T-368 Arcing Problems

[K1HH]

T-368 SERIES F TROUBLES

I’M HAVING SOME TROUBLES WITH MY T-3, TRW SERIAL #400.
TWO OTHER T-3 OWNERS, MARK KA4JVY AND MAURY NRGUI AND I INITIALLY BROUGHT THE RECENTLY ACQUIRED T-3 UP ON A VARIAC AND IN STOCK FORM, THE T-3 PERFORMED NORMALLY OVER AN HOURS PERIOD INTO A ME-165 DUMMY LOAD USING A STOCK CARBON MICROPHONE.

I THEN PROCEEDED TO MODIFY THE TRANSMITTER USING MODS WIDELY FOUND ON THE WEB WITH NOTATION TO TIMTRON, W3NP, KK4DF AND OTHERS. THE MODS ARE DETAILED BELOW.

UPON COMPLETION OF ALL MODS, THE TRANSMITTER TUNED AND LOADED PROPERLY IN BOTH CW AND AM PUSH TO TALK MODE. TUNING AND LOADING AT SOMEWHAT DIFFERENT NUMERICAL POINTS ATTRIBUTED TO DIFFERENT PLATE CHOKE.  PLENTY OF RF OUTPUT; PLENTY OF MODULATION.

NOW THE PROBLEM.

THE T-3 WILL GO INTO TRANSMIT WITH LITTLE DIFFICULTY. IT’S THE UNKEYING THAT CAUSES HIGH VOLTAGE ARCS USUALLY AT THE SAFETY INTERLOCK HV FEED POINT. SOMETIMES THE ARCING CAN BE NOTICED AT BOTH THE RF AND MODULATOR HV FEED POINTS. MOST TIMES YOU MUST RESET THE PLATE RELAY.
ON OCASSION THE PLATE CIRCUIT BREAKER WILL TRIP. AND RECENTLY WHILE TRYING TO FIND THE PROBLEM, THE TRANSMITTER PLATE RELAY STUCK ON AND THE ONLY WAY TO GET OUT OF TRANSMIT WAS TO POWER DOWN COMPLETELY. WHACKING K6, THE PLATE RELAY, WOULD THEN RELEASE THE CIRCUIT.

ANY HELP RESOLVING THE PROBLEM IS GREATLY APPRECIATED. I’D LOVE TO GET THIS BABY ON THE AIR.


THANKS,
K1HH
RODGER

DETAILS OF MODS

MODIFICATIONS INCLUDE ISOLATING THE FINAL TANK CIRCUIT BY PARALLING TWO 470PF/16KV DOORKNOB CAPS, WINDING A NEW PLATE CHOKE TO FEED THE FINAL (260 TURNS 24 GUAGE WIRE ON 3/4 INCH PVC FORM) IN ADDITION COIL L13 (PI CHOKE) WAS MOVED TO THE TOP OF THE DECK IN SERIES WITH THE NEW PLATE CHOKE; HIGH VOLAGE BYPASSES OF 470PF/16KV ARE ON THE BOTTOM SUPPLY SIDE OF L13 AND BOTTOM SUPPLY SIDE OF THE NEW PLATE CHOKE. THE SAGAMO TUB (FORMER BLOCKING CAP) HAS BEEN STRAPPED ACROSS AND FORMER LEAKING BROWN BRICK C10 BYPASS CAPACITOR HAS BEEN REMOVED. THE ORIGINAL L3 PLATE CHOKE’S BOTTOM END (ORIGINAL B PLUS FEED) NOW GOES TO GROUND. IN ADDITION THE PLATE METER WAS MADE A CATHODE METER AND BYPASSED WITH TWO BACK TO BACK DIODES AND A CAP.

MODS TO THE AUDIO DECK FOLLOW THOSE DETAILED BY TIMTRON,
W2ILA, KG2IC, W1UJR AND OTHERS. THEY INCLUDE BYPASSING THE FILTERS AND CLIPPING CIRCUIT IN THE SPEECH AMP AND CHANGING A COUPLE OF CAPS AND ADDING A BYPASS OR TWO.
THE CARBON MIC BIAS VOLTAGE WAS REMOVED FROM THE FRONT PANEL MIC CONNECTOR SO THAT AN AMPLIFIED D-104 CAN BE USED.

I’VE CHECKED MY WIRING AND BELIEVE IT TO BE CORRECT. PIX AVAILABLE OF MY CHOKE/ BYPASS CAP ARRANGEMENT, BUT FOLLOWS THOSE WHO HAVE VENTURED HERE BEFORE.

ANY IDEAS OUT THERE IN T-3 LAND?

RESPECTFULLY SUBMITTED TO THE KNOWLEDABLE AMATEUR RADIO FRATERNITY.

K1HH

.

[WQ9E]

Rodger,

On edit, it seems the T-368 has built in antenna switching.  Is there any way to temporarily defeat that relay so that the dummy load remains connected to the RF output to see if it is a relay / relay timing issue?

Good luck and you are sure to hear from some T3 owners.

73,
Rodger WQ9E

[K1HH]

Thanks Rodger. I'll look at the schematic and the unit tomorrow. It's in a warehouse, not here at home. I know it has a vacuum T/R setup.

Rodger
K1HH

[flintstone mop]

Your description of the trouble seems like something is going bizerk de-keying the transmitter. Try de-keying with the modulator tubes removed and see what happens.
The other suggestion is a good check too. There might be RF and a no-load condition on the tank circuit.

Fred
It's a great legal limit transmitter and can have B'cast quality sound with the stock  mod transformer...........yes it's good down to around 80 hz

Fred

[W3NP]

Sticking or welding of the the K6 relay contacts is a relatively common problem. My T3 now sports a very heavy duty 3PDT contactor with all 3 poles in parallel.

If K6 is sticking closed, the HV will be present after the antenna relay and other circuits have de-energized.

Check the HV interlocks closely for carbon tracks, burnt spots, dirt, etc. The arcing there could be a result of the other problem (or vice versa), but once damaged, the interlocks could be a continued souce of grief. These interlocks have vexed many a T3 owner.

Dave

[K1HH]

Thanks to all who are offering suggestions to my T-3 problems.  Today I replaced the 5933 tube and did some more cleaning and observations.

In CW TUNE mode the rig draws 130 ma CATHODE current with 75 watts out.
In CW OPERATE mode the rig shows 250 ma CATHODE current with 375 watts out.
I could turn the plate relay on and operate it brick down 375 watts out for over 20 minutes; it did not arc when turning off the plate relay. In and out of transmit several times with no visible sign of arcing, but a couple of times I thought I heard something.

In AM TUNE mode 100 ma with 40 watts out.
In AM OPERATE mode 250 ma with 375 out.

Each time I used the PTT I could hear arcing in the TUNE mode. I laid on the floor and looked under the mod deck while keying and I observed arcing at the spark gaps under K9 the relay located near the front center of the mod deck.

In OPERATE mode, using the PTT, there was no arcing at the spark gaps, but arc returned to the RF deck at the HV contact point. The arc occurs between the HV contact and the side of the case. Sometimes the plate relay would have to be reset after arcing, and finally the plate relay stuck on, and I had to power down. Smacking the plate relay would release the relay. I'll try to inspect the relay tomorrow. I'm not sure how its sealed.

Again thanks for all suggestions.

Rodger
K1HH

[W1UJR]

So, question 1 - No arcing before the mods were carried out?

[K1HH]

That is correct. No problems were observed during the original hours worth of testing before modifications were made. This transmitter (correct serial number 40, not 400 that I posted earlier) had been in storage for some time.

Rodger
K1HH

[W1UJR]

And the arcing only occurs when operating the rig in the fone mode?
Does not occur with CW?

[K1HH]

It ALWAYS arcs when unkeying in AM; it sometimes arcs when unkeying in CW. I have removed the driver tube from the speech amp to see if any of the audio mods were causing the problem. No difference was noted. I have not remove the modulator tubes to see if that has an effect. I can try that tomorrow.

[W1UJR]

The clamper tube operation and voltages have been checked?

Assuming your checking into a dummy load, and not some reactive antenna?

If it didn't do it before the mods, then ....

Sounds like something is keeping the HV on for a bit when you unkey.
There is a pot for adjusting the delay of something on the mod deck, if I recall correctly, take a look at the schematic. Or energy is being discharged from the tank circuit loading caps in the tank circuit in some odd way.

Assume the arcing is worse as the power level increases?

Does it do it on every band, or are some worse then others?

[w3jn]

Look for a parasitic in the modulator section.  Did you try Fred's suggestion of yoinking the modulator toobs and see what happens?

Could also be a parasitic in the RF section.  Once you have a carbon track from arcing, though, it's much easier for stuff to arc.

[W1UJR]

Look for a parasitic in the modulator section.

Good point, but won't you see that in modulator plate current?

[w3jn]

Yes, but if it only occurs on key up it may not be noticed.

[W1UJR]

Yes, but if it only occurs on key up it may not be noticed.

Good point again, would explain why it does not seem to occur in CW mode.

I never modified my rigs, both of the T-3s were stock, seemed to sound ok once the speech amp mods were done.
But I know that others have, and have good luck with the results.
Always a good idea to do one mod at a time, then test.

[K1HH]

To all:

Thanks for the continued comments. Some additional info to pass along today.

Line voltage 117
TUNE HV 1620 vdc
OPERATE HV 2740 vdc

Have tried both tube and solid state HV; no differences.
Tried different mod tubes; no difference.
REMOVED Modulator tubes.

Keyed in TUNE mode, on UNKEY RF drops to zero immediately but plate (cathode) current increase from 120 to 200 ma before relay releases.

Keyed in OPERATE mode, on UNKEY RF drops to zero immediately, full 250 ma final current drops without additional increase in current. HV overload trips, need to flip reset; sometimes plate relay sticks needs whack to release.

Still looking.

And thanks again to all.

Rodger
K1HH

And yes, I wish I had checked after each modification. I can go back and reverse the speech amp mods, but I discount that as a source of the problem SO FAR since I have removed the audio driver tube (last stage of speech amp) which feeds the modulators and it has made no difference.

On the RF deck I used 470pf caps at 16KV. Most RF deck bypasses are 500pf. And I used two of the 470s in parallel for the coupling to the tank circuit. These are supposedly new Russian caps found in abundance on the net.

[WA1GFZ]

sounds like you need to delay the antenna relay from opening so fast.
If that relay opens while current is flowing through the mod transformer there will be a large voltage spike from the inductive kick.
Just like pushing in the clutch with your foot on the gas

[W1UJR]

Antenna or dummy load OM?

Does it do this on all bands, or just one?

Do u have the T-3 mil manual for service info?

I had a similar problem with T-3 #3, one of the clamper tube pins had corroded off the socket.

[K1HH]

Bruce/W1UJR:

I have replaced the clamp tube with no changes. The unit is terminated in a ME-165 dummy load, and the problems are the same on 75 and 40 meters, the only two bands I have checked. I do have the Militray Manual.

Rodger
K1HH

[W1UJR]

Ok.

Frank has as point, the t/r relay is a vacuum type, operated by 12/24 volts.
So jumper out the relay, so that it is always in the transmit position, always connected to the dummy load.
Try try it, see if the same problem occurs.
Sounds like the transmitter is unkeying before the power is bleed off the tank.
Which is why I asked about the clamper tube. Understand you have a new tube, but are the connections ok electrically?

So try jumping out the t/r relay and see what happens.

[WA1GFZ]

Make sure you remove connection to the rX or you will be repairing the front end for Bruce's test.

[W1UJR]

Make sure you remove connection to the rX or you will be repairing the front end for Bruce's test.

Ah, yes, good point.
And, always, always wear clean underwear and socks in case you get into an accident.

[K1HH]

I'm about ready to go to the shop and look at the beast again. Do I have to wear underwear and socks? I am in Florida you know.

Maintaining my sense of humor, and not getting discouraged, I remain respectful of your knowledge and help. Thanks guys (and gals too to remain politically correct).

Rodger
K!HH

[W3NP]

HV overload trips, need to flip reset; sometimes plate relay sticks needs whack to release.

I am still suspicious of the K6 relay sticking and the HV still on after the load has been removed from the final. Even if it does not require a "whack" to release it, it still may be releasing very slowly while arcing. The more this happens the worse K6 will get.

K6 sticking could be a result of another problem or it could be the problem.

[W1UJR]

I'm about ready to go to the shop and look at the beast again. Do I have to wear underwear and socks? I am in Florida you know.

Maintaining my sense of humor, and not getting discouraged, I remain respectful of your knowledge and help. Thanks guys (and gals too to remain politically correct).

Rodger
K!HH

No problem Rodger, just be careful with the HV.
Political correctness destroyed this nation, so please DON'T be politically correct.

I don't have my manuals up here, so sorry, flying blind from memory, but I thought there was some sort of delayed drop out on the T/R relay, like a pot or something to alter an L/R delay circuit. Seemed to recall it was on the modulator deck, right hand side?