The AM Forum

I thought I would post some data on my T368C.  I recently converted to 4-250s for the modulator. 

The 4-125s on paper will fully modulate the rig to 100%.  In reality. They never reach 100.

Testing into the bird dummy load with the audio gen produced about 70 to 75% modulation positive.  The tips on the scope would flat top anything past this.  This is fine. The T3 runs geat and has for years.   

I got some 4-250s recently and decided to install them.  You can plug these right in but the filiment transformer runs hot.  The solution is to add another transformer under the chasiss. After this, Everything runs cool.  You can remove the stock trans and replace it wtih a 30 amp unit but I like to keep things fairly stock looking. I have heard from T3 owners that did not replace the filiment trans and it ran for years with no issues.

The tubes plug in but its nice to remove the threaded rods that held the clamp system for the 4-125s.

You will have to regap the modulation transformer.  The Gap is to tight for increased modulation and it will arc across.  I just adjusted it out a 1/16th.  No more arc.

The mod current meter on the T3 is adjusted to 2.5 for the 4-125s. This is 50MA. The 4-250s require 100 to 120 MA or about 5 on the meter.  There is plenty of range on the T3 to handle the adjustment. I tested different idle currents and saw no difference what so ever and decided on 80 to 90.

Numbers before on Power master NIST calibrated meter, Tech 2245 Oscope, and Bird dummy load:

MY numbers for MY tubes and MY transmitter. On 40 meters. Your results may be different:

4-125s
1260 watts PEP at flat top.  Scope shows 70% peak. While talking normaly 1100 watts or around 60 % mod.

4-250A
1880 watts pep before flat top.  Scope is showing 120% pos.

Carrier is 400 watts at 200 MA.  500 at 275 ma.  Peak modulation does not change much  with more loading.

Now I can load to 200MA and have 400 watts of carrier and I end up with a full legal limit power out of the T3 just under 100% mod. The signal is spotless clean on the Spec AN.  60! db down. Probably because of the Dual L network. 

I know this is nothing new and this has been done for many years.  I just wanted to post my findings and data on the change. People have swapped 250s and 400s in before I was even alive on T3s.


C

I would never be happy running any transmitter that wasn't capable of at least 100% modulation positive. Preferably even more, 120-125%, enough head room in the positive direction to allow 100% negative without flat-topping on the positive.  Flat-topping on the positive generates exactly the same kind of spurious distortion products (aka "splatter") as overmodulating in the negative. Limiting the percentage of modulation to 70% would cut your total sideband power by one-half!

I am surprised you are able to achieve 120% positive using the stock modulation transformer, even with the 4-250s.  The mod xfmr in the T-368 has roughly the same impedance characteristics as the one in the BC-610.  The total primary to secondary step-down ratio is something like 1.7:1 (3:1 impedance step-down ratio).  This amount of  step-down was deliberately in the design in attempt to avoid even the possibility of overmodulating in either direction, and usually allows modulation peaks to hit the mid 90s before flat-topping occurs.  Better positive peak response could be attained using a replacement modulation transformer with less step-down, 1.4:1 to 1.5:1 turns ratio, since the modulator and final run off the same +HV supply.  Many older broadcast transmitters and communications transmitters, as well as ham transmitters like the DX-100, incorporated the 3:1 impedance step-down in the design as a crude high level peak limiter to prevent overmodulation, but this fails to take into account the  natural asymmetry of the human voice. The mod xfmr in the Gates BC1-T is approximately 1.3:1 step-down turns ratio.  This was designed after the FCC let AM broacasters  run up to 125% positive, and most of the older  transmitters didn't have the capability.

Good information Don.  I can get over 100 easily now. I have a big transmitter that will do 150+ with external audio gear for that kind of thing.  The T3 just runs with very minimal modifications.  This tube swap was sure worth the effort. I too never liked running 60 to 70% mod.

C

A fellow out in Western Oklahoma has one and he had Peter Dahl wind him a custom modulation transformer to achieve the 100% level.  as I recall he made no other changes

The Timtron mods and removing HV in the Tank circuit does the same thing. B'cast audio and a little over 100% pos. with the stock iron.
It will go higher if you load the PA. for 300 out. No one will notice the drop of 100 watts.

You can push it harder by using 4-400's as modulators. And that will require more cooling and a bigger filament xfrmr.
Fred...............I know coz I did it with a T368 about 10 yrs ago.

Fred is there any other reason to do that mod?  Safety?  I am pleased with the sound and do not play music here. I just use a D104.

The one area I am going to address on the T3 is the HV power supply. I noticed that the oil cans are leaking into the bottom deck.  I am going to remove the deck next time my brother is over to help me. I want to get the deck in the backyard. Then remove the caps and clean up the mess.

How/where do I dispose of those caps? 

A buddy has a 32ufd at 5000 volt can.  Thats probably severe over kill here, Maybe it will fit. I am sure Robert has something for me.  Maybe in the 15UFD range?  The T3 uses two 4UFD caps now after the choke for a total of 8ufd.

I spent an hour on the T3 today.  It sure modulates nicely now.  Such a simple swap and an enormous gain in audio.  I had it set right at 1475pep on the air with a 380 watt carrier. I used the T'3s audio limiter and set it to never go over 1500. Thats all I am allowed to run here and the T3 seems happy to do that now!



C

The 4-400 is a 250 with fins, otherwise the same tube. You probably could get a bit more with PL-175A's

Good info thanks Carl.
Splains why Clark's fil. transformer runs hot.

  I recall my Globe King 500 had two filament transformers in parallel for the 4-250 filament. Those transformers ran hot as hell, and I often had issues with low filament voltage to the RF Final tube. Some of this was socket contact resistance where the tube needed a nudge on occasion to light up. The other part was just low filament voltage @ 15 amps from them transformers. This would reduce off resonance peak current capability, and lower the modulation peaks. I ended up installing a larger 6V transformer with a variac on the primary. Problem solved.

   I got no idea about the T368, but for sure the extra filament draw of the 4-250's will lower the filament voltage. But how low? Not sure but if less than 4.75V, I'd not want to leave it that way for long. If the voltage is OK, maybe fuse that transformer primary in case it shorts out inside. Possibly a 2A standard fuse would do? The idea is to have protection since the rigs main fuse or breaker might be rated such that the filament transformer just burns up in case it faults.

Jim
WD5JKO

PL-175As are a kool tube. They WILL out perform 4-400s by quite a bit if all of the operating parameters are correct. However, they are getting to be unobtanium these days. That is also the reason my Invader 2000 will way out perform a T-bolt (both have what physically appears to be the same RF deck.) Just a few extra volts on the plate and screen make a VERY noticable difference in outpoot with the 175As. Without trying to plot the plate and grid curves, I wold guess that the 175As mst have a lot more emission.

My trans does not run hot jim. The T3 has a sturdy Filiment transformer.  It also has a filiment voltage meter on the front panel and a filiment adjustment knob.  Turning it straight on had a voltage Drop of .2 volts on the meter with the stock fil trans.  I turned the knob up to 5.0.  The manual states to run 5.0 to 5.2.  I have plenty of range left on the knob.  Tim said you do not need to change the transformer out but it would run hot.  If you add another transformer, there is no issue.

C

From what I understand of the mod (don't have a schematic), taking the HV off the tank coil allows less rf by-pass capacitance plus loading capacitance to shunt the modulation transformer, thus producing a cleaner high end.  As I see it, one drawback to doing that is that the rf plate choke becomes more critical.  The reason for the original circuit is to avoid having an rf choke directly from the final amp plate to ground.  That is enough enough with a ham rig, avoiding resonances within a ham band, but the T-368 was designed for continuous frequency coverage from 1.5 mc/s up to something like 20 mc/s.  It would be virtually impossible to design a plate choke that did not have a resonant point somewhere in that range.  Tim's modification would work OK as long as the rig was used only on the amateur bands, but care must be taken in choosing the plate choke, just is it is with a typical all-band ham rig, to avoid resonances within one or more of the bands. With series feed, the choke can go to a low rf voltage point on the tank coil, where it is much less critical. Shunt feed relieves the modulator of most of the shunting effect of the tuning and loading capacitors, since the plate blocking capacitor can have relatively low capacitance and still be effective all way across the transmitter's tuning range.

I use series feed on both my homebrew rigs.  The choke goes from the mid-tap of the balanced tank coil, where there is theoretically zero rf, to the +HV line. My link-coupled final uses no loading capacitor to shunt additional capacitance across the mod transformer. Series feed requires the plate capacitor to be mounted on HV standoffs and use an insulated shaft coupling, since the full modulated +HV is on the frame of the capacitor.

It goes without saying that care must be taken in manually changing the final amplifier plug-in coils. I use both the Jesus Stick AND two layers of interlock protection with the HF-300 rig, and at least the Jesus Stick with the 8005 rig, which is not in a fully enclosed cabinet.

Thanks for the post Don.  I studied the schematic and I see that if you do the mod you will be modulating the plate of the tube directly.  In factory form, You are hooked to the other side of the coil at the low impedance tap I think.  I asked W0VMC if he thought it would help me in any way since I use a D104 mic, and he said he thought it would not be worth the effort.  The only real gain would be increased high end in the audio spectrum as you stated and at the cost of reduced frequency range.

C

The Ameritron choke is 225uH and $20 and holds together well on RTTY or digi even on 160 at 1500W with a 1000pf bypass and 2000pf of blocking/coupling.

Some other designs with less L use the choke as a part of the 160M tank where its a very high impedance at some point down the choke.

Alpha recently went with a design similar to the Ameritron for their 2X 4CX1000A amp.
http://media.photobucket.com/image/alpha+8410+/wsmc551/Ham%20Radio/_PAS0127.jpg

I use that plate choke in the 2x4-400transmitter with great success.

Here is my power supply out and in the back yard.  Are the Transformers and choke filled with oil also?  The choke has oil all around it.  The HV insulators are dripping oil. The bottom of the Chassis is wet and has the largest amount of oil.  Its hard to see in the pics here. The top side oil has alot of dust and dirt on it (live in AZ).

I got a 12.5 UF 4000 volt cap comming.  I am going to use it to replace the two 4UFs.  The choke has me worried.  If its filled with oil like I think you guys are going to tell me it is, I will have to replace it also :(

C

Another picture.

The choke is fine! AlL the oil is from one of the 4 UF caps. The trans and the choke are leak free.  ALL cleaned up now and waiting for the cap to arrive!

C

I bet it wall still play fine with only one 4uF capacitor in place if you feel the need to get on the air before the new one arrives.  Glad it wasn't the choke.

Thats how it was K3ya.  I just tested the caps,  One is 4.5 and other is.... 0.   Looks like i ran that way for years :( 

Gotta dyno tune a race car at 1 so no time to finish it today.  I will clean it up tomorrow, scrub the chassis down and then install the new 12.5/4kv cap :)

C

check the ripple with the bigger cap... I have had it happen where on some choke input supplies when the cap is made bigger, it does NOT help reduce ripple, it increases ripple. In which case you may need to "tune" the choke with a small cap in parallel.

Others may have more experience than I with this...

                   _-_-bear

That tuned choke idea sound interesting. Ever try it? Supposed to make a high impedance at 60hz by resonating the choke with the parallel cap.  I seem to recall needing a fairly high voltage rating for the cap.

Patrick Turner has a good analysis of power supply filter networks, including choke input:

http://www.turneraudio.com.au/powersupplies.htm

"Power supply resonance
Since we have an LC filter, it will have a series resonance and the RL of 800 ohms we have is not a low enough
R value to damp the resonance. For the power supply to have a flat response without a peak at Fo
RL must be = 1.41 ZC or ZL ( both equal reactance values at Fo ). This is something you need to remember for all LC
or CL low pass or high pass second order filters.
The LC filter is a low pass second order filter and behaves just like any other LC filter you may use at RF
or at AF in a speaker crossover. The frequency response of an under-damped LC filter will be flat at a couple of octaves below the Fo, and then a peak occurs at Fo, and then the response rapidly rolls off after Fo
at an ultimate rate of 12dB/octave."

The point is you want Fo well below the lowest modulation frequency, and certainly much lower than 120 HZ.

"Frequency of resonance for any LC circuit, Fo =  5,035  /   square root of ( L x C )     
                                                                                 
where Fo is in Hz,
5,035 is a constant for all equations to work,
C is in uF, and L is in milliHenrys." 

Jim
WD5JKO

The cap is after the choke. Should be ok.  Talked to a T3 owner that has a 20 UFD in there now. No trouble. Also common is putting in a larger cap in 610s. Same thing. No trouble, less hum.  I guess we will find out soon enough. Knowing my luck....   :-[

ANyone got any tips for cleaning out that oil grime?   I spent an hour scrubbing with dish soap, then Fantastic.  Then hosed it out.   I got some out but not all. Its very very stuborn.  Dennis gave me some tips that helped but still alot of oil in and around every nook and cranny.

Tomorrow, I thought of trying Acetone and rags.  That might be my only option here.. Maybe some non clorinated Carb cleaner? 

The new cap is here.  ITs less then half the size of one of the factory 4 UF caps.  Same format.  I am going to cut the brackets down and put them in a metal brake tomorrow at work.  Then, the new 12.5 UF will mount like factory.    I have to extend two 14AWG wires and she is ready to be tested. 

Thanks for any tips.

In a FW supply its 120 Hz that you want to filter and the cap better resonate it about 5% high unless you want fireworks as it comes up thru resonance at turn-on. Henry and a few others used resonant PS for years but as components aged failures became common as values changed.

Also its not easy to determine the actual Choke L with commonly available equipment or to select a precise resonating C value from whats available.

ITs an 11H choke. 

C

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.

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

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
 

  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

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

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.

So your smug?

C

Bah!! HUMbug!

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


C

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!


  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!  ;D



Jim
WD5JKO

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

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

Set the trigger to Line to get a better display.

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.

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

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. ;D

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

Who is making you listen?  ;)

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

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

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  ;D   Its now free of PCB oil caps and seems to run strong.

C

 

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.

No grid leak on the tube would do it

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

Random pics of my T3 with the oil slick cleaned up and the New non PCB 12.5 MFD 4KV cap in the PS. The pics also show the new Fil transformer installed in the open location on the modulator deck and of course the larger modulator tubes.  Pictured is 4-400s.  I later swapped back to 4-250s.  The 4-400s needed more idle current to hit the same power and since I was not going to run over 1500 anyways, I thought it was a waste.   

C

last one.

Update.  More complaints from the peanut gallery.  Hum on the signal.  WTF???

This time I called W8QBG and had him listen. He said there was a slight 60hz signal with some FMing. I cant even see it here. I could hear the Beat change a bit while using the BFO on a reciever to get a steady tone.  waw waw waw as I could modulated.

I started pulling tubes in the audio section. Then all the way back to the exciter. It was in fact comming from that exciter unit. I could even yank the intermediate PA tube (6000) out and still here.  I got down to the two PTO tubes.  I pulled one, Still there.  I pulled the last one and of course, I lost the signal.  I noticed that some FORKING $*($&# E*(#_)(*$ #_)(#!!!!!  put a 6ah6 in the left side PTO!  FORK!!!! I was SOFA KING pissed I missed that the other day! when I checked all of this.

I got a new 6ba6 out and installed it. The dials off. I had to then realign exciter.

Why do hams rig things up?  I mean really.  WHY?  A 6ba6 is $2.  Why not get one and put it in the PTO rather then just insert some incorrect tube and then Realign the Exciter to make it work? 

FMing is gone.  Hum is cut in half or better.  Keyed up 500 watt carrier on 1885 and QBG said its so low that he would not notice it. 

Hooked up the Audio tubes again and found some hum return with the insertion of the first and second preamp tubes. 

Zoomed in using Spec AN and see a blip at 60hz that is over 52 DB down from carrier.  I think thats pretty good right?  I mean is some band cop complains at a 52DB down Hum on my signal, Fork him  :D

I believe the only way to get this lower now is to DC the filiments on the speach amp.  I can vary the slight hum with the audio gain control.  Kinda like on a PA system or audio amp. Turn up the gain, The hum gets louder. I am going to make a new post about this so we can start a new discussion on common practices for filiments on Triodes.

Learned alot.  Kept me busy.  I think I got this rig running decent now. These issues bugged me for 4 years now and needed to be dealt with.

Thanks for all the help my fellow AM'rs

It's certainly something how a transmitter (and many other types of both electrical and mechanical items) can carry on and operate quite well even with a number of problems present. We think everything is okay until we really go through it and make sure everything is according to spec. I know it's nowhere near as complicated but that reminds me of my HQ-129X. I've had it for years and it has always worked very well. I've finally decided to go through it with a very fine tooth comb and I've found a few resistors that have gone 50% high!

Thats a very good point TAU.  These old rigs are very robust when it comes to things like this!

C