Yet *Another* Crapout

[Todd, KA1KAQ]

It happened last night whilst talking with W1LYD, W1IA, 'QIX, 'KBW and a few others up in the ghetto.

Was having no problems or anything to indicate such, when the audio went away. Oddly, the modulation current meter went from peaking down in the 100-200 ma range to bouncing up to full scale (500ma) with wild swings. I first checked the audio gain, thinking I'd bumped it up by mistake, then realized that no one was hearing me anymore, only my carrier.

Did a quick check of fuses and obvious things, all fuses were good and tubes were lit. Tried again, this time Brent could tell that it was me because the band was quieter, but still virtually no audio. He could tell it was me, that was it.

I suspected the driver transformer that worried me before, but it felt stone cold. No heating at all, not that this means anything. Tonight I'll dig into it more to see if it's an easy fix like a crapped out tube. It's seldom, if ever that easy for me. The strange thing is that, unlike last time when the mod current meter went numb, this time it's jumping around like a field mouse on crack. Looks like there should be plenty-o-audio, yet almost none is making it out.

Weird. Maybe this time the mod tranny ist kaput? Nein! Nein!

BTW, the last 'loss of audio' issue seems to have been caused by a grid to cathode short in one of the 6B4G audio driver tubes that didn't show on the tube tester beyond testing really, really high. None of the replaced caps seems to have been a big problem, but I'm still glad they're out of there. At least I'm getting familiar with the transmitter again. 

[Bacon, WA3WDR]

That sounds like a shorted modulation transformer.  The modulators will draw more current with a low-low impedance load.    Hopefully some capacitor across the primary or secondary shorted, and the short is not in the transformer itself.

Is it humid where the transmitter is?  I worry about humidity in higher voltage things like power and modulation transformers, and power chokes.

[The Slab Bacon]

See, its that Collins crap!! throw that nasty KW-1 away, just hand it to the junk man and get rid of it. Get yo self something nice like a nice Johnson Valiant or V-2 and you'll live happily ever after!!

                                            The Slab Bacon

[Todd, KA1KAQ]

That sounds like a shorted modulation transformer.  The modulators will draw more current with a low-low impedance load.    Hopefully some capacitor across the primary or secondary shorted, and the short is not in the transformer itself.

Is it humid where the transmitter is?  I worry about humidity in higher voltage things like power and modulation transformers, and power chokes.

Quite dry, actually. The woodstove is just around the corner, and despite having kettles of water on it, the house remains very dry all winter.

I had a sneaking suspsicion it could be the mod iron because pretty much everything else in line seems to be working up to that point and caps have been replaced. It's also the Achilles heal of the transmitter, so in the back of my mind it's always been a possibility.

The schematic is online here, page 7:

http://www.collinsradio.org/Manuals/KW-1(01-52)/KW-1-(01-52)Sec7,ant-ref.pdf


There's nothing between the modulators and transformer to crap out. I suppose one of the 810s could have crapped out, but both would seem highly unlikely.

[Todd, KA1KAQ]

See, its that Collins crap!! throw that nasty KW-1 away, just hand it to the junk man and get rid of it. Get yo self something nice like a nice Johnson Valiant or V-2 and you'll live happily ever after!!

But Frank -  I have a (32)V-2, ran it for years. Decided I wanted to be heard instead.

[The Slab Bacon]

Yea, but, Todd, Irb ran a lowly DX-ahundred for how many years?
And everybody could hear him (whether they wanted to or not)
Hmmmm................................

[k4kyv]

If you are lucky, it could be C503 or C504.  But KW-1's are notorious for blowing modulation transformers, and that sounds exactly like what happened.

If you still have the splatter filter in place, that is likely the culprit.  The Johnson Desk Kilowatt, designed by the same engineer, uses the same tube lineup and exact same mod iron as the KW-1, but lacks the splatter filter.  You rarely hear of someone losing the mod xfmr in the Johnson.  The filter causes extremely high audio voltage peaks to occur whenever a strong audio signal  component occurs around the upper limit of the filter cutoff frequency.

If you previously removed the filter, the insulation in the transformer could have already been scorched from previous voltage peaks with the filter in place.

A good replacement would be to shoe-horn a Gates BC1-T mod xfmr and reactor in place of the stock xfmr (if you can tolerate the Gates reactor talk-back).

But get rid of that splatter filter if you have not already disconnected it.

[The Slab Bacon]

And / or try checking your modder biass supply. When we were talking (I think it was friday night) You did momentarily loose audio a couple of times for a few seconds at a time.

[Bacon, WA3WDR]

I think that an intermittent is a actually a good sign.  I wouldn't expect a shorted transformer to unshort without major arcing and zorching.  But for sure remove the splatter choke circuitry.

[Todd, KA1KAQ]

There were plenty of times I couldn't hear Irb up this way, Frank. Generally late afternoon/evenings. Sometimes I'd hear him down in the noise. Of course, he was more conveniently-located for you folks down there.   I'm a ways further north, and I was quite often referred to as 'pissweak' when I tried to join in on 75 at night. If the band is quiet, sure. Otherwise... to borrow Don's phrase, you need the ability to 'turn up the wick'.

And / or try checking your modder biass supply. When we were talking (I think it was friday night) You did momentarily loose audio a couple of times for a few seconds at a time.

Yes, I dumped the overload relay a couple times because I didn't have it dialed in right, having just moved up from 3724. I did check the bias before shutting down last night, it was right on 50ma.

One of the first things I did to the transmitter was to remove the clipper and splatter choke, upon the advice of Uncle Ed, WA3PUN. Clippers piss people off and the splatter choke is famous for killing mod iron, as you said Don. So I don't think there's much doubt about it, I just wanted to see what others might think. I tend to miss the more obvious things while searching for something more complicated. 

John Foster was the guy who designed the KW-1 and the Viking Kilowatt, which may be why they bear such a strong resemblance. He was smart to leave the splatter choke out of the Johnson rig.

It's only a mod transformer. If it's zorched, I'll get it rewound. There's a Desk sitting across the room I can swipe the mod iron out of, but since it was missing the 810s when I got it, there's a possibility that it's bad too. I've actually been looking for a VM-5 for a while now, even advertised here for one, thinking it would be good to swap it out before it craps out. Considering the rest of the transmitter, the stock mod transformer seems too small, like it would be out on the edge. Of course, I ran it at 12,000 p-p too, instead of the more moderate 18,000. And yes, it already has/had talkback. 

What's the best way to test the windings other than for shorts to ground? I have no megger. Would be good to test the Viking iron before making a swap.

[The Slab Bacon]

I usually put 120vac across the full secondary (or primary if needed) and take a voltmeter to the segments of the primary (or secondary)and look for symetry. If they are not symetrical in voltage, you most likely have some shorted turns. This will also give you a good feel for the turns ratio of the transfoma.
                                                The Slab Bacon

[Todd, KA1KAQ]

Thanks Frank. I'll play with it some tonight and see what I come up with.

[WA1GFZ]

Gee Todd,
I had exactly the same failure in the V2-CDC Saturday while on 75. I had a bias filter cap open taking it from a cap input filter to a choke input filter. This dropped the bias supply from 89 volts to 60 volts. My modulation resting current went from 40 ma to 120 ma. Also was pulling 300 ma peak on the 807s. An 807 is a heck of a tube. The plates didn't even turn red. Check your modulation resting current for a clue.
My fault gave me a reason to rebuild the bias circuit. fc

[k7yoo]

John Foster spearheaded the KW 1 & Viking 500 projects but had nothing to do with the Desk KW. This is based on a personal conversation with him approx 15 years ago.
I have several mod transformers that will work in your KW-1 if you really get stuck and decide not to call Peter Dahl. I like Don's Gates idea.
The problem is getting something to fit unless you are willing to cut the vertical panel off that mounts the chokes. (I did !!) A CVM-5 works FB (this is what I used) and is much stouter than the original CMS 2. You could also mount stuff outboard--which I personally detest. Call me if yoou get stuck
Skip

[steve_qix]

Frank (slab b.) mentioned using 120 VAC to test the mod transformer.  This is generally accepted EXCEPT *DO* put a lamp or lightbulb or other resistance in series with the 120VAC and the transformer.  Otherwise, if there IS a short, you risk consuming huge amounts of power, and other undesirable (and potentially dangerous) consequences.

With the 120 VAC applied, the series lightbulb should glow dimly or not at all.  If you short the other winding (depending on whether the power is applied to the primary or secondary), the bulb will (or should) glow brightly.  If this happens, the transformer is most likely good.  You'll also have to test for primary to secondary shorts, and shorts to the case.

Hope it's not the transformer, although the symptoms do point strongly in that direction 

Regards,

Steve

[AB2EZ]

Todd

Some suggestions:

If the primary or secondary of the mod transformer were arcing to ground (or shorted to ground) then you would expect to blow one of the Edison fuses that are on the primary of the HV transfromer. [As an aside, I put in a step start circuit on my KW-1, and I also put a fast blow fuse in series with the primary of the HV transformer that is just large enough to handle normal conditions... including the initial surge of about 8 amps of current, that lasts for about 1 cycle of the 60Hz AC. The step start relay bypasses the dropping resistor after about 10 cycles of the 60 Hz AC. Better to blow an $0.89 fuse than a transformer]

I suggest that you see if the 6B4G audio driver circuit is acting up again, or if (perhaps) the audio driver transformer is arcing over when you talk (not very likely, but possible)... thus putting a large positive voltage on one of the 810 grids. The 810 grid voltage isn't that large, and the grid caps are readily accessible... so it is feasible to monitor the 810 grid voltage waveform with a scope (as always... be careful). If the 810 grid voltage is jumping all over the place when you talk, then that would suggest that the problem is not in the modulation transformer.

Best regards
Stu

[Todd, KA1KAQ]

Yep, it's not looking good, Steve. I do have a porcelain bulb fixture in the radio room for test purposes, so I'll be sure to use it.

Didn't have time to do much last night beyond swapping the 810s and a quick on-air test with some nearby ears. First Herb VH and Bill DUQ, Herb could make me out but said it sounded like it was cutting out and was distorted. Bill did his impression of the sound, but the band was crappy so I didn't make it out. Backing the gain down even more helped some, but not a lot.

Moved down to 3725 with Al VE3AJM who comes in here great and generally heres me the same. His description was that my audio sounded like a station close in when the band starts to go out, underwater and so on. He said a couple of times it sounded like it would straighten out, but didn't. Ed, VA3ES thought that if I was getting any audio out, it was a good sign the transformer wasn't blown. Mod cathode current meter still swings around a lot more than normal.

But more and more it's sounding like shorted turns. Some audio, but very low. Distorted, warbly, underwater-sounding. It was mentioned by Herb and others that my audio was cutting out, so That may have been what Slab was hearing afterall last Friday (tho I did dump the overload reset a coupla times).

Maybe the D-104 head just suddenly went bad.

[Todd, KA1KAQ]

I suggest that you see if the 6B4G audio driver circuit is acting up again, or if (perhaps) the audio driver transformer is arcing over when you talk (not very likely, but possible)... thus putting a large positive voltage on one of the 810 grids. The 810 grid voltage isn't that large, and the grid caps are readily accessible... so it is feasible to monitor the 810 grid voltage waveform with a scope (as always... be careful). If the 810 grid voltage is jumping all over the place when you talk, then that would suggest that the problem is not in the modulation transformer.

Thanks Stu, guess we were typing at the same time. I did notice that the driver tubes were really hot again last night, but the driver transformer was stone cold. That's as far as I got with it, hopefully I'll have some time this weekend to get into it more.

[AB2EZ]

Todd

If the driver tubes were "really hot" , then that is good news ... less likelihood that the problem is in the mod transformer. Things to look for:

As suggested last time, the problem may be in the biasing circuitry. As you know, Collins' engineers decided that a good place to save money (on a rig targeted toward CEOs who happened to be ham radio hobbyists) was to use a pair of potentiometers in series with some fixed resistors to obtain grid bias for both the 810's and the 6B4Gs. One can imagine all sorts of strange phenomena occuring if one of the potentiometers became "noisy", or if the -150 volt bias supply developed a problem that limited its ability to deliver current, etc. Some of these problems could even involve positive feedback interactions between the 6B4G stage and the 810 stage. Remember that the grids of the tubes conduct in only one direction... and therefore, when you drive the grids positive on modulation, you have to draw current from the bias supply... even though the modulating waveform has no average value. The bias supply (when it is working) uses a resistor chain with a total resistance of about 1000 ohms. So, with a few milliamperes of average grid current when applying audio... the (slowly varying) change in bias voltage is only a few volts. However, if something goes wrong with the -150 volt bias supply, or one of the resistors in the chain... leading to a significant increase in resistance... then as soon as you start to modulate, the tubes will draw grid current... which will drop the bias voltage (because of the high resistance)... which will lead to more grid current... which will also lead to more plate current... which will lead to hot tubes, and improper operation of the audio stages.

Stu

[WA1GFZ]

Yes Stu that KW1 bias circuit is a work of art.......like watching a monkey with a 4 inch brush.
I downloaded the manual the last time Todd had a problem and was very surprised. Looks like a good place to make changes and add some power zeners.
I wonder how many mod transformers bit the dust because of that J.S. circuit.

[AB2EZ]

Frank

Yes...

I built a separate -62 volt bias bias supply for the 810's in a small box that sits on the bottom of the cabinet. Since the required bias current is only a few milliamps (during modulation, when the grids are driven positive with respect to the cathodes) ... it was easy to construct a circuit from a small, 48 VAC transformer (48 volts x 1.414 ~ 68 volts). I used five 12-volt Zener diodes and a few forward-biased 1N4007s to get the bias voltage I needed (plus one 1N4007 before the input-filtering capacitor, to perform the required rectification). As long as the Zeners stay in breakdown at the maximum average current draw from the supply during modulation (I assumed that this is certainly less than 10 ma), the output voltage remains nearly constant. This implies that the series dropping resistor between the Zener diode chain and the input-filtering capacitor has to be less than (roughly) [68 volts - 0.7 volts - 62 volts]/ 10 ma = 500 ohms. The power consumption of the supply is around  68 volts x 10 ma = 0.68 watts.

I didn't build a separate grid bias supply for the 6B4G's.

All I had to do is remove the existing grid bias from the 810's, by unsoldering one wire... and attach the new bias supply in its place. I adjusted the new bias to get the resting current I wanted in the 810s by adding or removing one or two 1N4007 diodes from the Zener diode string. The voltage drop across the 1N4007's is a small portion of the total voltage drop across the Zener diode string... and it doesn't change much as the current being drawn from the bias supply (and away from the Zener diode string) changes from zero to a few milliamperes.

Stu

[K1MVP]

See, its that Collins crap!! throw that nasty KW-1 away, just hand it to the junk man and get rid of it. Get yo self something nice like a nice Johnson Valiant or V-2 and you'll live happily ever after!!

But Frank -  I have a (32)V-2, ran it for years. Decided I wanted to be heard instead.

Todd,
But if you got a "Cadillac" thats in the shop all the time,--would it not be better to have a Chevy or
Ford you can depend on.
                                                 73, K1MVP

[Todd, KA1KAQ]

But if you got a "Cadillac" thats in the shop all the time,--would it not be better to have a Chevy or
Ford you can depend on.

This from the guy who builds nice stuff but is never on the air. I think Ron needs to come over there and light a fire under you.

It's more like a Rolls Royce with respect to price, Rene. Quality is pretty good, too. I hear plenty of folks 'fixing' their Rangers, Valiants, DX-100s and so on. Keep in mind that I use it regularly and I don't mollycoddle it. It's an old transmitter that wasn't used much for decades, and being impatient to get it back on the air, I didn't go through it completely. No fault of the transmitter, it's higher quality than any other piece of amateur gear I've seen. Blame it on 'operator error'. The ratio of downtime to on air time I've gotten out of it since March 12th '06 is actually pretty good, when you consider how often I'm on. Besides, all the Chevys and Fords need work too.

John Foster spearheaded the KW 1 & Viking 500 projects but had nothing to do with the Desk KW. This is based on a personal conversation with him approx 15 years ago.

Thanks Skip! Seems I read it in ER or somewhere else and have heard it from a number of other sources, and considering the similarity between the two, I assumed it to be so. But knowing of your experience with John and both rigs, I appreciate the correction.

I was thinking along the lines of a CVM-5 also. Ed even listed it as a recommendation years ago when he sent me the audio documentation. Even if this ends up being something else, I'm still hoping to locate one.

Old or new, big or small, if you use it enough, it will eventually break.

[W2XR]

Hi Todd,

I seem to recall the modulation xfmr in the KW-1 that Joe/WA2PJP bought a number of years ago was blown by the previous owner.

Perhaps comparing notes with him as to what he saw when he first fired that rig up vs. what you are seeing will shed some additional light on the nature of this failure.

IMHO, sounds to me like the modulation xfmr blew, based upon the high modulator plate current excursions and low percentage of modulation. Hopefully, I'm wrong.

This is one situation where protecting the primary and secondary windings of the mod xfmr with spark gaps, and providing audio peak limiting to prevent overmodulating the xmtr in the negative direction, can minimize the possibility of modulation transformer failure. It is also good practice to float the xfmr above chassis ground.

Good luck and let's hope for the best outcome here.

73,

Bruce

[w3jn]

Check for parasitics in the audio circuits, Todd.