The AM Forum

Ok. This T3 has been acting up. Now its full blown.  I spent several hours trying to figure out the problem and we cant seem to find it:

The Transmitter makes full power, Tunes at the correct dial numbers, Makes full power, Has a clean signal.  I can talk on it all night.  

When I UNKEY the transmitter, I see a BIG signal on the O scope.  This signal lasts just a moment. It whipes the screen out. The Bird Hits 1000 watts or so during this time. The SWR meter on the KW matchbox flips up to 5 to 1.

So.. Its throwing trash the moment after ONE relay is dropped and until the next Drops.

The Plate current only rises up about 10ma during this time but it might be higher in reality. The meter is slow.

The INT amp plate and the PA Grid meter Drop INSTANTLY when I unkey.

Some notes:

nobody can hear this over the air.

No change in KW matchbox tuning changes this effect.

No change in tuning on the plate vac variable or the Load control effects this.  It even does it in the TUNE position but its very hard to see.

It does NOT do this in the dummy load.  However, My idea is that the dumy is taking the oscilation and it wont show reflections back.

I have swapped coax cables and lengths.  The T3 is looking at 1.0 at 52 ohms according to the 259B

I have cleaned contacts, checked caps, Inspected tank coil band switch ect. I have adjusted the Relay delay. You can only make this condition longer or shorter with the relay delay. I am confident that the Relay system is working as it works on 40 perfectly.

Here is the video..

http://www.youtube.com/watch?v=VBB617V2vIQ

Thanks for the help!

Clark

Looks like a neutralization or parasitic oscilation problem to me. The incoming drive swamps it out, then it takes off when the drive is cut off.

Does it have a parasitic supressor on the final plate?

Is the resistor fried out in the supressor? (if it has one)

Have you neutralized it properly?

Has a bypass cap or ground connection failed?

Just my $.02 worth.

Yea, 1000WATTS out of a lowly T-3..............but I forgot that your a P.E.P. kinda guy.  ;D
Just pullling your chain Clark. HMMMMM no fan noises.......is the T-3 in the same room??
The Slab may be on to something with runaway operation when unkeying.

Phred

Yes. The parasitic choke is there.  I have checked all of that. I have not checked bypass caps on bottom of 4-400. Maybe i should pull it apart again today and have a look. Thanks for the tip.

Fred.  No.  The Bird has a 2500 slug in it.  Not PEP.  When I unkey it hits 1000 watts or so on the meter.  The T3 is about 5 ft from me in the room.

C

I don't have T3 schematic any more.... but isn't there a clamp tube on the 4-400?  It should be shutting down the 4-400 screen voltage as soon as drive goes away.

Yes there is.. A good idea.. Let me yank the deck and check the old girl.  I tested ALL tubes about 6 months ago.  Maybe its shot.

C

YUP! Had a surprise here too with a tube that quit in my R390A. One of the 26Z5W 's were capoot and only getting 70VDC from the PS when it shudda been 190.
Fred

Ok. Now who wants to Drive over here and help me yank these Decks out? LOL

C

5933 clamper tube tests 80+ on the TV7.  Its good. 

C

Its better.. But not fixed. 

The Parasitic resistor checked at 78 ohms.  No sign of heat or failure.

I found two issues. 

1. The 6ah6 keyer delay tube was testing 40 on the TV7.  Min value is 30.  I noticed that it was very blue and gassy.  I replaced it with a used one that tested 52. 

2. The 4-400 plate was very grey and pitted.  In other words, It looked used and abused. I put a new Eimac 4-400a that was a different style plate.  I ran this tube in the GK500 for testing.  Long enough to burn it in.  Then returned it to the box for a rainy day.

The radio now makes a bit more power, Tunes smoother  I could not test each tube one at a time in the rig. I had to beg a friend to come over and help me pull the deck. He had to leave to pick his daughter up from school so this was a one shot deal. 

The radio worked perfectly for about 20 minutes.. Then I left it keyed for about 2 minutes and unkeyed it. The oscilation started up again when I unkeyed it.

I noticed that if I turn the plate tune cap to the right so its 40 miles out of the dip, The problem goes away.  If you tune IN the dip, It will oscilate.  Example..  Dip at 200, 225, 250, 275,  It will oscilate.  Tune for any of these, Then turn plate control to the right up 30 to 40 mills. No oscilation.  This really has no effect on power out.  Any real drawback to just running this way?


I guess I will run it out of the dip a bit. Not sure why the hell this radio does this.  I have now checked just about every part in the RF deck by hand.  Any ideas?


C

Possibility of a Relay hang up sequence issue.....? For some reason I'm thinking too fast or to slow keying.

Keying timing changed..? Have you Visually monitored the contactor sequence, Back Flash on contactor(s)
something bent not straight....? Are they the correct contactors for your set up.

Sounds like you checked all the important parts.

73
Jack.

Jack. I have testest the realy system. You can hear them working in order.  No funny noises.  I can adjust the delay and this just makes the problem longer or shorter. 

It also works FB on 160 and 40. 

c

Hi,

Okie Doke I was just wonderen.

73
Jack.

I agree with Frank (Slab Bacon) that this looks even more like a neutralization issue. I have seen similar behavior when un-keying with my 6146 RF deck. A slight change in my neutralization setting and it was fine. Not sure how the T-368 accomplishes neutralization for the 4-400, but it would definitely be worth a look IMHO.

I dont see any mention of it in the manual and do not see any caps or adjustments

C

Hi Again,

In thinking further, something is holding or discharging or both at the end of
transmission sequence.

If relay timing and contactors are in FB shape then it leads me to look at
what could be discharging/holding a charge then discharging in circuit at
end of transmission sequence, return of B+ to receive.

I've never owned one of those so I'm in the dark here somewhat but a
spike is a spike frequency is irrelevant due to the energy pulse it could
emit any amount or any number of frequencies.

At this point Me Myself I'd pull that thing off line and go over every
condenser in that rig because I know little about it, but I'd check
what I could get at.

And re-neutralize

I'm like that...all the best in your endeavor there OM.

73
Jack.

Are you sure it is gassy?  Or is it just the bleu  glow on the glass envelope.  I would check it using the Gas test on the TV7. No point in throwing away a good tube.

I doubt that has anything to do with the problem, though.

It failed the test don. I tossed it out. 

I have replaced every cap in the modulator deck with orange Drops. I wonder how neutralization is handled on a 4-400 like this.  The Grid dip is inline with the plate dip though.  I dont think thats my trouble. 

Maybe it does not like the KW matchbox? 

C

Clark,

   You say the turn off burst goes away if you de-tune the plate tuning. Does the 4-400 grid tuning have an effect as well, or is the grid tuning some sort of ganged tuning such that you cannot adjust the grid tuning by itself?

   It sounds like a TPTG oscillator to me. I wonder what your relay sequence is after transmitting? Is there a protective bias + grid leak on the 4-400? Yes I know it has an 807 series clamp tube, and what appears to be grid leak bias, but the schematics I've seen are all simplified such that much detail has been omitted. If there is a fixed bias component, maybe it has changed?

   Since you see the burst on your watt meter, the T3 is seeing a load. You say it is fine into a dummy load. So what does that mean? How about dipping the 4-400 plate proper, and detune the KW matchbox a bit to see what effect that has.

  Maybe you can link to or post the RF deck schematic?

Jim
WD5JKO

Wonder if the input to the matchbox is shot?

Jim. The Grid is tuned by the PTO.  YOu cant adjust it.. Only the two big Vac variables.  I can however watch the Grid and it seems to follow the tuning.

The KW matchbox is showing 1.0 at 52 ohms with the MFJ. It works with all other transmitters. But this T3 is an odd design and not a pi net like the GK500. 

I will try to find a schematic online. I dont have a scanner anymore. I might be able to snap a photo of the schematic and upload it..

C

Are you using the T/R relay in the matchbox, Clark?

I think I hear the matchbox T/R relay going clunk about the same time in the vid. Something going thwap anyway that sounds a bit out of place.

No. The TR switch is bypassed in the KW.  Its straight through. The T3 has a built in TR relay with RXer outputs and it mutes the R390. 

I have the GK500 back on the air. THe fan is rebuilt.  The station works perfectly with the GK500.  Flat, Full power out. Tunes smooth ect..

C

I ran the transmitter for 2 hours with it loaded up out of the dip by 30 mills.  Its rock solid.  FULL power out.  It really does not seem to mind. I just load it down to 220.. Then kick it up out of the dip to 250. This is still well under the suggested 275 loading for 75 meters. I hope this does not hurt the tube.

C

Clark,
        what is the grid current doing as you swing the plate through resonance? The grid current should PEAKas the plate current dips. If it doesn't you definately have a neutralization issue. Can you measure the screen current on the 4-400? What is it showing? If the screen current is high, you arent loading it heavily enough. If the screen current is way low, you have it loaded too heavily.

I have occasionally seen "something extra" in the scope pattern when running a transmitter loaded too lightly. This usually goes away as you load it heavier.

Have you or anyone else changed the taps on the final tank coil? If so, a little different L/C combo might make this problem go away. It is definately an OSCILATION problem.

Try loading it differently, and also make sure your class-C biass is enough to get the tube to cutoff. Also make sure your screen voltage isn't excessive.

                                                                        the Slab Bacon

Interesting problem here.
It's been a while since my T-3, can't remember if there was any neutralization. And I never had to change any taps on the tank. That TX with it's outboard antenna tuner was designed to work into anything that resembled a load.
There was a relay in the T-3 that was famous for malfunctioning/sticking. I cannot remember what its purpose was or how it screwed up the TX.
Do you think Clark could just do the slight "mistuning" procedure and let it ride?
Phred

The T 368 is not neutralized.  Neutralization is generally not needed below 20 meg when using a 4-400, the max freq. of the T 368.   Remember if you are not operating a high power transmitter to specified output all bets are off and there is no telling what you will have out the antenna port.  Same for the intermediate stages.

You mention it works into a dummy load.  Are you using the same jumper from the rig to the DL that you are using to go to the matchbox.  What is the intergrity of your jumpers, antenna, grounds, etc.  Do you have RF coming back into something?  Checking the input of the matchbox with a MFJ 259B is not like feeding 4-500 watts into it.  You can have a breakdown in insulation when the voltage or current is high like with this transmitter.

Ok. Thanks for the suggestions. 

The rig seems to be neutral.  The Grid is peaking with the dip.

The Taps have not been changed. I studied the pictures in the manual.

I am loading it correctly and have even loaded it beyond what the boost says. It still oscilates. 

The Jumpers are new and in perfect shape. I have swapped them all out one at a time to be sure. 

There is no RFI in my shack. 

I am unhooking the KW matchbox and installing the Dummy load there.  But you wont oscilate into a Dummy. Its will not show the reflected power back  as the antenna will.  Its a wide range sponge for RF.

Fred.  I think I am going to have to just load it out of the dip.  Unless anyone has any more ideas!  I think maybe next step is to check the burried Transmittings caps?

C

If it won't do anything strange into a 50 (or whatever) dummy load, then something is wrong with the circuit you are feeding.  Or something is wrong with the Pi network in the transmitter.  There are fixed caps there or maybe one of the vacuum variables has arced.  That has been known to happen which causes all kinds of problems. 

Are there an scorch marks on the plates of the vacuum variables?  If so replace.  Could you have lost vacuum in them?  That does happen.

They are shiny new. No corrosion.  Tunes smooth. No arcing.  How can something oscilate into a dummy load?  It will accept all Freqs shotguned into it.   The Tuner will not except Freqs out of that Freq range it is tuned for. Hence the High SWR.

Its possible some of the fixed caps are bad. I guess thats the next step here.  What a pain. Those things are burried. I will have to disassemble the entire RF deck. 

C

If your T3 has not had the PA B+ mod, that is the first thing I would do.  A stock T3 has the 4-400 B+ fed through the PI coil with the plate blocker between the PI and L coils.  It is a simple change & takes the DC and audio off of the PI components.

Rule number 1.  Dont modify a Broken Rig.  Performing mods like that could really cause trouble and heartache.  They worked for 55 years this way. I just need more ideas on how to fix mine.

C

That doesn't necessarily mean anything.  If you have a cap checker that used voltage to make measurments, attach it and tune the cap through its range.

If the transmitter is where the oscillaton is, then the dummy load will accept it up to the max freq of the dummy load.  So if you have a 50 ohm dummy load it looks the same to the RF as the input of your tuner if it is properly tuned, so there should not be any difference.  Is there something funny on the other side of the tuner that is being reflected back from somewhere?

Hmm..  Its a KW matchbox. It runs into 600 ohm line. 135 ft of wire. All the bands tune with in the middle of the range of the tuner now. I use it on 80 through 10 with the GK500, Icom, Champion and valiant.   I have not had a problem. 

The reason that I dont think the caps are bad jim is that the tuning chart in the manual for both caps is very close to actual loading in operation.  Your right though. They probably need to be checked.

C

Hi

Maybe your 4-400 ,with it.s input tuning and output tuning with the matchbox becomes a free running oscillator at the frequency You tuned.

Once I made a 833 amplifier transmitter I put a drive to it than tuning and loading I succeed  to get all the parameters as the data book. A "good transmitter"

But when I switch off  the driver , to my wonder the 833 is still running with it grid current and plate current   it's parameter as the databook( OscilatoR ?)
.
When I switch of the B+ a big spark jumps across the spark gap in the modulation transformer.
But to become an oscillator it needs a push from the driver,when it oscillates ( become an oscillator) it needs no driver anymore.

Maybe your 4-400 transmitter has the same symptoms  but lighter/very light  compared to my transmitter.

Just A "Maybe"


Gito

Dear OM Clark
we have been at this point about modifying the P.A. This is a known shortcoming in the T-3. The B+ in addition to modulating audio has stressed these components and now they may be defective.
N2BC was giving the number 1 modification that should be done to all T-3's.
 
The main purpose of the T-3 was for RTTY service not A.M. It was purposely designed NOT to make good sounding A.M. It was designed for military bozos that were'nt going to be very careful about operating procedures.
Try to reconsider the no modify stand.

Fred

Phred..  I am not saying that I dont like the mod or I dont like to modify radios.   I dont want to mod a Broken rig.  I really think thats a big mistake in trying to fix something.. 

I suspect that Gito is right.  Now it needs to come apart and have the caps checked.  One must be bad or leaky. 

C

Removing the HV  from the coil is not a major thing and anyone with basic skills should be able to perform it without causing other or futher troubles.  It is also a safety mod.  I would have never run the rig without making that change.

I am not worried about the Voltage on the coil.  I can see this thread has turned into how to modify a T3.  I was just looking for advice on how to fix this odd problem that is started on my radio.   Thats all...

Thanks To the people that offerered help. I really apretiate it. I made alot of headway with this problem. I will take the rest of the discussion offline.

We can just close the thread down as its no longer helpfull to anyone.

C

That's not helpful at all.  What will be helpful is to keep this discussion going and find the cause of the trouble.  Rest assured if you're having this trouble someone else will, and this thread will be a valuable reference tool in the future.

Another thing that will be helpful is to figure out the frequency the final is oscillating (if indeed it is), or what frequency your big spike appears at.  Then, using basic math and electronics theory we can probably pinpoint the root cause.

Can you beg/borrow/steal a spectrum analyzer, or barring that, a digital storage scope to catch the spike in the act?

I agree with JN that it would be helpful to see where it is oscillating. A frequency counter might be quick enough to catch where the energy is being produced during the un-keying process. Also, does this issue occur on all 5 ranges of the PA bandswitch? What happens if it is keyed and un-keyed in CW mode? Can the problem be duplicated if you remove or reduce excitation?

It only oscilates on 75 meters in the ghetto.  Low or high power. All other bands work fine. I will ask my brother about using the spec an

C

Understand that it on;ly oscillates when it's operating on 75M.  What we need to know is what frequency is that oscillation actually at.  It's obvious during the switching cycle it's taking off, but it's not a given that it's oscillating at the same frequency it's transmitting on.

If you nab the transient with a spec an, or a digital storage scope, it will tell more of what's going on here.  BTW make sure you do NOT hook the spec an directly to the xmitter, or your brother's gonna be pissed at you for blowing out the front end of his spec an ;D

I think he still has an eaton 2Gig spec an.  I will ask him to use it.  It shotguns Freqs out and I cant see it on the scope.

C

Have you tried the neon on a stick?  Put it in close to the tank circuit, unkey and see what color the neon flashes.

Purple = VHF

Won't give you the exact freq., but at least it would allow you to see if it's a parasitic or LF spike.

--Shane

You cant shane. This is a T3.

C

Never worked on one...  You can't see the RF deck, or get close to a RF relay or anything near the RF deck?  You only need to couple incidental RF into the neon bulb....  Remember, you can light one up near an antenna... Same principle here.

Matter of fact, would it work if he tried it on a tap off the dummy load?  Maybe not, wouldn't show if it was something AFTER the TX relay had shut off.... :(

Suckage!

--Shane

You hit it on the head Jim, no problem in a dummy load, its likely not in the rig.
My T-3 would spit and sputter when unkeyed on 160 meters, its was all the antenna I was using.
Never bothered on 75 or 40.
Very reactive antenna, and I could dial that out with the antenna tuner, had to add a lot of C, but was unstable
when I unkeyed.

My vote is its not the rig, but rather the antenna/tuner rig.
Change to a different antenna if possible.

When in doubt, use Occam's razor.

Ok..  I will hook up the 259B and test it. 

I did try to induce some SWR with the Tuner and that did not calm it down. I tried adjusting both caps.

C

shane, The T3 is full enclosed. If you pull the deck. HV is unhooked.

C

Remove the rear cover, jump out the contractors, and you can carefully probe various parts of the circut.
http://www.arizona-am.net/PHOENIX/W8QBG/Bauer/Refurb_356a_M.jpg

(http://www.arizona-am.net/PHOENIX/W8QBG/Bauer/Refurb_356a_M.jpg)

Yeah.. If you have your T3 in the garage like George does!   I can get some guys over here to muscle it out in the room.

C

I haven't run across anything with HV that didn't have an interlock.  If it has an interlock, it can be bypassed.

I'll take your word for it.  Kinda hard to troubleshoot and align that way, methinks.  What's the word, clustersomething or other :)

Off to start up a disney flick for the kids.  Then din din time :)