T-368 help

[W9BHI]

I am helping a fellow ham troubleshoot a T-368.
The HVPS voltage is on in the tune mode but kicks out the front panel plate supply breaker when switched from tune to operate.
I checked for shorts on the mod iron and the RF deck, all OK.
Could the stand off's that support the HV interconnects to the 3
decks be arcing to ground?
Do these units have a history of insulation issues in the transformers/choke?
I may have an opportunity to own this rig.
Any experienced owners have any ideas?

P.S. When he got this thing a few years ago He did have it fired up and it was working fine.

Thanks in advance,
Don W9BHI

[ke7trp]

On the bottom of the front is an adjustment for the plate trip.  Its possible that its just out of adjustment.  If you think this is the problem, Load the rig down in tune, Then, try operate again.  If you get it to load at say 200MA then you can walk it up until it trips and then set the trip higher.

Another sore spot is the HV connectors on the back of the RF deck.  If they have carbon they will arc. 

C

[WA3VJB]

Don,

I had a T368C for about 20 years and one of the few failures was traced to standoff insulators.

The ceramic "beehive" insulators have a cork cushion that -- when still resilient -- allows for some thermal stress and mechanical pressure between the chassis and the ceramic.

Over a period of time the cork hardens, and there no longer is any "give."

In my case, two or three of the ceramic standoffs suffered an arc between the threaded metal shaft and the INSIDE of the structure -- NO outward signs of a problem !  That took a while to figure out.

The Plate overload setting consequently reacted properly and saved further damage.

Another failure was a carbon path between an RF choke and the bakelite square it was mounted on.  That one was easier to find.  I initially used a Dremel-type tool to rout the carbon path clean, but intended to get a square of Teflon as a substitute. I see raw manufacturing stock at hamfests and it's worth buying a quantity.

I took the piece destined for the T-3 and instead put it under a mod transformer to raise it up from ground as a precaution.

The T-368 is a great transmitter and if you follow through on the chance to acquire it you won't be disappointed.

[Ott]

Had only two problems with my T-3.

Kicking the big breaker...

Had that here and it was a bad 3B28.

Kicking the reset...

Had that here and it was rf transfer shoe on the rf deck.

It had to be re-bent to make a much tighter contact with the relay box.

As in Paul's case the the plate overload did its job perfectly.

There is a problem on some unmodifed T-3's where dc is still on the tank coil.
As I understand it, they can arc to the chassis on the one end...

Enclosed pix to show you the shoe and the tank coil insulation point...

[flintstone mop]

If it is a virginal T-3 then it is prone to arcing and tripping. HV is present in tank circuit. Timtron in the AM Bulletin board , had great mods to make it almost B'cast quality audio and remove the HV from the plate tank circuit. The 2.5mh choke right at the very end of the tank before it goes to the antenna connector could be shorted. Modulation and B+ is too much to handle.
Modulating the TX can trip the breaker. You'll hear a big pop. Try to tune and very lightly load the P.A. and see if that helps. The 4-400 might be tired.
Do not start tweeking safety circuits to 'MAKE IT work". Take your time. It is, or was, quite a work horse in its day. You'll have to go through the usual trouble shooting as mentioned by Paul, and possibly more, to find the problem.
I still wish I had kept mine. Very noisy beast and you can not see the tubes glowing.
Fred

[W9BHI]

I will be going to my friends house this weekend and will start to take a closer look.(It's a 3.5 hr trip one way).
It's not the HV overcurrent that is tripping.
It is the breaker on the front panel.
It is OK in the tune position but trips in the operate position.
I have to see if the plate meter is jumping up before it trips.
I am trying to get a pickup truck or something so I can bring this beast home and work on it.
There are no mods to the rig except the usual removal of the voltage for the carbon mic so a D-104 could be used, the clipper, HP and LP filters have been bypassed but are still in place.
No hacking or extra holes, it looks pretty good.

Thanks for the help so far,
Don W9BHI

[flintstone mop]

I'll have to disagree that the Timtron mods are a waste of time. The transmitter with its mod transformer and the 4-125's can sound marvelous on the air. The mods to the speech amp are worth the trouble. Gather up a few parts and follow the schematic and the endless sentences of Timmy. When these things were overflowing Hamdom for $700. a piece from Fair Radio, it was so nice that a real engineer, Timtron, took the time to show how to make it a very nice TX.
The mod to the tank gets rid of the B+ on the tank circuit and the dangers of 2500 volts DC on your antenna. The changing of the loading caps opens the high frequency response. Changes the tuning for 160M BUT will operate ok there.
What we're doing is modding the TX from it's design flaws, which were on-purpose, BTW. Going from carbon microphone, space shuttle audio, to something really pleasant to hear on the air.
Isn't what we try to do with the AM mode? I really regret getting rid of my T-3. I did all of the mods and with stock mod transformer had a beautiful sound and easy 100% pos peaks. That military iron is capable of more than what is printed on the can.
If 'messing' with it is not your thing; then find your short circuit and enjoy what you got.
Fred

[ke7trp]

Don, Did you ever figure out what is going on with the ole T3?   

I want to remind you that you should replace the main PS caps and check for leaks.  The caps used cork Gaskets and will leak out PCB oil.  You can try to use O rings instead and then check again. But a T3 should never be used inside a home or near a sleeping area.  I purchased the test kit online and sent the sample of oil from the caps off to be tested and it came back with major PCB and a bunch of other compounds listed in red. 

I was able to wash the rig out and replace the leaking caps with a single modern cap on mine.  In a Garage, maybe it would be ok or a barn, Dont worry.  But we had major respratory problems and once that thing was cleaned out and fixed, The problems went away.   I know I am preaching to the choir here about PCB's and the manly men around here used to wash the hands it it. But it really caused me breathing problems. 

Good luck with it. 

C

[W9BHI]

I finally found a pickup truck to retrieve the T-368.
I'll probably do it this coming weekend.
The first thing I will do when I get it home is to check for leaking caps.

Thanks,
Don W9BHI

[ke7trp]

I have a spare speach amp in the closet. I got it so I could build the hifi unit but never did.  Let me know if you need it.

C

[Mike/W8BAC]

What was Crusher's (Clark's T-368 nickname) Bird watt power output and modulation percentage when you sold it Clark? I have it saved here, but can't put my finger on it right now.

Mike

[ke7trp]

Mine had the larger of the power transformers. If you look in the factory manual there is one with more voltage.  I am going from memory but I think it was 6336 volt.  On 160 it would load to 600 I think.  500 anywhere.  Tim told me to load it down to 200 ma which I did most of the time and it was under legal limit pep.  I put 4-400s in the modulator(plug in) and added a second fil transformer under the chassis.  It would not reach 100% in stock form.  After, it would reach 100 but you had to push it. The stock iron's ratio is a limiting factor.

[W3RSW]

Memory lane..  Sigh.
Yeah, wanted to get a trailer over to Fair Radio when i lived in N. Pa., but mummy had other plans for those 700 dollars back then.  Remember how I bitched when I found out Fair stripped out all the spare tubes and military kit from the crates, but still "sold you a pristine T3."   As though that was a deal breaker.  Well, had to swallow pride and blame something back then. 

JHC, a single 4x4 costs you that now.

Concerning keeping Potential HV off the antenna. -- Just moving the blocking cap from place to another.

[ke7trp]

Wow. Those where the days huh?  A new 390 and T3 for cheap. Wow. 

The some of the RF deck mods make sense but for voice,  Its not needed to do all those mods and when you do, you have trouble tuning some bands. The main problem with these rigs is they are FULL of oil caps, and oil filled transformers. Leaks can be a problem.  I enjoyed mine when I had it. I never thought I would sell it.  But it was time.   

C

[K1JJ]

In my case, two or three of the ceramic standoffs suffered an arc between the threaded metal shaft and the INSIDE of the structure -- NO outward signs of a problem !  That took a while to figure out.

I think a lot of those beehive insulators got circulated around over the years. Maybe they were from other sources too. But over time in various homebrew rigs I've had at least eight of them arc internally, break apart at the threads or simply blow up from HV.  I still use them here and there but make sure the threaded bolts on each end have a lot of distance between them internally.  You'd think I'd learn my lesson, but big insulators are not all that common - if they work right.

The 2" ones commonly fail. However, the 3" to 4" ones seem to last OK. This leads me to believe it is the arcing distance between the internal bolt threads that is the problem on the shorter ones. The bad cork probably starts the whole degrading process.

Those old beehive insulators have made us all better troubleshooters....  


T

[KB2WIG]

Are there any ways to  rejuvienate the cork insulators?

klc

[W9BHI]

How many different versions of the T-368 were made?
Any body ever see a parts list?
I can't seem to find one anywhere on line.

Don W9BHI

[W9BHI]

I bought the T-3 and brought it home about 2 weeks ago and after replacing both arcing 3B38's, the 6000 and a 6AH6.(both showed shorts on my Hickok tester).
It is now functioning.
I replaced the well fried plate relay with a 40 amp rated SS unit with AC control.
I have it connected to an Array Solutions Power Master meter and a Drake 1Kw dry
dummy load.
I am using an amplified D-104 mic.
I was looking at the manual and it says to dip & load it to 350Ma. in CW and it should be putting out around 450 watts carrier.
I am seeing about 640 watts.
The manual says to dip & load it to 275Ma. in AM and it should be putting out 470 watts carrier (why more output with less current, Typo maybe?).
I am seeing about 550 watts carrier and about 1400 watts PEP modulated.
Does this sound about right?
This is my first experience with a T-3 so be gentle on me
The excitation meter is sticky and wont go below 4Ma.
If anybody has a spare they would sell, I would be glad to buy it.

Thanks,
Don W9BHI