the cathode cap that was suggested to get changed may have been bad, but changing it showed no change since the problem(s) were elsewhere...prolly a sum total thing??...lol...Al/W8FAX
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w8fax
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« Reply #30 on: February 23, 2013, 06:21:28 AM »

Well, I have been on the air now quite a bit, and so far so good. Good audio reports and no drifting or chirping on CW, no arcs and sparks or smoke. I rcvd and appreciate the audio recording from Jim, KJ9T. I felt it was pretty much done until I can round up some caps to replace the band caps and layered mica cap, so I put it back in it's cabinet. A sure sign of optimism because that is not fun either. Thanks for the help everyone, now on to the next Valiant that I have.
One question....I have seen pros and cons about this, but what is the consensus these days of removing all the little coils that were meant to be TVI traps I think, and just wire up straight??? I do not see much use for them now. Thanks again....Al/W8FAX
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wa3dsp
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WWW
« Reply #31 on: February 23, 2013, 02:01:26 PM »

Al,

  Unless you are a purist I would remove them. There are several reasons why this is a good idea.

1. They are not needed in today's RF environment.
2. It is questionable that they even did what they were intended to do.
3. Other rigs of the era, like Collins, used simple capacitive filtering.
4. There have been stories of the open enameled wire chokes shorting and causing damage.
5. Johnson went way overboard putting these LC filters on pilot lights, even the lights that
went through the long tube to the jewel on the front panel. 

See the September 2011 ER page 28 Ranger article for an analysis of this and a better replacement should you decide to not completely eliminate them. I used the ferrite LC suggested there on the AC line, Mic PTT, and CW key lines as well as the meter leads in my Ranger. All others I completely eliminated. Even that was probably overkill.

The recommended replacement is an Amidon FB-776301 with 2 turns of Teflon and a .1 cap.  I used more turns  (like 6-8) and a smaller cap except on the AC line.

Doug
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