T368 power output

[ke7trp]

I recently picked up a T368. 

I would like to hear from T3 owners as to what knd of power they show in carrier and in PEP. 

My unit does 600 watts full loaded, About 500 if you tune it by the book. 

The mod current is correct and when I talk to shows 250 mills which is a bit higher then the books 230.

What concerns me is the fact that this unit is only outputing 1000 watts PEP.  The scope looks nice and clean.  I would expect this radio to produce 2000 watts PEP and was told it would do that.

The Final is a 4-400 that is new. The unit is restored and in great shape. The Modulators are 4-125s.  My Mod current is correct and when I talk is going to 250 mills.

The current is there.. This is 375 watts of audio.  This should be enough to full modulate a 400 to 500 watt carrier.  Why am I only seeing 1000 watts PEP?   The radio sounds nice but the audio is down.   If this is the way the rig is, Fine.  But if its not normal, I would like to fix it.

The Globe king does 300 watts carrier and 1200 pep on the same meter and dummy. My Valiant does 150 carrier and 450 pep when I talk.

Help me understand if I have a problem or not. The old timers are telling me no.. This is the way the radio ran.
 
Clark

[k4kyv]

Do you have a monitor scope?  The envelope pattern will tell you a lot more than one of those Hammy Hambone wattmeters ever will.

If the T-368 is making close to100% modulation on negative and positive peaks, I wouldn't worry about it.  Like the BC-610, the modulation transformer turns ratio doesn't allow much more than 100% modulation.

[K1HH]

My T-368 will certainly make 100 percent positive and negative modulation as verified by a scope. Easy way to calibrate is to use two vertical units for your full carrier and if the scope goes to 4 vertical units you made it to 100 percent positive; likewise if the carrier reduces to the center baseline you are at 100 percent negative. 

While it will do more, I load my T-3 to the 375 watt output using either a Bird, a ME-165, or a Drake W4 which has an aftermarket PEP kit in it. I don't think it swings much above 1 KW on the PEP position, but that can certainly be a part of the PEP circuitry and certainly the ballistics of the meter.

Again, a scope, and some form of tone source is necessary to get an accurate PEP measurement. Even the broqadcast AM Modulation Monitors suffered in the meter ballistic area; although their peak indicator circuitry was pretty good, but not as good as a scope.

Rodger
K1HH

[N2DTS]

The books say a pair of 4-125's will do 330 or 400 watts (AB1 or AB2).
If everything is right, you should be able to do 600 watts input or 650  watts input, about 450 to 500 watts carrier output and close to 2000 watts pep.

But that is only if the RF tube modulates well, the power supply is stiff, the mod trans ratio is correct, and the audio has a lot of asymetricality? in it and its phased correctly.

Other things impact it as well, compression can reduce the peak power.

As Don says, its hard to tell much without a scope and/or mod monitor.

You need plenty of grid drive and bias, good modulator tubes, a stiff power supply, and extra modulation power.

How does the 4-400 screen get modulated?

Brett

[WA1GFZ]

You might want to monitor your 120 volt line to see if it is stiff enough.
Maybe your low frequency response is pinched off making the PEP look low. A scope is your friend here.

[ke7trp]

Yeah. I have a scope hooked up now.  

If I load this thing by the book then I get about 500 watts of carrier There is not enough modulator there to get to 100%. It Does not close the carrier all the way.  

If I load it 75Mills LESS around 200 mills then it still wont close the carrier all the way. This is about 400 watts Carrier.  Its ALOT better loaded way down. But still very weak..  1100 PEP or so on the Bird 43P.  

Note to Don.  You really need to stop calling the 43P a hammy hambone meter. It was the industry standard for half your life

If I set the scope so the carrier is just taking up TWO rows,  then tone the Transmitter, I get ONE more row on top and one more on bottom.  

Carrier, A bit more then two rows.  TOTAL MODULATED 4 rows.  I guess this thing is just a weak modulator.  




Brett, I am like you.. I would expecct 2000 watts PEP.  It is NOT even close to that.   I have had three T3 owners tell me they make about a Grand PEP.  What a mess.  A 700 Lb box that makes less power then the Globe king 500C.

I need to get my Big Box done.  This one will sit on 160 and be a converstaion piece I fire up once in a while.

TimTron says to put in 4-250s, change the Fill trans out so it can handle the higher fill current and you get 150%.  Not a lot of work. I wonder if its even worth my time and money.  Probably not.  

Here are some Screen shot of the scope.  Sorry for the crappy pictures.  I ran the mic gain way up. It was flat topping. I was trying to get the carrier to close. It wont.. I guess this is about 50% mod?


 
Clark

[Fred k2dx]

Before you do anything, check your AC line voltage. Unless you have the model that could run on 220 VAC I'll bet your line voltage is dropping, especially on modulation. Check how much it drops on just keying the transmitter... how much it drops on modulating. What amp service is it on? Is it on a dedicated line, 20 amps or more? If it's a long run to your breaker panel, try running a 30 amp 120 VAC dedicated line for the T 368. I run a 20 amp 220 line to mine, then plug into a 5 kva step up/down transformer that is 110/220/440 volts located right next to the tx.

The rated power out is 450 watts AM carrier I believe. Loading up for a little less carrier will make for better modulation. There is an RCA modulation transformer Tim recomends - along with lighter loading of the transmitter due to the mod transformer current rating. There isn't much room for a larger transformer but Tim sez this RCA is a drop in and well worth it.

Forget trying to use a wattmeter, Bird or whatever! Use the scope.

[Gito]

Hi

Maybe what's Don wrote is true,check the modulator transformer turn ratio,maybe output impedance of the Transmitter and the output impedance of the modulator turn ratio is not what it must be.
 
Gito

[k4kyv]

The mod transformer turns ratio will allow 100% modulation on positive and negative peaks, but little, if any more.  This was intentional in the design, as with many military and commercial AM transmitters, to limit the modulation capability to just about 100%, a crude measure to make them "overmodulation proof".  Remember, these are GI Joe military transmitters and it is expected that many of the operators will have little or no technical knowledge or expertise about  such things as percentages, voice peaks and overmodulation.

The scope pattern definitely indicates an anaemic modulator.  Check for soft tubes in the audio chain.  Make sure every tube has the full specified plate voltage. Something is clipping the waveform way before it reaches 100% modulation.

It could be the modulation transformer, but if so, it would be due to something like shorted turns, unless a previous owner has stuck in a non-standard replacement transformer with the wrong turns ratio.  To get extended positive peaks, up to 125% or beyond, the modulation transformer needs to have less step down than the stock one, and this will load the modulator tubes with a lower plate-to-plate load impedance, making them run hotter and less efficient, but with higher peak audio power capability.  But clearly, the scope pattern shows that is not the issue here, but just the opposite.  It looks like something is clipping the audio at about 60% modulation.  Something is definitely wrong, somewhere along the way from the microphone to the modulation transformer.

I doubt the problem is sagging line voltage.  If that were the case, the RF HV would sag along with the modulator HV.  You might not get full positive peaks, but the negative peaks would still reach down to 100%.  There would be a noticeable carrier shift in the negative direction, as indicated by a downward kick in final amplifier plate current.

A useful check with the Bird would be to use the so-called "average" power mode.  The meter actually reads average rf voltage, not power, so for AM the needle should not budge with modulation, but remain perfectly still.  If it kicks downwards, that tells you that you have negative carrier shift, aka downward modulation.  It is extremely unlikely that the needle will kick upwards during modulation, but if it does, that may indicate self-oscillation in the final.

BTW, the best Bird wattmeter for measuring rf power output is the APM-16.  They are not cheap, but they give a true reading of real rf power even with a complex waveform.  The 43 is accurate only with constant amplitude, sinusoidal waveforms. It is rugged and built with quality construction and OK for FM and CW, but for any kind of amplitude modulated signal, including conventional AM or SSB, the accuracy is little better than that of the typical el cheapo hammy hambone "wattmeter".

[Jim, W5JO]

There is a guy out here using a T3 that went through what Don is describing Clark.  He had Peter Dahl wind 3 modulation transformers before he got the ratio the way he wanted it to get full modulation. 

[Gito]

Hi

 I think  all Don  that have wrote is true. But I think the problem is still in the modulation transformer.Since Clark wrote the current is there  ...the radio sound nice .....

So my conclusion is the modulator works with it's full power (the current is there) with a good sounded audio,
if I am not wrong if it ,s clipped ,with a full output power(the current is there) ,it still can modulate the transmitter 100% with maybe distorted sound.
if it's clipped in the driver tube before the final tube power of the final modulator tube can't get the full current it needs(low power/voltage input to the final modulator tube)

Even with full power output power /max power if the turns ratio of the modulation transformer is "wrong" .it can not supply the transmitter needed peak voltage.
For instance it needs a 1000 v peak voltage , but since the transformer turn ratio is"wrong" it can only deliver  600 v peak voltage

When Don wrote it's a military Radio,my conclusion ,maybe it 's not to important at that time to have such like 100% modulation ,"as long it can transmit the audio on that transmitter."

So if we know the plate impedance of the transmitter and the output impedance of the modulator,we can find the winding turn ratio of the modulator transformer must be.

 we can measure the turn ratio /the voltage ratio. of the present modulation transformer
does it has  the right turn ratio/voltage needed

Or it can also caused by shorted turns in the modulation transformer.


Gito

[N2DTS]

Yes, I would guess the problem is in the mod trans ratio.
The transmitter is a well made unit it seems, just limited in some respects, and seems like a good start on making a reliable good sounding transmitter, better than a globe king as a starting point....

Change out the filiment trans, put some 4-250 or 4-400's in, regulate the modulator screen voltage, run them in AB1, replace the mod trans with something closer to the correct match, or just use more modulator power with a so/so match.

It would seem like the T368 is a much better starting point than most transmitters you are going to find, a robust RF deck, stable easy to tune pto driver, good power supply parts, safety circuits and interlocks, etc.

Like everything else non home brew, it just needs a little work.....

Or, you could load it till it makes just short of 100% modulation and run it that way forever, and also not have to worry about over modulating....

Brett

[k4kyv]

I believe something is terribly wrong.  A T-368 may not quite reach 100% modulation without distortion, but it should do well over 90%.  There are too many of these transmitters in operation with the stock transformer that modulate well, for this to be an inherent problem in the design. The army would never accept something that bad.

One possibility is a defective modulation transformer.  If it had the wrong ratio (too much step down) the plate current wouldn't kick up to normal.  If there were a problem with the modulator tubes (4-125's?) or the screen voltage were too low, that would also result in less than full plate current.

It looks to be clipping at 50-60%.  With a sine wave, that would represent 25-36% normal audio power output from the modulator.  The plate current should be hitting about half normal value at that percentage modulation.

Unless... "normal" modulator plate current is showing up because the audio output is nearly a square wave as shown by the scope pattern. A square wave contains about double the average power of a sine wave at the same peak voltage and peak power level.  Try reducing the audio test tone until the waveform clears up to a sine wave, but before the amplitude of the peak begins to drop off, and note the modulator plate current.  Maybe it is the modulator tubes or mod screen voltage that is limiting the output, and once it saturates, the square waveform is driving up the mod plate current to the point that it appears normal.

I notice a tilt to the square waveform.  That is normal when the audio passes through a transformer, or through an R-C coupled amplifier if the coupling capacitors are small enough to limit low frequency response.  That tells you that the saturation is occurring somewhere before the secondary winding of the modulation transformer. Don't rule out a low level stage in the  speech amplifier.

[ke7trp]

Wow.. Thanks for all the help guys.

First off we have two issues going on.

1. How to repair the unit because I think we all agree that it is broken.


2.  How to modify the unit for better performance.


I am interested in number 1.  I just want to repair it.  I dont want to hack this radio up. This is not the one to modify. I can get another one that is beat down and mod it all I want.  But this one, Should stay stock or close to it.

Let me see if I can answer some questions.

I had a 30 amp 120 volt line run into the room by an electrician.  Its 126 volts as measured. 125 when I key it and 124 under full load. This is not my problem. T3s have a built in Filament adjust ment knob and needle.

The radio is bone stock.  stock iron.  I had YEC in san diego offer to send me some 4-125s he had. I will try them when I get them in the mail.

I will check all Preamp tubes again today in the TV7.  I will check coupling caps also with my cap checker.

I dont know how to test the mod iron.  I will need help on that one.

I cant load this radio low enough to get 100% Brett.  The scope photo was done with the load as low as it will dip. If I load it by the book, Its more like 25% audio.

I would love some information on how to add Line level audio to this thing.

I am using the Front mic with a carbon mic or, I put my Mixing board into that jack with bypass caps to remove the carbon mic voltage.



Clark

[Fred k2dx]

Look in the mods section. You can run line level audio into the speech amplifier through a phono jack mounted on the subchassis.

http://www.amwindow.org/tech/htm/t368.htm
http://www.amwindow.org/tech/htm/n9foy/n9foymods.htm
http://www.w1ujr.net/t368.htm

Here's links to the required T 368 reading. Read them before you go further!

[N2DTS]

Oh, the output power is that low?
I am with Don then, since you get the normal mod current, or even higher, its time to scope the audio stages.

Not sure its not something with how you are injecting the audio into the carbon mic jack.
If you dont want to modify it, maybe try a carbon mic and live with the audio...

Why would they use a carbon mic, there is no advantage to that, are they not sensitive to moisture?

Brett

[k4kyv]

It shouldn't take much modification to replace the carbon mic with a balanced audio line or at least a better microphone.  The  stock rig with carbon mic might make a good museum piece to look at, but to use it on the air in stock military configuration, it sounds worse than a tin-can telephone.  Hyelloweeeee!

I just looked over the W1UJR site under "tube use", and noticed V14 is designated as a clipper.

Just a thought, but maybe the clipper stage is simply out of adjustment, allowing the extreme clipping shown in the scope pattern to occur well before 100% modulation where it is supposed to  happen.  But that doesn't fully explain the normal modulator plate current.  I would think the current would peak well below the rated maximum value, unless the extreme flat-topping is producing the near square wave as explained earlier.

It wouldn't hurt anything to at least investigate this before digging further into the transmitter.

[ke7trp]

I got a carbon mic from fair radio. It has the same output as my outboard audio. Low.  Sounds like hell. I got it to hang on the radio and look original.

Don, I bypassed the clipper on pins 1 and 5 like the mod says. This got me a bit more audio and more mod current. But not nearly enough.

The 4-125a's are good. I verified that. 

The deck is now out and I am going to test each tube again in a Tv7 and then attempt the audio line level input. 

Keep in mind that this rig has "600 ohm balanced" input right on the back.  It has a seperate gain control. I wonder why someone would want to add another input. I will have to read the mod to see what they are doing.

Clark

[k4kyv]

I never dug inside a T-368, but am familiar with the BC-610, its predecessor.  The 610 used a separate speech amplifier, known as the BC-614.  It has 600Ω line feed to the transmitter.  I assume with the T-368, the speech amp is built into the main unit.

The mod xfmrs between 610 and T-3 are so close in specs that they should be directly interchangeable except perhaps for mechanical mounting, but the T-3 transformer is more robust than the potted version of the 610.  I once had a T-3 transformer and sold it to a guy who used it as a replacement for his blown 610 transformer, and he said it fit inside the 610 and worked fine.

The external antenna tuning units appear to be identical, except that the 610 model is painted black wrinkle while the T-368 model is painted grey.

[ke7trp]

Yes. The speach amp is built in Don.  I am about half way on the tube checking. All are good so far. 

Clark

[N2DTS]

I have a bc610 mod transformer but its the end bell type, like the power trans I have.
Maybe these were from early bc610 units?
Didn't they make different models?

Brett

[K1HH]

There are indeed two audio inputs on the T-3, one of them is marked 600 Ohm Line In and enters through connector J12 on the rear of the transmitter. This is NOT a balanced line in.  It is a 600 ohm line level input. To make it balanced you would need a 600/600 transformer. The two mic preamps are identical except for the value of the cathode resistor in V12a/V12b, a 12AT7. The cathode resistors set the gain. I have disconnected the bias voltage from the front panel carbon mic connector and run an amplified D-104 with a U-77 connector on it.

I have jumpered out the clipper circuit and bypassed the hi-lo pass filters and made minor capacitor value changes in the stock speech amp.

I am told it sounds good and modulates fully.


Rodger
K1HH

[ke7trp]

This is a C model.  

I just checked ALL the tubes. I found two weak but good tubes.  Replaced them with new.  One of the oA2s in the speach amp was flickering when tested.  It tested 10 on the TV7.  I put a new one in and it tested 80.  

So I had some weak tubes.

I loaded it up on 80 into the dummy. I have gained alot of power. It now does about 1300 PEP.  THe MOD current meter now goes higher to about 270 mills.  The Scope is showing the carrier is now closing but NOT all the way.. DAMN close though.  I now have peaks at the top of the scope and not flat tops.  I estimate about 80% mod.

Can we call this working as it should? Or should I still keep going?  

This is with the carbon mic..  I will try the external audio next.

Clark

[ke7trp]

I need a plan to run the outboard audio into the thing.   I read the mods and I dont see any options for this.

I have the connector J12.  Can I run the audio rack into this connector? 

Where do I tap the Speech amp if the answer is no?

C

[WBear2GCR]

I'd be checking the bias points on the grids of all the smaller tubes, up to the 4-125s.

Sounds to me like perhaps there are one or more leaky caps in the signal path?

Otoh, you could inject a proper voltage signal at any point along the signal path, and also look with a scope at the waveforms from front to back of the modulator.

I know this might be difficult on the T-368 because of the interlock system.

Be extra special cautious in there, since there are deadly voltages present!

                          _-_-