The AM Forum

Hum the carrier on the T3.  No hum in CW mode. 

Pulled the modulator deck out and found all the old caps in place. These caps are metal with studs.  Most test leaky on the tester. Figured I would just replace them all.

The 8.0 and 2.0 UF 350 volt caps are a no brainer. Going to use some modern 10s and 4.7s.

The others are .25MFD at 600V.  Kinda large, Metal can with studs. Says "good all" or Guddeman on them.  Can I use .22 orange Drops?

The screen bypass caps are for sure toast.  They are a paper tube type cap and they are swollen.  They are stamped .006.  There are two in P.  Can I use a ceramic disk here or do I need Silver mica? I have some Silver mica caps like the ones the valiant uses in the tank. Sangamo brand. Will these work or should I just use disks? 

The "rock crusher" as the T3 was nicknamed is running strong.. Just want the hum gone. Its slight but I can see it on the scope and sometimes people mention it.

Clark

I don't think it will matter what type caps you use, as long as the voltage rating is as good or better then the originals.

Brett

Ok.  These are "Mica molds".   I found two that where testing leaky.  So they have to go.  The Screen bypass caps are toast. They are bulged.  I will replace them with Mica sangamo's.

I think I found the real problem though.  When working on it, I noticed that the wire on the ground side of the Oil can cap was broken!  These two oil cans are the main modulator HV supply.  I tested the oil cans and they charged right up with no leak on the tester. They also test at the correct capacitance. It almost looks like someone cut this wire. So the HV supply had only ONE cap in service.  I hope this solves my problem. I am going to solder it back and test. If the hum is gone, I am not going to replace all those mica molds. I hope it will be back in service tomorrow. I really want to use this thing for the contest.

Clark

Ok. I used Orange Drops to replace the two leaky caps.  I used Silver micas for the Screen bypass caps.  I repaired the Broken lead on the Oil can capacitor with HV wire. Its ready to back in the rig. I just through my shoulder out trying to move this deck.  I am now waiting for my Brother to get over here to help me install the deck again. If its fixed, I will be on 7293 AM around 11 to 11:30. 

Clark

Its not fixed.  Even with the oil cans back in place and wired up, The Hum is still there. Must be the 8 UF Mica mold caps in the speech amp.  I ordered all new caps. When I get them tomorrow, I will sit down and just replace them all.

Clark

Here are pics of the O scope.   

The one with the peaks is the AM carrier. No audio.  Gains down and audio unplugged. 

The picture of the smooth carrier is CW. 

Clark

Clark,

   I wonder if that hum modulation is 60hz or 120 hz? If it is 120 hz that is power supply ripple, and your on track to a fix. If the ripple is 60 hz, your replace all caps idea will only fix the issue if there is a half wave rectified supply like a bias supply. If there are NO half wave supplies, then look for a low level tube with a heater to cathode short.

   I use a Gonset GSB-201 Linear amp (with 4 type 572B tubes), and this amp caused hum modulation (60hz) similar to your scope plot of the T3 on AM. I replaced the HV caps since they were old, and I thought the problem would be fixed. Nope, the hum remained. The bias supply was half wave, recapped that too. Still hum.  >:(

   The hum turned out to be a design issue. The 572B filament supply was not center-tapped, and the Gonset folks grounded one side. Some dumb engineer designed in 60hz cathode modulation into the GSB-201! I floated the filament winding, made a resistive center-tap (pretty low resistances), and the hum went away.

   So on your T3, making the assumption that a t3 should be hum free 'as-designed' might not be wise. After all, the BC-610 which preceded the T3 was certainly NOT hum free.

Jim
WD5JKO

Damn good Point.  It might be by design.  Thanks for the info on the Cathode and filament.  I get the caps today. I am goign to install them anyways. The old caps are old :)  Some are leaky. Maybe I can get the thing to work just a little better. If the hum goes away, I have won. If not, I will keep plugging away at it.

C

Jim. I looked, the T3 has a balanced fil system.  I am going to check all those .001s If one is bad, It could induce hum. Also going to check and or replace the coupling caps.


C

Ok. I replaced every damn cap in this modulator.  ALL of them.  I used Orange Drops and modern electrolytics.  I also put in Fil Bypass caps since this rig does not have any. I figured what the heck why not try it. 

The Hum is still there.  Man this is getting to me.  THe audio is nice, The power is stellar.  But there is that damn hum right on the carrier.

If I go to CW there is no hum.  Anyone have any ideas?  I have checked all grounds also. I just dont see anything that could cause it. I am told by alot of T3 guys that these old Rigs hum. ALL of them.  The hum is slight but its there.  I heard hum on another T3 today.  Its not just mine.  Should I just live with it or keep plugging away?

I could really use some help.
Clark

I'm not familiar with the chassis, Clark, so some of my ?s might be out there....

Does it have a mic gain?  If so, does it still do it with the mic gain turned to zero?  How about if you short the mic jack, still have it?

Have you tried checking the wire going from the mic jack to the pre-amp / modulator?  If it WAS sheilded at one time, something might have rubbed against it, etc. causing the shield to fray....  OR, maybe you should try a coaxial type audio lead to the mic jack..  If it's balanced, run dual, and short the shields together and ground them on both ends.  Some of these old radios used twisted pairs for shield...  Maybe this one?

What is the difference between CW and phone..... ie, What is the B+ path?  Is the modulator completely taken out of the circuit so it's a flat run from the B+ supply to the final? 

And a longshot, maybe you have a leaky diode in the supply feeding the modulator?  Since you've replaced the caps, and ripple is still there, something is inducing it.  If the fils are AC (and balanced), then I'd be looking in the audio path next.  If you can disconnect the lead(s) that run from the audio circuit to the mic jack, key the TX and no more hum, then you've isolated it to the mic run...  If not, then it's something farther up the chain.

Hard to troubleshoot a story, but there's a few I've seen.

--Shane

Clark,

   I was looking at the T368 mods by Timtron, and one sentence stands out. " ground loops that will occur between the modulator mainframe and the speech amp". If this is true, then there might not be an easy fix.
I am assuming the hum here is 60Hz. If it is 120Hz, then that still suggests power supply ripple.
 
Maybe others with specific T3 experience can chime in with their experiences.

   So how difficult is it with a T3 to expose the circuitry so you can probe the stages with a meter or scope while the the B+ is on? Do you have to make connectorized harnesses or something like that to get the audio stuff out on a chair, or on top of the rig?

    You could always pull one tube at a time from the 1st audio stage forward to see where the hum is coming from. Then zero in on that stage. It sounds like design issues are on the table here, so looking for that "bad component" might not pan out.

Good Luck,
Jim
WD5JKO

That would be what I would do, pull the modulator driver tube, if hum is still there, forget all the stages before that.
It could be vibration into a low level audio tube, a filiment to cathode leakage, induced hum for an AC field, or something else...

Brett

The Hum is internal to the Rig. No mics hooked up into a dummy. It has hum. 

You cant probe the thing with the power on. Its a 200lb deck and the connectors are two huge Cannons and the B+ leads. Must be 50 pins or more.

Shane, In CW mode, YOu are not using the Speech amp and modulator.

To pull tubes is a bear.. I will have to try this. I am going to BET this hum is in the speech amp.

I am told its 120HZ ripple.  Several people with scopes over the air said 120hz.  I have studied the schematic and I have noticed that the Oil cans are small. The Big PS only has 4 UF!   THe modulator has even less. I wish I could measure ripple, But I cant. 

Please school me on 60 vs 120HZ ripple please.

Clark

The big power supply must be OK since on CW you have NO hum. That said, on AM with only 4 uf, your lower voice frequencies might modulate the power supply somewhat instead of all of the audio going to the RF PA B+. Opportunity for improvement there.

If the Modulator power supply has less than 4 uf, and the ripple is 120 hz, then increasing that capacitance would be a good thing to try, especially IF the hum modulation remains with the 6AQ5 audio driver tube removed. Of course the 6AQ5 B+ could also have ripple on it from a different source.

So with a scope, 60 hz noise, one full cycle takes 16.67 ms to complete. With 120 hz noise we need 8.33 ms for a full cycle. That monitor scope on your desk won't do; you need a scope with a calibrated horizontal time base.

I would wager here that this hum you are experiencing is the sum of multiple issues, and not a single problem. Got to love this stuff!

Jim
WD5JKO

TFO came over this morning to Drop off some manuals. I showed him the old Chap.  He said I have multiple issues right off the bat. 60 and 120.  We poked around the mod deck and manual. 

In short, What a mess.  This thing was designed to never produce low end. The R390a wont hear the hum. So it was never an issue.  We now see why most guys GUT the mod deck and speech amp and start over.

I am going to try to put some more UF on the speech amp today. Its only got 4 UF.  Two 8s.  I have plenty of 22s.  Might as well beef this area up.  Also. I am going to see if I can get some more UF in the Modulator B+.  Not sure If I have the voltage required. The bias supply needs more filter cap.  I am going to put cap in there.  I have to look at the caps I have. 

The other big thing he noticed is the 4-125s have AC filiment right on them with a Center tap transformer. Big oportunity for hum here. I could build a Pot into the trans to offset this hum like he suggested.  I have seen audio amps this way. You turn the Pot until the hum goes away. 

I really dont want to hack this radio up and mod it heavy.  So I think I am done.  It is what it is. NOT a Broadcast rig.

Maybe hook the filters back up and put a D104 on it and call it good.  It wont be hifi but it will be nice to talk on once in a while.  Its a super nice,clean rig with lots of Key down.  It sounds decent and thats where it ends. 

Clark

Uh, maybe I am wrong, but are not most modulators using ac on the filiments with the center tap to ground?
That is how I do all my modulators and no hum...

Hack hack hack, you are always talking about hacking, there is no hack in improving things as long as you don't make any holes and do nice work.

Is it hard to slide the deck out and pull the modulator grid driver tube?

Anyway, when I did mods to the 32v3, I had hum with the d104 and hacked the thing up to run the mic amp filiments on DC. A diode and cap did the trick, its not pure dc, but it does not have to be to get rid of the hum.

Funny, but I always thought improving and even fixing the old gear was fun, otherwise, why not just run a flex into an amplifier?

What would your friend want you to do with the thing?

Brett
 

Yeah I know.  This rig has value to me.  I wanted to keep it near stock with some decent audio. 

I Got the rig back together. Its not as bad now. More cap on the bias and speech amp helped.  Its still there. Just about half as high. 

Thanks for all the help on this to everyone. I had some phone calls and some Stop ins from people trying to teach me how to handle this old rig.   I am on the air now with it on 7293.

C

I always thought some hum just gave people the big rig power sound.....


Brett

A few thoughts --

I hope you tried terminating the mic input as something among your tests.

The fact you have no hum on CW suggests it's not the power supply.

You might also try unplugging the Cinch-Jones connector on the speech amp at the left rear, and let it hang there (unterminated) while you button things back up and re-test.  This may prove out the modulator stage, since there would be no audio drive to it.

Did I miss whether you put the mic gain at minimum to see if that reduces the hum ?

And Brett's right about a possible microphonic tube. You could be getting some modulation from the mechanical coupling of the blowers to a tube vulnerable to vibrations..

Mic gain does not effect hum. Mic hooked up or not hooked up, does not effect hum.  I will try unplugging the Jones connector. First, I am going to try to remove the last audio tube in the speech amp.  This will tell me if the hum is in the speech amp or in the modulators.

C

I got another burst of energy and tested some things:

1. If i pull the last audio tube on the speech amp, leaving the 4-125s and power supplies active, The HUM IS GONE.

2. If I put the last audio tube back and then pull the one before it, the HUM IS GONE. This means the 4-125s and the 6C4 tube and the power supplies are NOT causing the hum.

3. If I put the 4th audio tube back and pull the 3rd, The hum is GONE.

4. If I put it back and pull the First tube of the speech amp, the hum is GONE.

This means that the HUM IS NOT being caused by the 2nd, 3rd, 4th or final audio tube, the 4-125s or the power supplies.

5. If I unplug the BNC inputs to the speech amp, the HUM IS SUPER HUM!  100% audio on the hum. MORE HUM.

6. If I put terminating BNCs that are shorted internaly on one BNC at a time, I get the normal HUM.

7. If I put terminating BNCs on BOTH BNCs, The Hum is normal and is there. 

What in the heck Should I do now?  I have proved that the hum is NOT comming from the the speech amp, modulator, or power supplies. I have proved that if you put ALL the tubes back in, The hum is there. I have proved that the terminating the BNCs is NOT effecting the hum.

Clark

check yer bnc chassis plugs to make damn sure grounded shell is really grounded?

I
something that need to be grounded on that chassis is not,  at least not good enough. It sounds like you're only getting a 'ground' when you connect your cables up.

check for ac voltage potential difference from your cables to the chassis.

I probably shouldnt offer up stuff as I've never worked on one.  :P

I think what I am dealing with is not enough UF on the Modulator speech amp B+.  10 UF might not be enough since I HiFi'd the Rig. I am going to beef this up to 47 UF so the B+ is super clean and try again. I have the caps. I am starting to think that this is not a problem.. But an effect of me expanding the bandpass of the Rig.

C

Did it hum before you modified it for "hi-fi"  ?

A lack of filter capacity on the modulator B+ shouldn't manifest itself until you try to pass some low frequency audio.  Perhaps what sounds like hum is actually a condition called motorboating, which indeed would stress the B+. 

But you would see significant changes in modulator current, which you have not reported observing.

I wonder if it is worth your while to move the focus away from capacitors and toward the possibility of RF getting into the audio chain somewhere.

Like I said before, when I opened the 32v up, I had hum from the first audio stage.
A simple diode and cap fixed that.

Any high impedance high gain stage will pick up hum if you don't do things to prevent it.
You could experiment with the coupling caps in an early stage to roll off the response above 60Hz.
I am not sure 60 Hz is of any use going out on the audio.

I never had a t368 so don't know what the bnc cables are doing, you can use better cable, or better grounds, but if you have a high gain stage, you might have filiment ac getting into the audio there.

A small amount of hum just sounds like a big rig, it might be interesting to keep it....

Blower noise is nasty, but a little 60Hz sounds nice...

Brett

Clark,

   I was on 3870 last night with the vintage SSB net which was ending. You came on frequency, and I heard audio but NO hum.  ;D

Jim
WD5JKO

SSB on 3870. How dare you :) 

It was humming last night.  I got ran off the band.  Its about 5 watts of hum on that carrier. I am suprized nobody has any solutions.  I guess I will try a big cap on the B+ to that first tube. Maybe that will help.

Brett what do you mean about the diode and cap?

C

5 watts of hum on a 300 watt carrier, and people are complaining?  Man, some people have SUPERB hearing!

I'd say you pretty much narrowed down the stages where the hum is coming from in your other post, where you pulled tubes and could get the hum to go away.

Check for wiring in the audio stages that goes near either the ac leads, or near a run to the filaments, since the radio has is AC powered on the fils.

Since the hum is there when you put shorting BNC's in the radio, I'd say that the first tube is where it's picking up the hum.  Can you try putting a shield in place?

Wiring is king!!!!  MAKE SURE you have the audio leads going to / from the mod / speech amp away from everything carrying AC or hw rectified DC.

Another thing you could try is balanced feed, but since it's getting in there even with the BNC's shorted, I HIGHLY doubt it would fix it.

Could be simply ground loops.  All grounds TIGHT!  I know you've fought them in auto installs, we all have.  Maybe a GOOD ground run from deck to deck inside the chassis would help out.  1 volt of difference would cause a hum, methinks.

With a shorted BNC on it still humming (to the normal level), I'd expect it to be either wiring issues or something getting inside the first stage.

The other thing you can do is after the 1st stage, build a 60 or 120 hz notch filter, depending on the frequency of the offending hum.

--Shane

It has an AC fil trans Right next to the tube. How stupid can an engineer get?  I mean these really must have been some retards at B and W. 

I have a Great idea. Lets put an AC filiment transformer Right next to a HIGH gain Preamp tube!

I am going to work on it some more today. I wanted it up and running for tomorrows AM transmitter Rally.  I am going to Tripple the filtering to this deck and Try a 12AU7.  Ths should lower the gain down. Maybe the hum will go away. If not, I am going to have to move this Fil trans to the Deck, Put a connector on it, Make it DC and then the trouble will go away.

C

Yes, why operate an ssb net in the AM window??

To get rid of hum in the 32v, I just used one diode (half wave rectifier) and an electrolytic cap to filter it a bit to get rid of the hum. The value of the electrolytic cap sets the voltage, a bigger cap gives higher voltage, and I tried a few till I got 6 volts.

To get rid of the hum, you don't need pure DC, just smooth it somewhat so its not 6 volts ac...

Been running that way for 10 years....

Does the tube have a shield?

Maybe just live with the power hum, makes signals sound strong!
I need to find a way to ADD some big rig hum to my signal...

Brett

  Well this is kind of a regional thing. Here in Central Texas we run AM primarily on 3880 and 3890. This includes much of Louisiana, Oklahoma, and Arkansas. The W4's on AM come in strong early evening on 3885.

   Below 3880 there are many regionally STRONG SSB groups, and NETS. There is the group on 3878, more on 3875, a Net on 3873, and the 3870 bunch that often play with Hi-Fi SSB using big QRO amplifiers.

   I do operate both modes, but on SSB I use my Central Electronics 20A. We had two CE rigs on Wednesday's net. That to me is always a lot of fun.

   Right after the net I went to 3880 and checked into the Collins CCA net running AM where the net control was Brian K0EFJ. That was a lot of fun too.

Jim
WD5JKO

Yes,  and those "Hi-Fi SSB using big QRO amplifier." folks completly tear up the Western AM window at 3870 Kcs.  They will NOT stand by for an AM net.  Seem very thick headed about it.  What are they running  7-10 KW??
 OOPPSSSSS  too much attitude !     Vic

Hi

Clark why don't You monitor the output of the first speech amplifier with a Scope,if I'm not wrong the gain control is after the first speech amplifier,
so the speech amplifier is always "open" and amplified all the noise that "injected" to it with it's full gain .
If this speech amplifier amplified the hum that induced to it with it full  gain, than maybe there' a"big" AC current flowing in this tube modulating it B+,So it can also modulate the B+ of the speech amplifier as a whole (LV voltage)
I get the conclusion because You wrote with the gain is off (mic gain doesn,t effect hum) and the gain control is after the first speech amplifier.

So the hum is always there because the B+ is "modulated" ,causing the gain control has no effect on the hum.

Gito

This radio is now fixed. Thanks for the suggestions. 

Robert W0VMC told me how to fix it in 2 minutes on the phone.  I changed the 8UF cap to a 47 and then swapped the AT7 for a AU7.  This lower gain tube does not pick up the AC hum.  I actualy gained audio quality doing this. I can Drive the Rig a bit harder and its not so touchy. The HUM is 100% GONE.  I also changed the Screen cap from a 2200 to a 1000 and gained even more audio bandwidth. I have not measured it yet, But it sure sounds better on the RXer.


Just in time for the AM rally tomorrow!

Clark