Need some brainstorming help. T368C, Odd problem. Video inside.

[The Slab Bacon]

The Grid dip is inline with the plate dip though.  I dont think thats my trouble. 

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

[flintstone mop]

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

[Jim, W5JO]

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.

[ke7trp]

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

[Jim, W5JO]

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.

[ke7trp]

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

[n2bc]

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.

[ke7trp]

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

[Jim, W5JO]

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.

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?

[ke7trp]

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

[Gito]

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

[flintstone mop]

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

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

[ke7trp]

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

[Jim, W5JO]

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.

[ke7trp]

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

[w3jn]

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?

[W1AEX]

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?

[ke7trp]

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

[w3jn]

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

[ke7trp]

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

[KD6VXI]

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

[ke7trp]

You cant shane. This is a T3.

C

[KD6VXI]

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

[W1UJR]

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.

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.

[ke7trp]

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