Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 [2] 3 Next   Go Down
  Print  
Author Topic: Heathkit DX35 controlled carrier modulation  (Read 23813 times)
JS6TMW
Member

Posts: 1255




Ignore
« Reply #15 on: March 01, 2016, 06:55:52 AM »

The point has been well made, but I'd just add that if you look at any ARRL Handbook from the "tube" years, most voice transmitters used plate modulation. Modulation transformers were relatively costly but almost any power tubes could work in push-pull Class B, and their HV supply could be cobbled together easily.

Anything else lacked serious AM punch. I used a DX-35 and spent many hours talking myself hoarse into the mike with few contacts to show for it.

Steve in Okinawa (that was when I was K2RDP)
Logged
QRP4U2
Member

Posts: 262




Ignore
« Reply #16 on: March 01, 2016, 08:07:38 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.

Thanks.

Ham SGM transmitters can be fun with which to experiment.

If you keep it stock, don't expect miracles.


Phil - AC0OB

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

AC0OB - A Place Where Thermionic Emitters Rule!
--------------------------------------------------------------------
Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #17 on: March 01, 2016, 05:22:36 PM »

Discovered one more interesting item. The DX35 was probably meant to be used with a the VF-1 vfo. The VF-1 actually supplies 160 M output for 80 M. When I used a vfo with 80 M output for 80 M, it was hard to stop the plate current from pinning the meter all the time. I had to attenuate the vfo output to get the plate current back under control and set to 120 mA. With the vfo set for 160 M there was no problem ( except I could see significant 160 M leakage ).
Logged
QRP4U2
Member

Posts: 262




Ignore
« Reply #18 on: March 14, 2016, 09:39:01 PM »

Discovered one more interesting item. The DX35 was probably meant to be used with a the VF-1 vfo. The VF-1 actually supplies 160 M output for 80 M. When I used a vfo with 80 M output for 80 M, it was hard to stop the plate current from pinning the meter all the time. I had to attenuate the vfo output to get the plate current back under control and set to 120 mA. With the vfo set for 160 M there was no problem ( except I could see significant 160 M leakage ).

Not necessarily.

i use both crystals and vfo's.

You may want to check the vfo's output voltage on an oscilliscope.

i have found that too high of a vfo output voltage can cause the first RF amp stage to overload it to the point of making it a multiplier.

Phil - AC0OB
Logged

AC0OB - A Place Where Thermionic Emitters Rule!
--------------------------------------------------------------------
Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #19 on: March 16, 2016, 06:47:04 PM »

I will do that. I had to set the project aside for a bit while waiting for parts for my oscilloscope probes. The grabber tips seem to have spring fingers that slip over the probe tip but mine have become so worn that they are annoyingly intermittent. Just got the parts this week. Also, another local ham says he has a high voltage probe I can have which will be easier for my work around tube units.
 
I want to have that information because I am thinking about a DDS vfo for the unit but because a DDS ( that goes to 30 MHz ) does not really need doubling, tripling, quadrupling, etc. it would be interesting to see what output level is needed if the DX35 is driven with a fundamental at all frequencies.

I blew a scope probe awhile ago working on my Yaesu FT101 even though the ACrms specification for the probe was not exceeded. What I did not realize was that in "tune" mode if I set the output for 120 watts into 50 ohms, the peak AC rf output was about 110 volts. The FT101 can momentarily have its output voltage hit a peak of 160 volts which does exceed the specification before it settles to 110 volts peak. A former Yaesu engineer said it can do that because before the power supply is loaded and drops to its steady state voltage, it can supply more voltage momentarily. He said it was normal. Fortunately, I found the right surface mount resistors at a local surplus store and was able to rebuild the probe. Works fine again.
Logged
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #20 on: March 17, 2016, 03:38:02 PM »

Rechecked the DX-35 on 80M. Found 1 Vrms at 3.8 MHz results in 4.7 mA grid current. Dropping to 0.14 Vrms at 3.8 MHz brings the grid current down to 2.5 mA. 1 Vrms at 1.9 MHz results in 2.6 mA grid current. The buffer tuning capacitor is all the way in at 1.75 MHz and all the way out ( minimum C ) at 2 MHz. At 1.75 MHz grid current is 2.4 mA but plate current is 150 mA. Antenna capacitor is at max. C. At 2 MHz, same but plate current is 140 mA. I added a 220 pF capacitor in parallel with the antenna capacitor which supplied enough capacitance to bring the plate current down. So the high plate current is the result of insufficient C and overdrive with a VFO that actually has output on the 80M band. Seems most VFO's that output on 80M can overdrive the DX-35.

Curious if you have had any experience with the DX-35 having insufficient antenna C. All tests are into a 50 ohm dummy load.
Logged
QRP4U2
Member

Posts: 262




Ignore
« Reply #21 on: March 17, 2016, 07:13:29 PM »

I have.

I just looked at my mod. schematic.

I had to add a 47 pF 1kV ceramic across the Plate Tuning cap.

I also had to add a 330 pF cap across the Antenna Tune cap, since the stock antenna tune cap has only 900 pF max capacitance.

I eventually replaced the antenna tuning cap with a 3x450 pF variable tuning cap.

The antenna tune capacitance really needed to be about 1300 pF with the stock Pi-Net coil.

Sounds like you're making progress.

And if you use a DDS, you need to use a voltage amplifier to increase output voltage, since the raw output voltage of a DDS is around 3Vpeak.

Phil

--------------------------------------------------------------------

Besides, when you are a Ham, you experiment with and improve boat anchors - that's what you do!.  Grin
 
« Last Edit: March 17, 2016, 07:27:13 PM by QRP4U2 » Logged

AC0OB - A Place Where Thermionic Emitters Rule!
--------------------------------------------------------------------
Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #22 on: March 18, 2016, 02:18:36 AM »

I increased the capacitor in parallel with the antenna tuning capacitor to 330 pF but is is still not quite enough as you mentioned. I can see why you changed the 2x450 pF to 3x450 pF. There is no way to leave a fixed capacitor in without it affecting all other bands.

If I drive with 1.75 MHz for 3.5 MHz output, with the antenna tuning with the added 330 pF set to maximum, the plate current is 135 mA. with 47 watts output. Still a bit higher current than desired.
With 2 MHz drive for 4 MHz output, the antenna tuning with added 330 pF can be set to "17" so plate current can be set to 125 mA with just under 50 watts output.

3 Vpeak is 2.1 Vrms. Only 0.12 Vrms was needed for 47 watts output at 3.5 MHz without any doubling. The higher bands are the most problematic. The crystal oscillator stage only has tuning for up to 20M so a direct 10M input signal for 10M output will be attenuated. The DDS output will probably fall off that high as well.
Logged
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #23 on: March 19, 2016, 06:58:54 PM »

Interesting new issue. As I lowered the 4 MHz drive of the VFO, the buffer stage breaks into oscillation at about 3.5 MHz. There are numerous resistors to lower the Q of the 80M tank circuits. Most of these carbon composition resistors will drift higher in value over time so maybe the tuned input/output Q has increased. Did you experience anything like that?
Logged
QRP4U2
Member

Posts: 262




Ignore
« Reply #24 on: March 20, 2016, 08:47:34 AM »

There are number of Q reducing resistors that can change in value.

I also decoupled the OSC/Buf. string with separate power feeds which increased stability.

Rather than describing each and every stage, send me an email and I will send you a set of schematics reflecting the latest modifications.

Phil - AC0OB

--------------------------------------------------------------------
Besides, when you are a Ham, you experiment with and improve boat anchors - that's what you do!.   Grin
« Last Edit: March 20, 2016, 08:55:34 AM by QRP4U2 » Logged

AC0OB - A Place Where Thermionic Emitters Rule!
--------------------------------------------------------------------
Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #25 on: March 21, 2016, 01:23:27 PM »

Thanks for your offer. I feel I am pretty close to the end of this restoration so unless something new surprises me, I probably will not need the schematic.

I took both the 80 and 40M coils out with their de-Q'ing resistors. The 3.9K for the 40M coil was dead on. The 4.7K for 80M was about 5.25K. I replaced it. Now the only time I get oscillation is when the meter is in the "plate" position. When the meter is in the "grid" position it knocks out any oscillation. The 22K in series with the meter shunt for the grid is also out of spec.  ( about 24K instead of 22K ). I will try to replace that today.

I noticed something else. When the DX35 has no drive to the final, it draws about 120 mA of plated current because the 22K and meter shunt of 500 ohm are too small to create much grid bias.

I just got a Drake 2NT in for my next restoration. The Drake uses a fixed -80 volt supply to the grid to prevent plate current in the absence of drive. Also, I noticed that Drake makes available 1460 pF at the output of the Pi network instead of the 900 pF of the DX35 on 80M
Logged
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #26 on: March 24, 2016, 01:57:45 AM »

The DX-35 is full of surprises. Maybe you can tell me if you have any of the following experiences.

1. The oscillation without any crystal of vfo drive is solved. The fact the the oscillation only takes place with the meter reading plate current was the final clue. I have some ferrite beads designed to have a minimum loss of 75 ohms at 10 MHz. I inserted a few of those in the meter lead to the 500 ohm shunt, precision resistor in the grid. No effect. I added only one to the meter lead from the 12 ohm, precision cathode shunt. The oscillation stopped. There is some resonance formed between the .02 mfd cathode bypass capacitor and the long leads to and perhaps including the inductance of the meter.
2. Once the oscillation was stopped something else appeared. I was measuring the voltage across cathode 12 ohm resistor. It measured 1.2 volts yielding 100 mA. cathode current but the DX-35 meter was reading 120 mA. The manual says the meter is actually a 500 ohm, 3 mA  full scale meter. 1.2VDC across 500 ohms yields a meter current of 2.4 mA. Calculating the meter reading as ( 2.4/3.0 ) * 150 mA = 120 mA. The meter seems to be designed to read 20 mA. more than is really in the cathode circuit.
3. To correct the meter, I removed the ferrite bead and inserted a 100 ohm resistor in series with the lead from the 12 ohm cathode resistor. Now my scope, Fluke DMM, and the DX-35 all agree.

Next I placed my Fluke DMM to measure the high voltage to the 6146 then calculated the input power for the plate current set for 100 mA and 125 mA vs. output power.  

Plate current set for 100 mA except on 80M where is was not possible

1. 40M plate = 681 V, 68 watts input, 42 watt out
2. 20M plate = 687 V, 69 watts input, 40 watts out
3. 15M plate = 657 V, 66 watts input, 36 watts out
4. 10M plate = 515 V, 52 watts input, 30 watts out

Plate current = 125 mA

1. 80M ( 4.0 MHz ) plate = 650 V, I = 125 mA, 81 watts input, 50 watts out
2. 80M ( 3.5 MHz ) plate = 626 V, I = 150 mA, 93 watts input, 45 watt out
3. 40M plate = 665 V, 83 watts input, 48 watts out
4. 20M plate = 665 V, 83 watts input, 48 watts out
5. 15M plate = 636 V, 80 watts input, 45 watts out
6. 10M plate = 515 V, 64 watts input, 35 watts out

Insufficient Antenna Tuning capacitance makes it impossible to bring the plate current to the point where the old novice 75 watt input specification could be met on 80M; that the 6146 is nearly at its maximum plate dissipation with low efficiency at the low end of 80M.

The last item I would like to ask you about. The output power on 10M drops but it looks like because the HV drops down to nearly 500 VDC from over 630 VDC on every other band. Have you seen this? I suspect that perhaps the process of quadrupling might add to the power consumption but did not expect to see that much. Wondering if you think this is normal or is something remains to be investigated.

If I had not corrected the meter, 120 mA on the old meter would really be 100 mA. which would put the DX-35 close to its specified 65 watt input power on every band but 80M. Curious. Wonder if it was intended that way.
Logged
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #27 on: March 25, 2016, 02:19:58 PM »

Last night I went to look into the reason the HV drops to 515 VDC on 10M. The function switch began a continuous arc'ing. I took it out and found that the contacts for grounding the HV transformer center tap where completely black including the ceramic under the contacts. The stuff on the ceramic was so hard it took an X-Acto to scrap it off with final cleaning with isopropyl alcohol.

Now I get,

Plate current = 100 mA

10M plate = 605 V, 60 watts input, 35 watts out

Plate current = 125 mA

10M plate = 605, 76 watts input, 36 watts out.

Almost no additional output power as plate current increases. It happened to be a coincidence that I was testing the 10M band when the HV started to fail due to the function switch.

New problem.

As I put the DX-35 back into its case, the meter needle starts moving up ( DX-35 is unplugged ). It appears either the meter or the case is magnetized. The farther into the case, the higher the meter reading to the point that the adjustment screw is barely able to zero it. Looking into how to demagnetize the meter or the case.
Logged
HAMHOCK75
Member

Posts: 621




Ignore
« Reply #28 on: April 05, 2016, 02:17:59 PM »

I received the little de-magnetizer in the mail but although it worked on all my screwdrivers and other small parts, it did work on the meter. It might be that the de-magnetizer can't get close enough to the movement to have an effect.

I gave up and bought a pack of small ferrite magnets and glued one in the proper orientation to cause the meter to deflect enough in one direction so I could get back into the case and zero it. I re-checked the accuracy and all is good.
Logged
QRP4U2
Member

Posts: 262




Ignore
« Reply #29 on: April 05, 2016, 08:20:54 PM »

I received the little de-magnetizer in the mail but although it worked on all my screwdrivers and other small parts, it did work on the meter. It might be that the de-magnetizer can't get close enough to the movement to have an effect.

I gave up and bought a pack of small ferrite magnets and glued one in the proper orientation to cause the meter to deflect enough in one direction so I could get back into the case and zero it. I re-checked the accuracy and all is good.

As far as the meter, I had to place a 1k 1/4W resistor in series between the red lead and the meter positive to get accurate current readings.

To protect the meter, I placed a 5W 2.4V zener across the meter posts.

After reworking the LV distribution to the two RF amp stages, i have had no more problems with instability, as mentioned in a previous post. Highly recommended.

Phil - AC0OB
« Last Edit: April 05, 2016, 08:25:54 PM by QRP4U2 » Logged

AC0OB - A Place Where Thermionic Emitters Rule!
--------------------------------------------------------------------
Besides, when you're a Ham, you experiment with and improve boat anchors - that's what you do!. Smiley
Pages: Prev 1 [2] 3 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!