The AM Forum

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

.

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

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

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

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

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

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

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

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

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.

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?

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.

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

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.

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.

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

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.

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

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.

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. ;)

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

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.

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?

I thought the "F" series had sealed relays. Not that a sealed relay would not stick, but should be less trouble.

I'll look thru my junque and see if I kept a K-6 relay. Help my memory banks.....it's a grey metal plug-in type on the HV chassis???
If I have one from a former T-3, I'll send you one fer free.


Fred

Slow release relay, K1A?

The test I mention should tell you if the T/R relay is dropping out too soon.

Check your service manual, used in early series of TX, not sure if its still there on the F model.

See page 39 of depot service manual, the heory is explained there.
You'll have to check with someone who has the "F' supplement to see if it still applies.
Go back and check your work, see if you might have disabled said circuit on your rig.

http://atlanticmotorcar.com/w1ujr/t3.jpg

I'm at the shop now and will start checking some things again. 
Bruce:
I'll look and see if I have the timing circuit.

Fred:
K6 in this F Series rig is a sealed relay, apparently is bolted in with three bolts.
In this rig it is a 2 X 2 and 1/4 X 3 inch relay made by Price Electric, Type 5701-37HS 115v 220 ohm relay with 20 amp 120 volt contactor rating.

Bruce's test is the first thing you should do to verify the TR relay is opening too quickly. If that is true then you can find out why. Define the problem before you try to solve it. Also with the relay shorted you will see how much time it takes to discharge the system giving you a clue as to the delay you will need need.
A sealed relay will stick if it has been stressed too many times.

I do have a T-3 K-6 relay. If you need it PM me.
I think it's ok, but I may have shotgunned it and used a Potter Brumfield just as an insurance type thing. The problem you are working on has been around since we started picking up on these T-3's when they were within reach of reality $$$$. Middle 1990's. I paid $750 + shipping from Fair Radio and it worked right off the pallet.


Lemme know

Fred

Haven't heard from Rodger for a couple of days, hopefully all is all right?  ???

Keep away from the HV OM.

Thanks for your concern. I have avoided the HV. I try to be real cautious around the stuff. Earlier in my life as an engineer for 50KW-9 tower WDGY in Minneapolis two other senior engineers always let me clean the arc blocks each week. And I had to use the chicken stick to discharge the HV caps before reaching to get the blocks. I knew it was going to spark, and each time I discharged the caps, I'd jump.They'd laugh. Every week.  Actually I am spending some time with the XYL this weekend. It also gives me a little time to think about the T-3. Friday I did swap out the tubes in the timing circuit (page 40 for the lettered model units) and varied the pot but noticed no difference. Plate relay hung up again. May get to the shop Monday or Tuesday. I'll keep everyone posted.

Rodger

Hello all:

I'm still alive and I have replaced K6, the high voltage relay.
Relay is in and works well. In the CW mode at full power it goes in and out of transmit with no arcing and no relay sticking.

In AM mode still having the same problems. I have disconnected the plate leads to the mod tubes and removed the audio driver tube from the speech amp and still have the arcing problems. So I don't think its any mod I made to the speech amp. Maybe its the HV routing through the mod deck.

I still have not confirmed the antenna switching relay timing, although I am not currently having a problem in continuous carrier mode.  I'm not using a key, but turning the carrier on with the plate relay switch, which is in parallel with the push to talk switch.

I'll keep plugging away.

Going to Hamcation in Orlando this weekend. See what other junk I can acquire.

Again thanks for the relay and support.

Rodger
K1HH

Today I finally solved my arcing problems, and the solution was far simpler to solve than see. My thanks again to all who resonded to this forum and especially to Mark KA4JVY who brought his T-368 RF deck 150 miles to my place so we could confirm I had an RF deck problem rather than a mod deck problem, since I had modded both. I walked away from the T-368 for a while but it came to be yesterday and took 15 minutes to fix today. It had to do with where I was feeding the HV and I have taken before and after photos and hopefully its easy to see the error of my ways.

Rodger
K1HH

Wow, no wonder.
You could have just moved the suppressor to the other end of the strap near the caps. This would have reduced the lead length to the plate tuning cap. Consider that if you have any stability problems

WOW!
That would have been a looong time for me to pcik up on that. A lot of long head scratching.

Fred

Glad you found it Rodger. This illustrates why it's important not to shotgun an entire rig - receiver or transmitter - but better to work on each section and then stop to test functionality before moving forward. It's always easier and faster to go back to the circuit you just repaired or changed when a problem crops up.

Should be pretty much bullet-proof going forward as you've checked everything else. ;)

Thanks guys! During this whole procedure, I actually tried a National 175A plate choke and, of course the original 470 pf Russian caps. When that didn't work I searched for the 500 pf 30KV caps that are in there now. And I've had other caps clipped leaded in various places convinced it was a bypass problem.

In my original, stock T-368 the supressor was connected directly to the plate on one end and the other end connected to the tank coil because B Plus is fed in from a low imedance point instead of the high impedance point where it now comes in. For some reason, (brain flagalation, I suspect), I hooked it up the way I did. It would send out RF fine, but it didn't like coming out of transmit. In hindsight, I wish I had posted my original configuration photo. One of you guys would have spotted itin a heartbeat, I'm sure.

Rodger
K1HH

DC was fed into the low Z end of the tank coil for a reason. When you change to the high Z end the RF choke can resonate and fail. This is not an issue for the ham bands but operating over the whole range there could be a frequency that turns the choke into a fuse. It is good to get high voltage off all the caps and switches but there is a price. It is a good idea to check the circuit with a grid dip meter to make sure there are no issues on the ham bands. This could be the reason the R175A failed.

So, there was a method to their madness in the design of the T-3.
The same can be said for the modulator and its "shortcomings". (For Ham A.M. there were hurdles there to overcome)

Fred

This entire project started with the idea removing high voltage from the Ip meter by making it a cathode current meter, and to remove B Plus from the tank circuit, and along the way decrease some of the shunt capacitance in the tank circuit. Also implemented are some changes to the audio driver circuits, to hopefully, expand the unit's frequency response. The only way to really know if this works is to run "proof of performance" tests before and after the modifications. I did not "proof" it in its stock shape, but I hope to in it's current state. Just need to borrow a THD meter. I didn't burn out the R-175A choke; I only inserted it in place of my home wound coil and obtained the same results (arcing). Had the R-175A been installed in the correct configuration by me, I think it would have probably worked for the 80-10m amateur bands. I do acknowledge that the designers of the T-368 knew what they were doing when the fed DC in at the low impedance part of the circuit. Their choke design setup offerred wide frequency coverage in a very rugged package. My 1947 Ford worked just fine as it came from the factory, but I had to put Edelbrock heads and multiple carbs on it too.

Rodger
K1HH