The AM Forum

If you've been listening around 3880 +/- and heard me on the air, you'd likely have heard me have an intermittent carp out from time to time. I usually run the  rig that KE1AV modified many years ago, it's Valiant II, with 6DQ5 modulators.

Anyhow what would happen is that I'd key up and the plate current would DROP like a stone down to about 100mA and then it would not like to load up. Keying and unkeying would sometimes "bring it back". Twiddling the FINAL LOAD dial seemed to bring it back too. Also it seemed to me like maybe it was due to carbonized contacts on the Dowkey ANT relay. Or maybe arc'd contacts on the HV relay in the Valiant.

When it first started happening I did clean the Dowkey contacts and that seemed to help. A few days ago I thought it was that, and changed out the Dowkey - got to rebuild some hamfest acquired Dowkeys. Don't ask what was up with them, doodling with them is a topic in itself.

Then finally two days ago, it just CARP'D OUT, and would not "come back".

I tried all the positions of the Fine Loading switch, doodled everything else, including the bandswitch. I was convinced when it went down that it was a leaky/arc'd HV cap.

So earlier today I spent time looking at all the caps that are switched in and out.

I'm pretty darn sure that I found the problem in the course of all of that. To be upfront, I actually did not CHECK to see that the problem that I found IS going to make it run properly, but I'm pretty confident, and I will be banging my head on something hard if it isn't what I just found! Did I write "confident" I meant very foolish.

Ok, so what was it??

Two clues:
- the problem that I found islimited to the parts shown in the schematic I've posted
- I do run the Valiant, usually without the case on it at all.

I've got pix and an explanation that I'll post up as soon as I get the pix off the camera and through Photoshop. Or, if no one hears me on the air tomorrow at all, I'll 'fess up and still post the pix, because what I found is/was definitely NG, and pretty interesting too... in a moderate way.

So in the interim, feel free to jump in and who knows, maybe someone will hit the proverbial nail on the head! :D

               _-_-

hi Bear ... I'm sure you've rounded up the usual suspects but here goes anyway ....

1. loading caps have a poor reliability history as well as plate tank switching

2. no safety rf choke on rf output ... add a 1 uH or larger without fail ... coupling cap failure drives this !

3. is there any way to monitor 6146 screen voltage during operation ?  looking for +180V or so

there is more but I am enjoying your literary bent on problem description ... bon chance, mes ami

Not having the KE1AV mod schematics puts one at a disadvantage, but my experience with stock Valiants is that L12, the 4.7 uH "pre-" choke, also an L11 fuse, forms a dark band before carping out and then becomes intermittent as it opens then heals temporarily.


Phil - AC0OB

The finals are as pictured... the mods are in the audio section only...

Phil, ur pretty warm... very close.
The clues I gave will give Mr. Sherlock Holmes all the information he needs...

I was about to suggest looking at R13, the clamper adjust pot, but if you're looking in the vicinity of the above mentioned choke, I dunno...

Fine bunch of detectives!  ::)

I'm running around today like a chicken on c---k, so still not on the photoshop work, I'll dispel the great mystery as soon as I can post the images (yes, another clue!) which will make it all immediately obvious and self-evident!

Hint, put urself in the position of being the person (me), who has done what he has done; thus the clues!  ;)  Ok, to clarify a bit and narrow it down... I had to DO something to cause the fault!

                    _-_-

1. Leaky or loose C37;

2. solder splatter on the plates of  C8. (Don't ask me how I know that one  ;D).

Phil - AC0OB

Excellent work Phil, but your conclusions do not take into account the clues!!
(although the solder splatter might...)

Is it possible you accidentally tried transmitting with the antenna load switched to a different rig, thus damaging one or more of the fixed loading capacitors?  If the Pi net coupler is working into a dead short, the plate current will still dip, but the rig will not transfer any power into the load, thus the fixed plate current. 

Sure, quite possible. But as I mentioned, the plate current would drop to 1/2 and sometimes come back up, and the power out would drop precipitously when the current dropped. It was not possible to load it higher once the levels dropped... sometimes keying it repeatedly would seem to bring it back.

The problem actually started showing up when I moved the rig in the late fall from my lab building to the "main" building, aka the winter operating position...

            _-_-bear

A couple of possibilities:

1.) You pulled out an Oc3 and in doing so the base to glass seal broke, and the envelope rotated till the wires in the base touched, lowering the screen voltage by half. Not seeing more of the schematic, I bet them Oc3's are for the modulator screens. oh well.


2.) The final RF output is at 50% because something is arcing out; possibly on the loading switch where the arc is not to ground, but to the next loading cap in line, hence the reduced plate current. If this is true, perhaps you did something like trying to clean the rotary switch with an eraser.

I am good on Pay Pal, so you can send me my prize by that method.  ::)

Jim
Wd5JKO

My guess is a loose ground connection on the loading caps.

What clues? ???

An actual picture of this area might help.

BTW, I also take PayPal. ;D

Phil - AC0OB

Assuming the pi network starts out tuned to resonance, a drop in plate current would result if the output load resistance (including the effect of the loading capacitance) dropped. This could be a result of a resistive leakage path across the output coaxial connector.

Stu

No cover. Insect. Fried.

John, W1TAG

Just a wild guess, because it happened to me and my Viking II:

The funny little plug and socket that connects the Valiant to the Dow Key works it's way out and you lose the RF load.

Could it be Satan?

KLC

He said it was a valiant,  so good possibility.

--Shane
KD6VXI

Now I can't find the card reader for the SD card - the one in the laptop doesn't like newer cards (don't ask why that should matter), but it will show up.

I can see that most of us have been down a road like this before!

So far one of you has actually hit one of the two problems on the proverbial head! :D

As soon as I post the pix, all will be revealed!
I did get to go through the entire output stage in wonderful detail though.  :-\
And I can report that at least thus far none of the caps were faulty - they were likely all replaced at the time of the previous owners modifications. I did spend quite a bit of time checking them and testing, and substituting fixed values - just to be sure.

Remember now, this is ALL due to the fact that I use the unit with the top off, and I gave the follow up clue a few posts back! I think that when I write it all out, it's rather amusing stuff. Btw, it goes back into the cabinet later today... no screws though!

                _-_-bear

Piece of hair / dander / dust bunny?

--Shane
KD6VXI

"Leaves of Grass"
A wet leaf stuck in the plate of the load cap.
Blown from the wind during transit, or you got a late fall snow shower which wetted a previously dry leaf or other crud already there.

Or the load cap's rotor ground wiper intermittant.  Got skewed during case warping while carrying it without outer case support.  Or just about any circuit trace or wire connection opening during transit without proper case support.  If you now operate it with case on, just don't move it around.  ;D

So we're awaiting pix with baited breath.

Stu AB2EZ was closest, but I think on the basis of the symptoms, not the clues.

The key to the clues were that while I did say that I ran it with the case off, I did add that it was moved. Actually it was moved twice. The second time up to the /2 QTH.

The first problem came because I decided to get the center of gravity for the rig closest to my body when I picked it up and carried it about 75 feet from one building to the next, down and up 4-5 stairs as well. This turned out to be a mistake - even though I knew that I needed to be careful. By leaning the rid into my body as it was carried, I put pressure on the small loading coil (seen to the edge of the chassis rear in the stock manual image) which in turn put pressure through the two blocking caps and to the top of the RF choke. This caused the base of the RF choke to crack.

I'm sure that when KE1AX was doing the mods, this looked like a good idea, since he used a nice stout bit of silver plated angle to do a solid attachment that held the two blocking caps (see the pix to follow...) But, the RF choke is ceramic and can't handle the slightest pressure against the far end. :(

Now Johnson apparently had an even better idea! They put the GROUND LUG for the RF bypass caps on the RF choke between the bottom of the RF choke and the chassis - at least that's the way it is on this rig, and it looks like the original solder lug...

So, once the ceramic cracked, and the screw fell out, only gravity was holding the bypass caps to ground! This was fault/problem #1.

I fashioned a repair to the choke, as seen in the pix, and held it in using flowable silicone rubber, giving it just a bit of give... also replaced the solid connection from the blocking caps with some braid. Not the neatest job, but it serves the purpose and now there is no hard connection to the top of the RF choke, and there is very perty lacquered brass mount on it! :D

The second problem next - Stu more or less nailed this one... and again at least part of that was due to the move!

Hope everyone had some fun with this, I was not having much fun isolating these two problems...

Making the RF Choke mount.

The installation...

The confounding thing was that once the RF choke was mounted and repaired it did not FIX the problem, but it made the problem STABLE! Ha!!

The second part, that Stu hit was actually the coax connectors!

Turned out that in the process of moving it WITH the Dowkey on the back, held on with an angle coax and a female to female coax, it got banged. You'll see in the pix, the broken inner insulator of the chassis mount SO-239, and the broken collar on the Ampenol angle connector.

The reason I did not remove the Dowkey is that the key line is soldered to it, and does not plug in. Well, that ought to be changed... anyhow...

I replaced the chassis mount. It still acted squirrely.  Of course what led me to the SO-239 was Stu's comment, and I noticed the broken end on the angle adapter too... So, I grabbed another angle connector. Working with it, I found that it worked fine with a direct connection to the SO-239, adding in the angle, it went strange. Tried a second one. Same thing! So, what's the chance of having TWO bad connectors?? Yep, two bad ones. Then the Dowkey was going funky, as I had put the nicer one that I had in there initially, removed at first because I suspected that it was the fault. (putting a different Dowkey in did not fix it, which is what started this whole expedition)

Oddly, I found that it did have some oddness... it seems that Dowkey made two types of contacts on those SO-239 barrels that they use. One is a sold connection, the other seems to have a spring loading and moves!

Apparently the one that moved was on the XMIT side. It was not being pushed quite hard enough, or so it seemed. Anyone know what the intention of that moving contact/spring loaded bit is??

Some of the Dowkeys that I have the spring loaded thing on one side, the others are all solid.

I switched them on mine, so the solid contact is now the XMIT side. Works FB now.

So that's the whole puzzle - assuming it stays solid and keeps working. Remember this was working fine with only an occasional carp out, so it may be laying in wait until I think I am in the clear, waiting for the opportunity to MESS WITH ME AGAIN!

Bear

It is interesting that either of the following will significantly increase the RF load impedance (at the operating frequency) that is seen by the RF output tubes... looking into the pi network. Therefore, causing the average plate current, at resonance, to drop significantly... due to the drop in the instantaneous plate voltage on instantaneous plate current peaks:

a. A significant reduction in the RF load resistance that the output RF connector is looking into (e.g. an RF short circuit)

b. A significant increase in the RF load resistance that the output RF connector is looking into (e.g. the antenna system becomes disconnected, due to a bad RF relay or a bad RF connector)

The RF load resistance (at the operating frequency)... that is seen by the tubes looking into the pi network... is equal to:

Q² x R_eff = [2 x pi x f x L]² / R_eff,

where L is the inductance of the pi network's inductor, and R_eff is the effective RF load resistance across the output of the pi network. Note that it is inversely proportional to R_eff

R_eff is always less than R_ant... the RF resistance of the antenna system that is connected to the output RF connector.

R_eff = R_ant/[1+ (2pi x f x R_ant x C_loading)²]

For a given amount of loading capacitance, the maximum value of R_eff occurs when:

2pi x f x R_ant x C_loading= 1

Separately, I believe that Dow used the spring loaded connector design, with the internal gap that opens and closes (intended to be on the receive side of the relay), to provide extra isolation of the receiver when the relay was in the transmit position.

Also note: I have found that the SO-239 receptacles on DowKey relays, and also SO-239 chassis receptacles, tend to make intermittent contact with the center pin of a PL-259 plug... particularly if a PL-259 plug with excess solder on the outside of the center pin has been previously inserted into the SO-239. To fix this problem, I have resorted to inserting a 1/4 inch pin... made of rolled up aluminum foil... into the SO-239 socket... to improve the contact.

Stu

Nice post, with great pic's. I liked giving the clues, and seeing who can come up with explanation.

Thanks!

I thought that doing it this way - and assuming I was actually able to find and fix the problem - would be more interesting and fun than just posting up a repair. I have to admit that I had an underhanded motivation too, that was to keep me from wandering off the repair. Once I went "public" I had to save myself the embarrassment of not getting it done or not getting it fixed.

Stu, thanks for the theory. I assume that R_eff means "effective"?? Not "efficiency".

So, what is the relationship between Q and R_eff? Looks to me like those two equations can be substituted, and it also looks like the L value replaces the C if it is done that way... them dar matty maticks be like cipherin'!

                   _-_-

Bear

Q= L/R_eff, where L is the inductance of the pi network's physical inductor... adjusted to take into account the reactance associated with the series equivalent circuit of the loading capacitor in parallel with the antenna system.

For example, if j x 2 x pi x f_resonant x C_loading = j/(50 ohms)= j x 0.02 siemens,  and

if the impedance of the antenna system is 50 ohms...

then the equivalent series circuit for this parallel combination is

[50 ohms ] x [ 1 - j] /2 = 25 ohms - j25 ohms

Therefore, in the equation for Q,  L = L_{physical inductor} - L₂, where:

L₂ = 25 ohms/(2 x pi x f_resonant)

The impedance, at the resonant frequency, looking into the pi network is:

Q² x R_eff

For example, if the loading capacitance is set to minimum (~0pF), then Reff is the resistive component of the impedance of the antenna system.

If R_eff = 50 ohms, and Q=10, then the impedance looking into the pi network at the resonant frequency is 10 x 10 x 50 ohms = 5000 ohms.

Also, since Q= L/R_eff, it follows that:

Q² x R_eff = L²/R_eff.

All of the above can be derived by applying simple circuit theory (e.g. converting the parallel combination of a loading capacitor and a 50 ohm antenna input impedance into an equivalent series circuit), and algebra.

Stu