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Author Topic: Heathkit DX35 controlled carrier modulation  (Read 23816 times)
HAMHOCK75
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« on: February 28, 2016, 12:02:12 PM »

I restored a DX60B recently. The AM looked decent. The AM carrier would about double and pinch off symmetrically. The DX35 is another story. The AM carrier will about double but not pinch off much at all leaving very asymmetric audio on the carrier. The manual says there should be +40 volts on the 6146 screen grid in AM mode. I find double that ( +80 volts ). The DX60B only had +65 volts and I think that was with modulation present to bring it up.

So far, I have tested with two 12AU7A's ( both tested good on a tube tester ). Every capacitor has been removed so the 12AU7A is just DC coupled to the grid of the 6146. The cathode of the first half of the 12AU7A ( with a 1 megohm grid resistor to ground ) is at +44 volts with its 2.2 megohm cathode resistor dropping to +40 volts though a 1 megohm resistor feeding the grid of the second half of the 12AU7A acting as a cathode follower with a 220K cathode resistor. The cathode of the second half is at +77 volts.

From drawing the load lines for the 12AU7A with Heathkit's resistor values, these voltages seem to be what one would expect but it is double the voltage described in the manual. Any ideas about whether this is normal, the manual is in error, and the AM is really awful ( compared to the DX60 )?
« Last Edit: February 28, 2016, 12:37:00 PM by HAMHOCK75 » Logged
QRP4U2
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Posts: 262




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« Reply #1 on: February 28, 2016, 12:05:33 PM »

See if this helps:

http://amfone.net/Amforum/index.php?topic=40779.0

Phil
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HAMHOCK75
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« Reply #2 on: February 28, 2016, 12:29:56 PM »

Thanks Phil. Unfortunately it does answer the question. He replaces the 2.2 megohm cathode resistor of the first half of the 12AU7A with a variable 2.5 megohm resistor which allows him to vary the voltage from from 0 to +45 volts ( with the fixed 2.2 megohm cathode resistor I get +44 volts ). This allows him adjust the output of the cathode follower half of the 12AU7A from +25 to +75 volts ( I get +77 volts with the fixed 2.2 megohm resistor in place ).

So if he sets the adjustable resistor to the value of the fixed 2.2 megohm resistor, he gets almost exactly what I measure which really makes the AM ugly!
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QRP4U2
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« Reply #3 on: February 28, 2016, 12:49:46 PM »

Are you looking at the second cathode follower's output across the 220k or are you looking at the modulated AM's RF envelope?

How does the audio appear coming from the Speech Amp?

Phil
« Last Edit: February 28, 2016, 12:52:35 PM by QRP4U2 » Logged

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HAMHOCK75
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« Reply #4 on: February 28, 2016, 01:16:47 PM »

Good questions Phil. I have a 100 MHz dual channel Tektronix oscilloscope so I am looking at both at the same time. This is what I see. The +77 volts on the 220K cathode follower resistor drops to +64 volts probably because the B+ drops from +530 to +450 volts. The problem is that the audio ( which looks fine ) is riding on top of the +58 volts. That definitely shifts the average screen bias up increasing the carrier but the carrier will not go down much from the value with +64 volts on the screen grid.

Are you the person who made the improvements? Looks like you might have been posting under a different handle.

edit: I had posted that the screen grid of the 6146 drops to +58 volts. Just put everything back together. It drops from +77 volts to +64 volts.
« Last Edit: February 28, 2016, 02:34:53 PM by HAMHOCK75 » Logged
HAMHOCK75
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« Reply #5 on: February 28, 2016, 04:04:58 PM »

Ok, there is a solution ( actually the Heathkit solution ). The DX60B and DX40 both have a high pass network between the cathode follower and the screen grid of the 6146.

The following tests were done. The screen grid of the 6146 was measured at 120 VDC in CW mode. The RF amplitude measured at this voltage was arbitrarily adjusted for 2 divisions peak on the oscilloscope. I then selected some resistors to put in series with the cathode follower output and screen grid of the 6146. It took a 24K resistor to drop the cathode follower voltage to +55 VDC. At this voltage, the RF output measured about 0.6 division peak ( or about 1/3 the peak CW amplitude ).

Applying modulation causes the the carrier level to come up but now when 2 divisions peak carrier occurs with AM ( same output level as in CW mode ) the carrier is almost cut-off in the valleys so the modulation is symmetric. At 90% the modulation does look a bit distorted both top and bottom though.

The 24K resistor was bypassed with a 0.1 mfd capacitor same as used in the DX60B.
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K2OWK
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« Reply #6 on: February 28, 2016, 04:51:50 PM »

When I had the DX-35 the best solution to get excellent audio was to build a plate modulator. I used a pair of 1625 tubes running about 25 watts of audio. The controlled carrier modulation is an inexpensive way to modulate a transmitter with minimum audio drive. It works OK if your signal at receiving end is strong, if not the audio is very difficult to hear. Plate modulation allows a full 100% regardless of the signal lever, making it much easier to hear.

73s

K2OWK
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HAMHOCK75
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« Reply #7 on: February 28, 2016, 05:15:31 PM »

I know what you mean. I have built a plate modulator before from an old ARRL handbook. That's not what I am trying to do right now. Just trying to restore the DX-35. At the same time understand the design. I was not expecting the controlled carrier modulation to be much different than the DX60B but it is rather inferior. The DX-35 was two generations before the DX60B so I guess Heathkit did really refine it. The confusing part is that the voltages described in the assembly manual differ from the unit itself.

I haven't used load lines in decades but it was a fun exercise. Phil's post showed his DX-35 behaves exactly like mine confirming the analysis is correct. I doubt that I will ever use this rig on AM.

Now moving on to plastic welding the cracked plastic knobs. Heathkit did not use knobs with metal inserts so tightening the knob cracks them eventually. I embed stainless steel wire into the knob to act as a poor man's collet.
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QRP4U2
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« Reply #8 on: February 28, 2016, 10:24:31 PM »

Good questions Phil. I have a 100 MHz dual channel Tektronix oscilloscope so I am looking at both at the same time. This is what I see. The +77 volts on the 220K cathode follower resistor drops to +64 volts probably because the B+ drops from +530 to +450 volts. The problem is that the audio ( which looks fine ) is riding on top of the +58 volts. That definitely shifts the average screen bias up increasing the carrier but the carrier will not go down much from the value with +64 volts on the screen grid.

Are you the person who made the improvements? Looks like you might have been posting under a different handle.

edit: I had posted that the screen grid of the 6146 drops to +58 volts. Just put everything back together. It drops from +77 volts to +64 volts.

Yes one and the same. I don't use the same Handle because same Handles are boring. Wink DMOD was short for, "Digital Modulation."

I do have the same call letters.

The DX-40, DX-60, Viking Challenger, the Knight T-60, the Knight T-150, the Halli HT-40, etc., all have "controlled Carrier" screen grid modulation using either cathode follower modulators or shunt modulation modulators.

The DX-35 was an exception in that it had direct screen grid modulation.
 
The latter circuit modifications to SGM are called, “Controlled Carrier” (CC). Controlled carrier transmission can be defined as, “a method of modulation in which the average carrier output varies with the audio level, instead of remaining constant as in conventional plate modulated systems.”

A properly designed CC transmitter can be designed to modulate at >= 85% while automatically adjusting the average carrier level to accommodate the audio input signal. When the audio level increases the “average” carrier level increases as well. This action also prevents over modulation. The CC version of SGM was designed to improve operating efficiency.

In the latter CC designs, the cathode follower circuit is basically a voltage divider with a high-pass (parallel RC) circuit. What you really have out of the cathode follower is a DC level with an audio pedestal. The quiescent DC voltage level is set and then audio rides on the DC levels modulating the screen voltage. The quiescent DC voltage level sets the average power level.

The design flaw in most CC SGM's is that the operating SG voltages are too high for a tube (6146) whose SG curves are non-linear in those regions. That is why the Hallicrafters HT-40 had such good stock audio, the 6DQ5 was operated in the linear region of its SG curves.

For the DX-35, the best you can do is to improve the speech amplifier so the best quality audio reaches the cathode follower. 90% modulation is about the max I have seen with my modifications.

In the "V3A" schematic, you will see the Modulator supply circuit has extra energy storage to maintain a more constant voltage during high modulation peaks.

Properly designed and or modified SG speech amplifier and modulator circuits can produce very good quality audio.

Phil - AC0OB

« Last Edit: February 29, 2016, 10:49:38 PM by QRP4U2 » Logged

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HAMHOCK75
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« Reply #9 on: February 29, 2016, 12:19:48 AM »

I checked out the specifications for the DX35, DX40, and DX60B. All of them claim controlled carrier AM but they are not of equal performance. The DX35 says 65 watts input CW mode with 50 watt peaks in AM mode, the DX40 says 75 watts input CW mode with 60 watt peaks in AM mode, only the DX60B claims 90 watts input CW mode and 90 watt peaks in AM mode.

It is not very clear that the CC modulation in these earlier rigs could not really achieve a high modulation % due to the inability to reduce the carrier power when that was necessary. The peak power limitation in AM is not that it is impossible to hit 65 watts input power on AM but if you do, the distortion is awful. I have not worked on a DX40 so I am not sure what limitations it has. The DX60B could put out as much power on AM peaks as in CW mode with nice symmetry straight out of the box.

The high pass filter I introduced ( ala DX60B ) allows the DX35 to hit the same peak power in AM as in CW mode ( but there is more distortion than in the DX60B ). Not sure I will leave it that way though because my objective is to restore not necessarily to improve the rig. I can see why you did all the fixes in the link. There is a nice arc every time I move the function switch from Standby to AM.

I am glad to see the work you did. Really very nice.
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VE3LYX
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« Reply #10 on: February 29, 2016, 03:36:01 AM »

First I must say I am SO SICK of hearing plate modulation, plate modulation etc and 100% modulation too. Plate modulation is where you build another radio  to modulate a radio.(And is rarely used commercially BTW if ever) Wonderful if you are into the biggest the best the deepest etc etc. I am not. Decent understandable modulation as it was when the radio was made is fine. It is not necessary to have 100% mod. Considerably less with still do a fine job in communications. Is it an ideal to shot for? Perhaps. Is is necessary? No. Many rigs including many old WW2 rigs fall far short of that and were more then adequate for the job.
So the Heath DX  series of CW/ AM transmitters. I have found the choice of microphone to be the biggest single important choice. Casting aside all the wonderful mods that supposedly make it sound better,(Personally I can't hear "the wonderful improvements" nor do I really care.) However the right mic will change an also ran into a performer. When my amplified Cardon mic popped its element I borrowed the turner mic off my old Heathkit twoer. Not because I though it would be wonderful but because I was in a group when it quit and wanted to finish up. It was nearby on display (I dont use it I just have  it) and had the same goofy connector so I could pop it on in a minute to finish. Immediately I could tell from the rigs meter and the shacks FS meter I had done something that made a difference. Huge difference to be honest so before I went cutting and  modifying I would try a few different mics. I used to think the speech amp in these was a bit light but that concept is now history. BTW I appreciate your desire to keep it original a much as possible. One also has to sort of think if the DX35 and DX 40 were perfect there would not have been a DX60 and a DX60B. If it was possible it would be neat to compare it on the test equipment to another known good DX35 unit to see how they match (or don't)
GL .
donVE3LYX
« Last Edit: February 29, 2016, 03:46:57 AM by VE3LYX » Logged
QRP4U2
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« Reply #11 on: February 29, 2016, 09:58:22 AM »

 
Quote
The peak power limitation in AM is not that it is impossible to hit 65 watts input power on AM but if you do, the distortion is awful. I have not worked on a DX40 so I am not sure what limitations it has. The DX60B could put out as much power on AM peaks as in CW mode with nice symmetry straight out of the box.

One can't expect high powers or low distortion from weak power supplies or minimally designed speech amplifier and modulator circuits, respectively. The only powerhouse CC rigs were the Allied-Knight T-150 and the EF Johnson Challenger.

If you look at the various CC transmitter upgrades on AMFONE, you will see that most modifications are to first improve the power supply's energy availability, the speech amplifier (to reduce distortion in that stage), and thirdly to improve the linearity of the modulator/final circuit.

While a good microphone of high impedance (>50k) helps, without 'linearizing' the speech amp and modulator/final circuit, the result is less than desirable audio.

Besides, when you are a Ham, you experiment with and improve boat anchors - that's what you do!. Grin

Phil - AC0OB
« Last Edit: February 29, 2016, 10:10:29 AM by QRP4U2 » Logged

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G3RZP
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« Reply #12 on: February 29, 2016, 01:08:09 PM »

 
Quote
Plate modulation is where you build another radio  to modulate a radio.(And is rarely used commercially BTW if ever) Wonderful if you are into the biggest the best the deepest etc etc. I am not. Decent understandable modulation as it was when the radio was made is fine. It is not necessary to have 100% mod. Considerably less with still do a fine job in communications. Is it an ideal to shot for? Perhaps. Is is necessary? No. Many rigs including many old WW2 rigs fall far short of that and were more then adequate for the job.

But the BC191, the TCS series, the ART13, the BC610, the SCR522, the later versions of the Command series using the later modulator, the RCA ET4336, and then the Marconi TGS541 and 571 transmitter and the Marconi Marine 'Globespan' and 'Worldspan' and 'Kestrel', the Ajax marine Ajax 25 and an Ajax 100 marine transmitters, the Pye Dolphin marine transmitter, the Pye Telecomms AM base station and mobile VHF transmitters, the Marconi BD372 series BC tx, the BC transmitters from Collins and Gates and Harris, the Labgear LG300 amateur transmitter, the Panda PR120V, Explorer and Cub transmitters, the Minimitter transmitters, the KW Vanguard and valiant, the Tiger TR100 and TR200, the Collins 32V and KW 1, the multitude of Johnson transmitters, the Heath DX100, the RAF TR1192, the T1509 and the UK Army 53 set all show this claim to be rubbish.

More of them used anode and screen  than used any grid modulation including controlled carrier, and in total, far more were made than any WW2 tx using grid modulation.

So the claim "(And is rarely used commercially BTW if ever)" is absolute poppycock!
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QRP4U2
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« Reply #13 on: February 29, 2016, 02:16:38 PM »

Just to add some Historical trivia:

Amelia Earhart's Radio, the 1935 Western Electric 13C transmitter that flew aboard Earhart’s Lockheed Electra 10E, NR16020, used a direct SGM design because of the weight factor.

Continental manufactured a number of standalone, pure SGM Broadcast transmitter’s in the late 1950’s with transmitter powers up to 10 kW. One 5kw SGM unit fed a Doherty linear Power Amp for 55 kW output. With no "iron" in the intermediate (driver) stages, the audio response was 30Hz to 20kHz.

The first television transmitters used SGM in the final's to add synchronizing pulses to produce the composite video signal.

Phil - AC0OB

Besides, when you are a Ham, you experiment with and improve boat anchors - that's what you do!. Grin
« Last Edit: February 29, 2016, 02:19:39 PM by QRP4U2 » Logged

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Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
G3RZP
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« Reply #14 on: March 01, 2016, 12:31:30 AM »

All the big Marconi broadcast transmitters used plate or plate and screen modulation. That's right up to 750kW of carrier out - the BD229. The 100kW and 250kW carrier HF BC transmitters used plate and screen as well: the 250kW one had an oil cooled modulation transformer which was about a 9 foot cube and weighed in at 13 tons.

Not quite an 'off the shelf' from Stancor!
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