T368 with 4-250s

[ke7trp]

ITs an 11H choke. 

C

[K3YA]

My Collins 30K's use a "Tuned Choke", a small capacitor across the input choke of the plate supply. They also use little 2uF filter capacitors  in a two section filter.  Never a problem even though the components are very well aged.  The tuned input choke keeps the supply output voltage from soaring when the load is removed and allows the use of a relatively small bleeder resistor. 

I've been planning one increasing the size of the 2uF plate supply output capacitor for abut 25 years now, but never seem to get around to it.

[ke7trp]

update

I got the PS done.  The new cap looks great and is mounted.  The oil was cleaned up with NON clorinated Brake parts cleaner. That works the best.  It simply cuts the oil and runs it off into rags.  The real stubborn areas and grime took elbow grease. 

The PS is now oil free and spot less.  The new 12.5 MFD 4kv cap is mounted and installed.  I measured it with my new cap checker and it tested at 14.6 MFD.

I ran new HV lines from the choke up using HV wire and ring terminals with heat shrink tube.  Looks very nice.

I set the supply aside and focused on the modulator deck.

I decided to get the Oil caps out of that deck as well.  This T3 is in the house and I dont want the PCB crap in here with me.

I replaced the two 4UF oil cans and the two 8 UF oil cans.  I used 16 UF 800 volt electrolytics.  Mounted and installed with heat shrink tubing. It looks great.  I hope to have less hum.  Two 4UF caps is not enough for a bias supply in my opinion. 

The new filiment transformer is installed.  There is plenty of room. I now have a 30 amp 5 volt supply for the two 4-400s.  I decided to take the 4-250s out for now. One was an eimac and one was an amprex.  I had two matching amprex 4-400s so why not use them?  If they dont work out, I will go to the 250s again. I know I had a second Eimac here.. I just cant find it right now.

Tomorrow is the smoke test. i cant lift the PS deck myself.  A friend is going to help me in the am and then i will fire the rig up.   

C

[ke7trp]

Tested rig out today. The 12.5 MFD 4KV oil can worked perfectly. No AC at all on the HV supply. Its clean.  The tuned choke thing is out the window on this cap and my T3.

The 4-250s work great. The new filiment transformer runs cool.  There is a spot up front where you can mount it.  Things look factory the way I like it. I had to replace the 3 amp fuse with a 5 amp unit on the front socket.  On start up the 3amp blew. After that, The rig fired right up. 

I still have some classic T3 hum on the signal.  Its minimal.  I tracked it to the Exciter. I turned the HV off and listened to the Exciter and can clearly hear and see the hum on the scope.

Robert suggested a fil to cathode short.  I tested all tubes.  The 6000 tube showed  a short on the Tv7!   I tapped the glass and it went away.  The tube tested full output. I put it back in, Same hum.  I need a new 6000. This one has some rattling parts in it. But seems to work fine after I knocked the particle loose.

I removed the 6000 tube and and ran exciter without the 6000 intermediate tube. The Hum is still there on the monitor rxer. 

Frusterating.  I measured the plate voltage at the 6000 tube and it shows around 400 volts DC and 0.580 AC.  So over half a volt of ripple on the exciter power supply.  Is half a volt considered alot here?? 

I am considering the fact that one of the chokes is bad on that supply.. I replaced the oil caps and installed 22UF 500 volt electrolytics here. I gave up for the day. I await further instructions on the amount of AC hum in relation to the DC output of that supply. Thanks for any guidance here guys..

C
 

[WD5JKO]

Frusterating.  I measured the plate voltage at the 6000 tube and it shows around 400 volts DC and 0.580 AC.  So over half a volt of ripple on the exciter power supply.  Is half a volt considered alot here??  

I am considering the fact that one of the chokes is bad on that supply.. I replaced the oil caps and installed 22UF 500 volt electrolytics here. I gave up for the day. I await further instructions on the amount of AC hum in relation to the DC output of that supply. Thanks for any guidance here guys..

  Clark,

   I'd be a rich man for every time I've heard hum in a monitor receiver that wasn't there as the transmitter went to the antenna clean without hum. Was that hum present before you did all the work on the big 'T'?

   I tend to use a monitor scope on the RF output (not receiver IF), and set the horizontal timebase to "line trigger". Is there 60 or 120 HZ modulation there? If there is, 60hz might be heater-cathode issue somewhere, and if 120 hz it is likely power supply related. If there are any half wave power supplies in the 'T' then this would also be 60 hz, but the pattern on the scope will be different than a heater-cathode issue.

  Keep in mind that a class C RF chain tends to "limit" any AM modulation passing stage to stage much like the "Limiter" does in an FM receiver.

   Any chance the hum you are hearing (if real) is FM? Switch on the BFO, and listen to the carrier.

  So the ripple number you gave is way below 1%, and even closer to .1% rms ripple. I would consider the 400 v supply clean.

   Is that new filament transformer using the centertap like the original?

Jim
WD5JKO

[ke7trp]

Jim, Thanks for the reply.

I am using an O scope on the RF of the T3 to see the signal. I am also using an HP spec AN.  It shows two 120hz spikes and two 60 hz spikes.  Also, My SDR reciever can see the signal.

You can also hear the hum on the air. But I agree 100% with you about hum on a nearby receiver!   Good point.

This is not a new problem.   All T368s hum.   The power supplys are not filtered very well, they use a string of Triodes with AC filiments.  Every T3 I have ever heard hums.  My Buddy has three of them. I listen to his all the time, They all hum.  I listen to 3 or 4 more them hummming right along. 

Robert confirmed that everyone he has dealt with hums like crazy.  Its just the way they are.  He said he got one of them not to hum but it took major surgery including DC filiments.

So the radio is in working order.  STill.  I would like to find out where the majority of this hum is comming from and eliminate it.  Its just something to keep me busy.

The Hum is there with the HV off.
The hum is there with the Modulator OFF
The Hum is there on the output of the Exciter
The hum is there with the 6000 tube removed.

This means to me that the source of this hum has to be inside the T3 exciter itself.

If the 400 v supply is clean, Then I must have a bypass cap inside the Exciter that is leaking.  That or the Tubes themselves are giving some hum?

I took a video of the signal on the scope.  I picked up the factory carbon mic and modulated the rig a bit.  I am told that if I excite the rig externaly, this will go away.  I attemped to do that, But could not get enough Grid drive from my FT450.

http://youtu.be/Fq2ckq3v_xQ

If it cant be reduced, It will go back on the air.  I used it this way for years without to many complaints.  Sometimes, A a band cop would come on and scold me.  But all of us have dealt with Band cops from time to time.

Clark

[KM1H]

I would have thought that any self respecting op would have taken pains to fix that hum problem once and for all and published it long ago. Likely a ground loop.

Im not a band cop but I dont answer Humming Hammies CQ's or join a QSO unless its to discuss the problem.

[ke7trp]

So your smug?

C

[KX5JT]

Bah!! HUMbug!

[ke7trp]

Its the HUMTASTIC T368 JON!  Providing a source of discussion for band cops since 1951!


C

[WD5JKO]

Clark,  I remember a similar issue at ARS W5PYT, Ozona Bob. He had a Tek-Rad military transmitter that was a pair of 813's modulated by 805's. This rig always had hum until Bob tracked it down.

The rig is now two owners later still modified with a piece of plywood shoved under the exciter chassis. Hum gone!

This means to me that the source of this hum has to be inside the T3 exciter itself.

  Ya know, the BC-610 had hum when using the internal VFO. With that rig the turn on "Klunk" and then the hum heard on QSB dips was the signature of the BC-610. So if it cannot be easily fixed, then feature it! 

I would have thought that any self respecting op would have taken pains to fix that hum problem once and for all and published it long ago. Likely a ground loop.

Im not a band cop but I dont answer Humming Hammies CQ's or join a QSO unless its to discuss the problem.

Jim
WD5JKO

[ke7trp]

I posted right when you did... Sorry Jim.

Thats a great idea.. I think I might place tape on the exciter rails and then some rubber o rings on the front where the exciter hits the chassis. I bet that hum is 100 % gone.  Its low level and I am being picky.

The hum on the video is from the speech amp.  Even in CW, that speech amp is still lit up and running.  I wonder if I should try to isolate that for a test.

C

[WD5JKO]

Strange ripple pattern...see attachment.

I wonder what the time / division was set for in this snapshot from the video?

Looks like you got a symphony instead of a single baritone. 

Jim
WD5JKO

[Steve - K4HX]

Set the trigger to Line to get a better display.

[flintstone mop]

it looked like an RF kinda hum. Ground loop?
Hopefully the plywood will stop it.
I agree that it would be nice that the carrier would not have any jagged edges, when there is no modulation.

[ke7trp]

That ripple is gone with the speech amp is out of the rig.  My back hurts from lugging this damn modulator out.

[KM1H]

So your smug?

Far from it; I just dont enjoy listening to poor signals on AM especially when the carrier is always there. Windbags like W5PYT made it even more objectionable. There are enough rotten signals on 11M and SSB to go around.

[ke7trp]

I am not above anyone.  I dont down talk people.  I dont care if a guy has a couple cans and some string. I will say hi.

C

[Steve - K4HX]

Who is making you listen? 

So your smug?

Far from it; I just dont enjoy listening to poor signals on AM especially when the carrier is always there. Windbags like W5PYT made it even more objectionable. There are enough rotten signals on 11M and SSB to go around.

[ke7trp]

Does anyone have a T368 speech amp laying around?  I need some photos of the under side.  I found two white wires that are cut.  I also see someone changed some caps in the bottom. I did not notice this before when I went in to jump the filter about 5 years ago.

Some pictures would be helpfull.

C

[ke7trp]

I spent a few hours on the speech amp.  Found a wire cut.  On the schematic it goes to ground.  Its pulled from the harness and cut with snips and then tucked back in.

I also noticed that this speech amp is the A model.  My transmitter is a C.  The A is very different.  The rest of the models are the same.  That wire might be snipped to make this work in my C rig.  No idea. 

I have decided to search for a speech amp replacement. Then maybe use this one as a chassis to build a hifi unit from scratch.  I hope I find one.  This speech amp is really not the right unit for this rig and its more work to convert it over to the C style amp. 

If I cant find one, I will probably just purchase an 8 ohm to 15K audio transformer and run the Speech amp I cobbled together last year.  I hate to rig things up like this.  But I am sure it will sound better then this POS A speech amp anyways.  That speech amp I(we) made last year is very clear and has a D104 input on it.  I just like things to work the way they where supposed to work.

C

C

[ke7trp]

After a nap I got some more energy.  Ever lift a T3 modulator deck in and out of the Rig 9 times in a day?  I think I will be in traction tomorrow. 

The cut wire was some hams idea of removing the High pass filter. He cut that wire and that removed the ground on the filter terminal 2, I put that wire back.

I toned out each wire and traced them on a schematic printout.  I hooked the High pass filter back up.  This filter has 50 DB at 100HZ.  I dont care about sounding like Ted baxter and dont want hum.

Then I started tracing the path of audio through each tube one by one.  I found the Grid Leak resistor for the second half of the Second audio tube was missing  The coupling cap was there. But no grid leak.  A ham changed this cap from a .01 to a .05 and forgot to put the Resistor back.   I have no idea how this effects things.

I then changed the remaining coupling caps to 047 orange drops.  They where 01 and 02 factory. Some where leaky.  Some had burn marks from some yahoos Iron.

The last thing I did was to test each resistor in the rack one by one. I found two, way out.  I changed them.

I tested the 6al5 for the clipper and it was dead.  100% dead.  I replaced it. I had this working before.  But it died or I killed it in the process of working on the S.A

Back in the T3 for the last time I fired it up and had strong clear audio. The Hum apears to be gone on the scope with the audio turned down. I listened to the rig on a near by receiver and of course, here the hum that jim talked about.  I sure do not see those spikes on the Spec AN now though.  I loaded the carrier to 385 watts,  I set the clipper at 1450 and turned the audio up to baseline on the scope and backed it off a touch.

To my suprise, I found full scale AM signals on 40 meters here at 9PM,  The east coast guy gave me a good report,  Clear, Loud audio, no hum.  The station on the west coast said the same thing.

I will test tomorrow after work on the air. This thing may be good enough now.  I am going to do a right up on how to remove hum from that exciter unit, bias supply and Exciter supply.  The speech amp issues apply to my own crappy speech amp. If you have a good working Speech amp, Then there should be no trouble there.

I am looking for a spare Speech amp and will buy one when I find one.  Then, I can swap them out and have a spare.

Just one guys weekend with a T3     Its now free of PCB oil caps and seems to run strong.

C

 

[flintstone mop]

Hi Clark
That is good news you found the culprit.
I have had that hum pickup on a receiver that was not part of the radio system.
Radio system meaning, not actually connected to the T/R relay, just picking up the stray RF in the shack.
So you're good to go.......ONE DAY I might catch you somewhere on the lower bands.

[w3jn]

No grid leak on the tube would do it

[ke7trp]

While researching this issue I found this webpage to be helpfull:

http://www.lynx.net/~jc/hum.html


I was going to try the hum balance pot on the T3 speech amp. I think this would probably help.

I noticed the second circiut at the bottom of the page where they run 4 volts DC to the center pin of the hum pot. 

Anyone try this???
C