Tram D201A week modulation!

[crambone]

Ok my Tram is showing 4 watts but I need to scream into the mike to see any modulation swing. Mike works good another radio so i checked that off as a possible issue. The mike gain is all the way up. I am also using and external frequency counter/ watt meter/ modulation meter into a 50ohm dummy load. I have also noticed that the frequency is all over the place when I key up with the mike gain past 50%, when turned down it shows the correct frequency? I checked ALL the tubes and they all checked 90-105%. R647 AM Audio Comp is turned up all the way also, I also tried swapping BA Boards with one I know 100% works. 
This radio does have a bad power/ RTC knob I have the power hooked to a toggle temporarily. The RTC and volume do work just not the power on/off. 

The radio was recapped about a month ago and all new High Voltage Resisters installed and was tested and working.  

Any suggestions were to look? I even swapped out the Balanced Modulation Board just because I have a new 100% working one to swap? 

[2600]

There are two resistors on the audio board that routinely cause this. R646 is a 330k half-Watt. We routinely replace it with a 2 Watt part.

R620 is a 470k half-Watt. We use a 2-Watt part for that one, too.

73

[crambone]

Thanks I’ll pull um and check them tomorrow, will keep informed. 

[crambone]

I pulled both ^620 & R646 as suggested.

R646 330k tested 330k ohms

R620 470k tested .564m ohms ************

I also tested the Modulation Transformer across the white and orange wire coming off of it and took them out of circuit. I was told it should test 335 ohms or higher. It tested 65 ohms. 

[crambone]

Update as of today is I replaced the modulation xfmr and resisters suggested. No good news to report still have very low/ hardly any modulation. I’m waiting on some microphone connectors to hook up a d104 I know is 100% working then will give that a shot just to rule that out also. 

[SIX-SHOOTER]

Did you check the transmit audio tube? That would me my 1st guess but off the top of my head without pulling out the schematic I can't even remember what that tube is? That happens when you get OLD! I have a D201A sitting here on my desk also but it's to much of an effort to pull the top cover off also. LOL Also that is WEAK modulation not WEEK modulation I am guessing? LOL

SIX-SHOOTER

[2600]

I should probably have asked if the receiver audio is as strong as it should be.

My advice assumed that the receiver audio is okay.

If it isn't okay, that suggests trouble in a section of the audio shared by transmit and receive both.

There is a .005uf disc capacitor C627 strung between pins 4 and 3 of the 6L6 audio tube. If this one shorts, it cripples both receive and transmit audio.

If you have decent receiver volume with the volume knob all the way up, then C627 is NOT the problem.

This is the kind of fault where an oscilloscope pays for itself. Tracing the mike audio point by point would reveal where the fault is located pretty quickly.

73

[crambone]

The receive audio actually is week I didn’t even notice till you mentioned as it’s on a dummy load and the volume was down. I turned it up a hardly anything. As the 6l6 tube and all the 600 tubes I checked on a Mighty Mite V and they all tested above 95%. 
I did check a few more resisters on that board by taking out and 4 were way out of spec so they are on order. I will look at the ceramic cap tomorrow when I have some time. 
I can get an oscilloscope to use but I’m not sure how to really use it to follow a trace. I’ve used it to check my audio on it a while back just to play around. I took the signal off of a small breakout that reduces it to 1/4 watt. It was pretty cool to see. 
Im sure I can borrow again, I would assume I need to use the bnc to alligator clips to follow the trace? 

[crambone]

Oh and yes weak not week as in it’s taking me weeks to find my modulation problem 

[2600]

Awwlriiighty now!

Shoulda asked about the receiver audio to begin with. If that's weak, the AM transmit audio can't be sufficient.

Time to find out which version of the D201A you have. There is a circuit board plugged in alongside the 6L6GC audio tube. Tram calls this the "Auxiliary Audio" board. Commonly called the "BA" board. There are two versions of this board.

Thought I had a pic of each kind on my ImageShack account. Can't seem to find them. One version has a fuse on the rear-most facing edge of the board, two 220-ohm 2-Watt resistors on that rear corner of the board, and a 4uf 160-Volt electrolytic cap C625 just below the fuse. If that electrolytic cap is original, it SHOULD have gone bad by now. We always use a part rated for 450 Volts to replace it. I'll skip the reasoning for now, but it's just safer.

A 'scope probe placed on the positive lead of that cap should show around 30 Volts of DC, with no more than a couple of Volts of audio riding on it. I'll predict that you have ten or twenty Volts of audio on that spot, maybe more.

This is what happens when that cap fails from old age and becomes an open circuit. It reduces the gain of the 6L6 tube in a drastic way.

On the other hand, if your BA board has only a couple of jumper wires on the rear-most side of the board, it's the later version and that cap C625 isn't on there. Can't blame a part that ain't there.

I'll check back to see which version you have.

73

[crambone]

Attachment Deleted

[2600]

That BA board looks like it was rebuilt recently. None of the factory resistors or capacitors is on it, they all look new.

If you can measure the DC voltage on pin 8 of the 6L6GC audio tube, that would be the next place to look.

73

[crambone]

Pin 8 had 58.5 VAC when radio off and 42VAC when radio on? 

[crambone]

Also I have receive now someone had the RF Transformer adjustment way off! I am showing 2.5 watts when keyed on the Trams external meter. I am seeing power out of the Trams PL259 on my external watt meter and seeing the frequency on an external counter that is taking the reading from a break out. Am getting zero audio or any type of swing just dead key at 2.5 watts on Trams meter and 5 watts on external. I’m guessing the Trams meter needs to be zeroed I will do that next. I can here the rely when key/ unkey it just seems I have no modulation. Also when I go from AM to LSB or USB I get a tone both in Crystal and Manual. 
I Can see the radio key on the ossiliscope also but NO modulation. Also I took the modulation transformer from a good tram and also swapped BA boards and tubes from a working Tram just to verify and it changed nothing. 
I will take a video of the issue to YouTube and post a link to it. 

[crambone]

youtu.be/2y4sFQZ_N0c

[crambone]

Forgot to give you what you asked, Pin 8 VDC! 
33.8 VDc Standby
32.8 VDc Keyed.

[2600]

Well, that's not the problem.

Have a look at pin 3 of V601. Should have between 25 and 30 Volts DC on it.

If it's low or zero, this will kill the mike audio, but won't disturb receiver audio.

A low reading may be a bad R620, 470k 1/2-Watt. Another likely culprit is R646, 330K. The AM limiter trimpot R647 should have between 70 and 80 Volts DC where it connects to R646. If R646 goes bad this will hold the modulation down in a big way.

Both of those resistors are on our "hit list" when this model gets new capacitors.

73

[crambone]

V601 pin 3 is 37.7 VDc

R620 & 646 are new.

R647 trimpot to R646 goes from 0-93VDc when operated it was maxed at 93 when first tested. 

[2600]

Good grief!

We're running out of "routine" failures to blame. I'm in the habit of following the mike-audio signal with a 'scope step by step from the mike socket to the 6L6 modulator tube. This serves to pin down where the signal path is being disrupted. That makes finding the cause simpler.

Checking the most-common things that fail is a good way to skip having to do this.

But only when the fault proves to be on that "common" list.

So far, your problem isn't on that list.

Makes me wonder if the mike-gain control could be bad? Of course a 'scope can settle this question in a hurry. But only if you have one.

The one way to settle that question is to turn the mike gain full up, and then short the clockwise lug to the wiper (center) lug with the tip of a screwdriver. If the mike audio comes up in a big way, the control is bad. Make sure the tone knob is turned full clockwise for this test.

I'm gonna guess you have used the mike on this radio with another one. A bum mike wasn't on my radar, but I never did ask.

How loud is the receiver audio with the volume full up? Should be pretty deafening, and you won't want it turned up that high for long.

Just grasping at straws. Not many of them left to choose from.

73

[crambone]

I picked up an Osilicope EZ  OS-5060A from a friend but not really sure how to use it. I also ordered a VTVM RF Probe - for Heathkit IM meter I have. 

I did put an ohm meter on the mike gain and it did go up and down, not sure if that’s a good way to test but was just trying to see if it worked or not. 

Im slowly trying to learn a little. I will be out of State for 3 weeks for some R&R so everything will be on hold for a while after Saturday. 

[crambone]

Ok I changed every, every resistor on the audio board! That didn’t do anything but make it look better. So I ordered the probe for the Oscilloscope and I’m going to try to trace out the circuit but it won’t be till I get back in town in 3.5 weeks. Any recommendations on we’re to start my trace from? I also had the schematic blown up today so my blind but can see it! 😳

[mattbee]

Did you replace C627?

As mention earlier in this tread, if your radio volume is low, and your modulation is suppressed, I'm betting this is your problem.

If your dead keying 3-5-watts, and when you modulate it swings backwards or barley moves, C627 could be your problem.

Good Luck,

Mattbee

[air1]

You can measure the resistance of the audio transformer right in the radio. Despite what you've seen on the internet groups or videos, the measurements are as follows. These are from a new one and properly working ones that I have measured over the years will be close to these. The inputs are the blue and red wires and should be about 53 ohms. The outputs to the final tube are the white and orange wires at about 63 ohms. The outputs to the speaker are both green and about .3 ohms. Whenever I work on one of these radios that has audio issues, I always measure these, so I know if the transformer is good or not. Then I can go from there.

[2600]

The audio transformer sounds like it's probably okay.

This is where an oscilloscope becomes an unfair advantage. I would probe the cathode of the 6L6GC tube on pin 8. There should be no more than a Volt or two of peak-to-peak audio riding piggyback on the 31 Volts DC you should see there. Likewise pin 4 of the 6L6 should have only 355 Volts of DC, no audio at all riding on it. If either of these capacitors fails as an open circuit, the audio you'll see a these two points gets SUBTRACTED from the drive feeding the audio tube, makes the output weak.

Changing all the electrolytic caps, both large and small saves the labor devoted to identifying them as they fail like a row of dominoes one after the other.

73

[2600]

Um, okay. Does this mean you have receive audio back working?

Still no mike audio?

Lost track.

73