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

[ke7trp]

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

[The Slab Bacon]

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.

[flintstone mop]

Yea, 1000WATTS out of a lowly T-3..............but I forgot that your a P.E.P. kinda guy. 
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

[ke7trp]

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

[n2bc]

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.

[ke7trp]

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

[flintstone mop]

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

[ke7trp]

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

C

[ke7trp]

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

C

[ke7trp]

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

[ka3zlr]

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.

[ke7trp]

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

[ka3zlr]

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.

[W1AEX]

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.  

Any ideas?

C

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.

[ke7trp]

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

C

[ka3zlr]

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.

[k4kyv]

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. 

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.

[ke7trp]

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

[WD5JKO]

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

[Jim, W5JO]

 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.

 Jim
WD5JKO

Wonder if the input to the matchbox is shot?

[ke7trp]

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

[w3jn]

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

[N3DRB The Derb]

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.

[ke7trp]

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

[ke7trp]

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