T-368C trouble

[w3jn]

Just got around to hooking up a T-368C I bought for my lake place. 

Issue:  when unkeyed and plates on, and in TUNE mode, the plate meter reads 250 mA; when keyed, the plate current goes down to a normal level and the tune/load controls give about a hundred watts output when adjusted to 150 mA. 

When unkeyed, plates on, and in OPERATE mode the plate breaker trips.  If you hit the plates with the xmitter keyed and in TUNE mode and then switch to OPERATE the xmitter operates normally.

Obviously the final is receiving excitation despite not being keyed, and the increased plate current is because the ant relay is open.  There are some buffer stages that receive cutoff voltage when unkeyed, but I haven't checked the presence (or lack thereof) of this cutoff voltage.

Where should I focus my efforts?  Clamper tube?  Relays? 

Thing was working before it was disassembled and I hauled it up here...

I *hate* working on transmitters   

[WA1GFZ]

John,
Sounds like a bias problem. When you key the rig it gets drive so it generates grid leak bias. Send me a schematic and I'll take a look for you. I'll trade you T368 problem for a 300 HP motor control I'm trying to break. 

[w3jn]

 

Well this one was easy.  Someone had wired the plates on switch so it turns everything on no matter if it's keyed or not.  Turning the plate switch OFF and keying it normally got it going.

However, it downward mudulates only now.  Found a plate cap on one of the 4-125s that had apparently heated up enough it came unslobbered.  Perhaps a new set of mudulator toobs is in order.  Dunno what else it could be, tried loading lighter and could get no lower than 225 mA plate current.

And modulating this bad boy drops my line voltage from 117 to 107 (yep, it's a 20 amp circuit).

[N3DRB The Derb]

hambone strikes again! 

[N3DRB The Derb]

If the bias on the final still aint right, the mod xformer + toobs will still be lookin into a freaky deeky load. I bet thats why that 4-125 plate cap got so hot.

BTW - if you need some hi temp silver/tin solder I got 20++++ ft of it. send some down for you. 

[w3jn]

It's a JS self bias circuit.  I think one problem is the drive is a tad bit low.  If I change to the 2-3 MHZ band I can load it a bit more lightly.  At 200 mA plate current there's about 30% upward and 100% downward mudulation.

[WA1GFZ]

John,
Home Depot sells Silver Solder in the plumbing isle. I think it is 3%.
The small roll is cheap. you will need flux. I agree with Derb the bias needs to be right.

[N3DRB The Derb]

Jonny, dont you dare go buy any solder ! The stuff I got is from Measures n it's bad to the bone.

The 430ºF/221ºC, eutectic, silver solder we supply in the retrofit-kits is: corrosion resistant, approx. 3.5-times stronger than 60/40 tin-lead electronics solder, flows well on difficult to solder materials, and contains no toxic hazards. It is FDA-approved for use on food utensils. It surpasses Mil-Spec purity standards. 1/8" / 3.17mm diameter silver solder: $0.16 per inch /25mm, pn--41.2}. This material works well with liquid rosin flux for soldering copper, silver, tin, brass, pins on 3-500Zs, and printed circuit boards. It is excellent for use on antennas and ground systems.

I got 20 feet of the stuff to do the nichrome and my upcoming rev. 2.0 ground system with more STRAP!  gonna be home today? I'll run some down special D, or I'll snail mail sum to you.... you will need some rosin flux.  what say ye?

self bias? sounds gay. throw a grid leak rx in there and add a touch of fixed bias enough to cut it off when unkeyed. but I've never worked on a T368. probably a lot of things I'm unaware of. 4-250A final stock?
 

[w3jn]

Don't need any silver slobber, but TNX for the offer.

Yep, it's a self-bias arrangement with a clamper.   It seems to work well enough, there's lotsa T-3s out there working Fine Business.  Could be any number of things.  Drive to the final is a bit light, and goes DOWN when keyed.  Could be weak 4-125 mudulators, gotta score some for spares anyway.  I have 10 or 15 spare 4-400s but no 4-125s.  I'll be on the lookout for some @ Tim-onium.

[WA1GFZ]

John,
A weak clamp tube could also be your problem.

[Steve - WB3HUZ]

If you aren't working  CW, do you need a clamp tube?

[WA1GFZ]

Low Drive or no drive the clamper would protect the final.

[The Slab Bacon]

Definately check yo clampah!! I had a gassy 6AQ5 clampa in a ranger drive me nuts some time ago. The clampa wuz suckin the grid drive from the final.

[Todd, KA1KAQ]

Yep, the clamp tube should be easy enough to check to see if that's the problem. Does sound like something is breaking down under load. At least it's not followed by a loud *BANG!*, which means tracking it down will be less of a problem and a lot easier on the nerves.

I have some spare 125s in the pile somewhere, John. Will see if I can dig some out for Timonium. Stuff starts coming out of the other garage this week, so I'll be able to get to the tubes in the back.

[w3jn]

Clamper's a 5933, pretty sure I have one or two kicking around.  The 807 won't fit as there's a metal ring around the base.

TNX fer the offer of 4-125s, greatly appreciated!

[flintstone mop]

It's amazing how fast a piece of equipment 'forgets' how to operate, when it's left in a cold dark place too long.
John, did you do any audio mods to it?? The T-3 was an amazing TX after the mods and the re-work of the plate tank circuits.

Fred

[w3jn]

Fred, the audio's been bypassed and the mudulators are driven from an external amp via a driver xformer