The AM Forum

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.  :)

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.

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!! ;D ;D

                                            The Slab Bacon

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.

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

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

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.

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.

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.

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'.


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.

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

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

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

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

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

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

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. ;)

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.

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

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.

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

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

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.


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.

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

Check for parasitics in the audio circuits, Todd.

I wouldn't recommend a fixed regulated bias supply for the 810's, because the resting plate current will change excessively with small changes in a.c. power line voltage.

Better to substitute a heavier duty bias transformer.  An excellent choice would be a 150 v-a "control transformer".  Those have split primary and secondary windings with taps, much like a "multi-match" modulation transformer, so it is easy to set them up for a secondary at 220 volts each side of midtap, good for better than a half amp. Use a full wave rectifier, or a bridge circuit per your preference, several hundred mfd of filter capacitance, and bleed down the power supply to near the full current capacity of the transformer.  Take the bias off a tap on the bleeder. There should be no more than a few hundred ohms from the bias tap to ground.

That will substantially reduce the internal resistance of the bias supply, but when the line voltage decreases, so will the bias voltage, which will tend to stabilise the resting plate current.

Or else design an electronically regulated bias supply so that the output voltage is controlled proportionally by the a.c. line voltage input.

Todd,
Hey, just cause I am not on phone these days does not mean I have not been on (cw that is).
Right now I am finishing up some unfinished qrp projects,--like my 40 meter cw tranceiver, after
building another regen.
I have just as much "fun" building if not more, than getting on the air,--besides I need a "sabatical"
for a few months.--Man you are one to talk,--you were off for over 10 years, or was it 12?

                                          73, OM, Rene, MVP

P.S,--and after I finish my qrp rigs,--guess what, I got a bunch of parts, and cabinet to start my "big linear" for 160,--gee I might be off the air almost as long as you were,--(ye gads).
 
                   

QRP? Life is too short for QRP. At least from an AM perspective, on 75-80m, at night.  :)

Being off the air is like punishing yourself. I don't recommend that for anyone who enjoys operating. I actually brought my logbook to work today to do some QSLing which makes it easy to check back. Looks like my last AM contact before the station went dark was 3/10 of (I think) 1995 with Jerry, K1GUP. Worked W2VJZ the day before. I got back on 3/12/06 (not the 21st), so that makes it eleven years almost to the day. Too long!

Just remember, those rigs are only radios if you use them. Otherwise they're just interesting doorstops. Yes, I've used some for that. ;)

Quick update: Didn't have a lot of time last night, but I checked the bias on the 6B4G drivers and the resting current on the 810s. -60V on the drivers, 50ma on the 810s, right where it should be. I did not check anything under transmit load because of the time needed to do it properly and safely. Maybe tonight. But the drivers both got quite hot just sitting there with no load, which must be normal for 6B4s, 6V6s, etc. The line voltage meter was definitely wandering the evening of the crap out, from around 117 to 122. Usually it hangs right on 120.

Bruce: The mod transformer does indeed have spark gaps on it, set where they are supposed to be set. This was checked last April when W1UJR was visiting. Joe's transmitter was a mess when he got it from Columbia. Parts missing, holes gouged into it, broken glass, blown iron.....as a result, his is the only complete and accurate restoration I'm aware of. Been trying to catch up with him this week to get Deerfield plans set, no luck so far. He's gonna smack me for breaking the transmitter again.

John: check for a pair of what in the audio circuits?  ;)

Well Todd,
Resting current right sounds like the iron has a shorted turn. Me being a bit of a nut in your shoes. I would find what size wire is inside the transformer then find the thickness of the insulation paper. I would rewind it with kapton insulation and see if I could increase the wire size due to thinner insulation. This would keep the inductance the same but reduce the resistance. Kapton is good for 7000 volts per mil so 2 or 3 mils thick would do it. Paper insulation could be 10 mills thick.
I would also buy a spool of the best wire I could get.
Not being crazy you could send it to Texas Pete D. Joe PJP will not allow you to use different iron in a KW1. (or be tried for treason )

Gary WZ1M rewinds and is closer and cheaper than Dahl, so I'll have him rewind it for me. No idea what he uses for materials, but folks are happy with his work and prices. I'll include your notes when I email him.

Joe won't beat me up too badly as long as I don't blast holes in the rig. ;) I'm of pretty much the same mindset (use it as a 1950s transmitter, don't try to make it a 2007 model), but don't mind doing the mild mods that are reversible if they won't let me be buried in it and someone else ends up getting it. At the current rate, it may deserve to be retired and admired after another few decades. Meantime, I fully intend to run the crap out of it. Looks like I'm succeeding?

Frank,

I think Joe did send the bad mod xfmr from his KW-1 to Peter Dahl for a rewind. But if I recall correctly, although the rewind worked OK, the xfmr talked so much it was unusable. I think he had to send it back to Dahl one or two more times before they assembled that thing tight enough so that it stopped singing.

73,

Bruce

Yes, I seem to remember the same story. I designed transformers years ago and it is all about potting. A mod transformer I would pot with a vacuum pulled on it so all the gaps get filled. I would also make sure the last center E was well clamped. Today wire insulation is much better and insulation is thinner. I've used wire you can melt the coating off with a soldering iron. It took a solder pot set pretty high. 

Todd et. al.
I was thinking about this last night, while cooling my heels in a bed and breakfast here in South Jersey... like trying to solve a mystery.

It seems to me that if there is a problem in the modulation transformer, the observed symptoms (fuse doesn't blow, resting current ok, higher than normal modulator plate current under modulation, very low audio) would not be caused by a moderate number of shorted turns in the modulation transformer. A moderate number of shorted turns in the primary would result in higher than normal modulator plate current under modulation, probably some imbalance of the class AB modulator... but not such a dramatic drop in the modulation at the output of the KW-1.

Check the following (with appropriate safety precautions)

1. Is there a short between the primary and the secondary of the modulation transformer?

2. Is the blocking capacitor between the plates of the 4-250's and the tank circuit shorted out?  [I don't have a schematic with me, but depending upon the details of how the tank circuit is designed, a shorted blocking capacitor could lead to a very low audio frequency load between the plates of the 4-250's and ground, without producing a DC short to ground.]

Separately, since the secondary of the driver transformer normally looks into the very high impedance associated with the grids of the 810's, you could remove the 810's, leave the plate supply off, and measure the output of the driver transformer with audio supplied... to see if it looks reasonable.
 

When I got to office I use here in South Jersey, I downloaded the KW-1 schematic from the CCA web site:

Updates on my last post:

I looks like a short in the 4-250 (x2) plate blocking capacitor would also cause a DC short to ground, via L117, if L117 is still in place and not open.

There are four (4) ways to place a short across the modulation transformer secondary:

1. The rear shorting switch S502
2. Either of the 6000 uF capacitors that are part of the high-level splatter filter (if they haven't been removed... which would normally be done if the splatter choke has been bypassed)
3. The AM/CW relay. NOTE: this relay is normally in the CW position, and is moved to the AM position (removing the short across the secondary of the modulation transfromer) when current flows through its coil. If the relay contacts were sticking, or it the relay coil was open, or if the AM/CW switch contacts weren't working properly, then the short across the secondary of the modulation transformer would remain in place.

Best regards
Stu

Stu,
A shorted turn in the mod transformer would not effect the final except to reduce the modulation swing. It would reflect a low Z back to the modulator tubes if the short was on either side. A shorted turn would suck up the AC swing reducing modulation. Primary to secondary short would cause some weird distortion I would think.
This would be the least common fault because usually there are a numbers of layers of insulating material between windings.
Anything is possible at that age though.
Might be time to take it out and connect it to a variac and series bulb as Steve suggested.

FB Stu -

I actually had a bad blocking cap years ago, it would blow out one of the 15a screw-in fuses anytime I tried to apply HV. Once it was replaced, I could get on to the other issues, like swapping out the 872s for 4B32s.

The AM/CW relay also lights the 810s, and only switches in upon power up, unless you change to CW and back. If time permits, I plan to give it a thorough cleaning this weekend anyhow, while I'm in there. But it has no problem latching and holding, so any issue would have to be related to a specific contact.

Since recapping and repairing the audio deck, I've had to keep the audio gain down even lower, so I'm still a bit surprised at the likelihood of a shorted transformer. It's what happens when you run break-in up in the ghetto. ;)

I'm going to do some testing this weekend if I get all the house chores done as planned. Several stations did report that my audio was weak and distorted when I tested two night ago. Al VE3AJM compared it to the sound from close in stations when the band starts to go long, that warbly, underwater sound. The meter jumps around a lot more than normal too, as if the gain is cranked waaaay up. If the transformer was shorted to ground, I'd expect no audio and fuses to blow.

Hi Todd, 

Very sorry to hear about the rig crash!  I know how it feels.  :-(
When the  mod transformer in my 30K-4 failed......the audio level went down and became very distorted.  The mod current, on average, appeared to be a little high, but the mod idle current was near normal.

I tested the transformer by comparing the winding resistance measurements to what I had recorded when I originally got the rig and had been doing some troubleshooting.  I measured the primary resistance and found that one side of the winding had a reduced resistance reading (went down about 20 ohms or so).  I then tested the transformer with a small AC (filament) voltage on the secondary and looked at the voltage across the primary.  I found quite a voltage imbalance in the readings between one side of the winding and the CT and the other side and CT.  Probably a 30% difference.  This led me to believe that one side of the primary had shorted turns...probably between layers of the winding.  I believe this is a common failure mode in those old mod xfmrs and it is hard to be sure of, without the transformer actually being tested under working conditions (with HV applied).

A megger is also useful in looking for insulation breakdown between the windings or from the windings to the core.  This does allow you to look at it with a higher potential applied, but is not very useful for looking at the winding itself.  Using an RCL meter to look at the inductance of the windings, might also be a feasible approach, but would probably require some before and after failure readings to be certain of what you are really looking at.

I highly recommend Peter Dahl for replacement mod iron, but as you know, it is a rather expensive solution.  My 30K-4 now sports a new Peter Dahl "Heavy Duty" mod iron and it works quite nicely.  :)

73,  Jack, W9GT

Todd,

Another approach is to hook an audio signal generator to one side and a scope to the other.  If you have shorted turns, the AF generator is going to have difficulty building up voltage across the transformer and of course you can hook the generator to the secondary side and see if you have equal amplitude on both halves of the primary.  I made a copy of a great little article on this process, of course now I cannot find it.  I should digitize everything and put on the computer so it is harder to lose one thing (but easier to lose everything at once if you don't have good backups!).

Rodger WQ9E

Double Crap - I just nuked a pair of 6LQ6 output tubes or I think I did.  I know less than zippo about tubes.  They're not drawing any cathode current, but the tubes are lit up with 900 Volts on the caps, 300V on the what-ever-it's-called grid and about -60V on the control grid.  Even with 0V bias on the control grid I get zippo current. They're toast, right?  Or, how can I be sure?

How does a tube fail? I just looks like a bunch of glass and iron to me.

I would expect some current with 900V plate and 300V screen and 0V control grid, even from very badly abused tubes - but zero current says something else failed, maybe the metering shunt and the meter, or the line to the meter, etc.  Check for continuity from the cathodes to ground, then try removing the tubes and see if you can get some current reading by placing a power resistor of maybe 10K from the plate circuit to the cathode circuit.  Then make sure the voltages are applied to the right pins of the tubes, etc. 

Rodger, I'd sure like to see that article if you come across it again. Unfortunately a lot of my test equipment is already in storage (some never came out from the last move), so I'm limited in what I can do.

Alas, no time this weekend to do any radio work beyond getting the HRO-5C assembled in its rack and out of the way.

Did exchange email with Gary, WZ1M. He is standing by to receive at least one zorched transformer, maybe two if the spare in the Viking KW is also bad.

More to come, as time permits. Many thanks for the great suggestions and commiseration.  ;)

Hi Todd,

I haven't found the article yet but here is a link to a website that provides a simple test method using a scope:  http://www.repairfaq.org/REPAIR/F_flytest.html#FLYTEST_016  Although the article in general is structured towards flyback testing the "ring" test will work great for AF and power transformers.  If your scope has a "gate out"  you can probably use this for the pulse source.

Second thought, if you have "zorched" the transformers in the KW-1 AND the Desk KW maybe you need a priest and some holy water in addition to a transformer rewinder.  Or do you by chance have some CB genes buried deep somewhere in your family tree???

I spent the weekend turning $600 worth of wood into a swing and slide set for my daughter instead of finishing my Viking 500 restoration.   I hope next weekend I don't turn several hundred dollars worth of Viking 500 into smoke!

73, Rodger WQ9E

Be careful runing a flyback test. You and or the signal source can get belted.