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NEW Project - 6146B Plate modulated by Hammond 1628SEA and 85W SS Amp




 
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Author Topic: NEW Project - 6146B Plate modulated by Hammond 1628SEA and 85W SS Amp  (Read 14945 times)
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KB2WIG
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« Reply #175 on: April 19, 2022, 01:37:36 PM »


That rig looks ballsey and very stout.


The meters, control knobs, window and the labels dare one, no Demand one to to sock Atomic Chainsaw Yeah-A-Lows and thereby Command the aether.

Very fine work.

klc
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« Reply #176 on: April 20, 2022, 12:12:51 PM »


That rig looks ballsey and very stout.

The meters, control knobs, window and the labels dare one, no Demand one to to sock Atomic Chainsaw Yeah-A-Lows and thereby Command the aether.

Very fine work.

klc


Thanks for the comments Kevin!   Yep, I think you coined it... Commando might be considered a ballsey and stout looking rig. Reminds me of a big frog, ready to leap... or even a face like Ernest Borgnine.   Hopefully he can take rapid, punishing chainsaw yallos...  Grin Grin  I'll use that description later on when on the air with him.

Bob:  Yes, I'll experiment with artificial lighting as well as daylight to see what works. Even a polarizing filter may help. Right now, no matter how well I hold the camera, without the flash I get a tiny wobble that shows up as blurriness. The tripod will help a lot.  Your labels worked very FB. I'm now putting them on the older rigs as band presets.


More TDA7293 details:    (The Commando 140W modulator)

Rick emailed me some more great info about mounting the TDA7293s.  The kit boards arrived yesterday via USPS from China and went together easily as shown in the pic below.  He suggested a bracket to hold the center of the chips onto the heatsink with better even pressure. The normal chip screw is off center, favoring the top of the chip.  The bracket cannot touch any part of the PCB board or parts. It is grounded into the heatsink and sits across both the plastic TDA chip covers to give an even downward pressure. Better heatsinking using the bracket.  The three screws all need to be torqued evenly.

** The chip has -VCC (-45V) on the metal tab, so needs to be insulated from the heatsink.


I also ordered some nylon screws and sil pads to better custom fit the chips.  The TO220 silpads are too small.   I need to cut a clean screw hole into the silpad. Anyone know how to do this? I suppose a paper punch wud work.

So in summary, to mount I will use a nylon screw, washer , then the chip -  and silpad into the heatsink.  Then on top a small bracket with a screw in the middle to add a more even pressure.  No paste needed.

Hopefully everything will work when the nylon screws and silpads arrive from Mouser. I hope I didn't damage the chips during installation.  Rick gave me some refined installation precautions after I did the job.. I will post them here for those who are building a TDA module.

I plan to add a small 2.5" 120VAC muffin fan next to the heatsink blowing air thru the fins. It will be powered off the same Variac that feeds the 6146B fan. At 20-30% spin rate both should be very quiet and effective.


I see there are no more dual TDA kits or assembled available. But there are THREE chips in parallel TDA boards for about $30 assembled on eBay.

T


Pic #1 below:  Two dual TDA7293 KIT boards (one as a spare) almost ready for testing into a 10 ohm dummy load, showing the loose holding bracket.  (140 watts)    (The two outer screws will be nylon - the center holding the bracket will be stainless; all 6-32)


* DSCF0001.JPG (320.44 KB, 1280x960 - viewed 71 times.)
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« Reply #177 on: April 20, 2022, 02:21:20 PM »

Valuable info from Rick / W8KHK  about mounting and installation of power chips.  (TDA 7293)

T


------------------
Written by Rick and posted with his permission as follows:


Hi Tom,

Good news on receiving the kits.

They normally do not provide mounting hardware if the kit comes without heat sinks.  The mounting is usually done with a machine screw ( i prefer stainless steel) and a mica insulator or sil pad.  To insulate the tab on the IC from the screw head, a nylon or teflon washer with a flange is used between the bolt and the IC mounting tab, thus insulating the screw heads from the  tab, which is at minus 45 ( on the VEE bus).

The sequence I prefer is the screw head, flat washer (to allow sufficient mounting pressure) nylon flange insulating washer, IC tab, si lpad or mica, then the heat sink.

If you can get enough pressure with a nylon screw, give it a try.  Use the largest screw that will fit through the tab hole.  Heat sink compound normally is not used with sil pads, only with mica insulators.  Most of my experience is with mica and heat sink compound.

The device has overload AND thermal shutdown, so as long as you do not do full-power tests very long, you should be ok with a sil pad and nylon screw.  Use a flat washer to allow the screw to be tightened with minimal turning friction on the screw head, with the goal of making maximum pressure on the tab-to-heatsink junction without breaking the screw.

Often that type of tab transistor or IC is held down to the sil pad with a bracket that rests on the top of tie IC body, with a screw to the heatsink on either side of the chip.  This allows much more complete contact and thermal transfer.  

I have seen (and extracted) these types of brackets from used PC power supplies.  They attach the switching transistors to the heat sink metal bracket, which is usually just a plate of aluminum.   You might be able to make the same type of bracket with a scrap of thick aluminum.  After mounting the devices with the nylon screw, place a piece of aluminum across BOTH devices, and with a hole in the center of the aluminum strip, run a hole with a 6-32 or 8-32 machine screw into a tapped hole in the heat sink.  This method provides even better heat transfer than just a screw at the top of the tab (which only applies contact pressure at the top end of the device, not the best by any means.)
Hope you have success in your tests.  

---------

 It is really better to use ESD protection but you'll probably get lucky this time without a problem. When I attach heat-sinked devices to a PC board, I normally attach them to the heat sink first and then solder one connection on each device.   Then i make sure everything is aligned.  I solder the other terminals alternately to give it time to cool. One of the things we need to keep in mind is that even though these devices look very strong, they are susceptible to damage if too much strain or stress is put on the individual leads, especially when mounting to a heat sink.

 Another thing to try might be to use metal instead of plastic screws but insulate them in a different manner. If you have some nylon or plastic or fiber flat washers to insulate the screw head from the device, it's possible insulate the screw threads from the hole in the device with a piece of sleeve, this is often done with nylon or Teflon but you might consider a short piece of shrink sleeving over the threaded portion of the bolt along with just a flat washer to allow you to still use metallic grounding screws for the devices.


----------------
 I saw your post with the photo of the proposed bracket. Looks VY FB, thick aluminum bar.  As long as it is thick enough so it does not bend or distort, it should apply very even pressure to the tops of both chips.  Just do NOT tighten it so much that it bends, snug is fine, heavy duty torquing is not FB.

Yes, you may post my comments, and even the photos of the PC power supply sample if you believe they would be helpful.

That chip is pretty well protected internally, I doubt very much if you damaged it with ESD or soldering temp.  Just remember in the future, to solder one lead, let it cool, then do the next one, etc.  But if it was wave-soldered in a manufacturing plant, it would probably be hotter than what you caused with your iron.  No worries!

I would be VERY CURIOUS to see how warm that nice heatsink gets WITHOUT the fan.  It has lots of surface area.  If it is oriented so air can flow upward through the fins, maybe a fan is not needed.  The amplifier should be very efficient, ans with the voice duty cycle, it should not get very hot.  Steady-state audio will warm it up, but voice should not be a problem, even with moderate compression.

GL on the RF TESTING!

PS, if you have some sleeve and non-metallic washers to insulate the machine screw from the tab, I would use a metallic screw, instead of nylon.  But with the clamp it may not matter.  Notice that on the PC sample, they used one big sil pad, and NO SCREWS in the tab, just the metal clamp.  But then again, the devices and the heat sink were all secured to the same PC board.

As long as your interconnect wiring does not put a strain on the board position, you should be good just mounting it the chip leads holding the board.  But for mechanical rigidity, I would prefer an angle bracket holding the PC board stable in relation to the heat sink.  You do not want vibration or bending forces constantly moving the position of the chips and board, held only by the chip wires.  With a double row of pins, they should be rather strong, but the bending point where the leads enter the chip is the failure location if stressed regularly.

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







* Rick's Chip Bracket.jpg (104.01 KB, 1007x755 - viewed 72 times.)
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« Reply #178 on: April 20, 2022, 04:57:42 PM »

you're experiencing a slower shutter speed in low light conditions.  Camera is trying to compensate but can only do so much. I've experienced the same and I've tried messing shutter and aperture priorities and ISO to override it but it doesn't help really.  the tripod will definitely take care of the blur in low light.  Glad the labels are worked out.  Did the white on black work out good too?
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« Reply #179 on: April 20, 2022, 10:16:54 PM »

Hi Bob,

Finally!  The tripod arrived and what a difference it makes. The following samples were all made with a single lamp for light, no flash.  Steady tripod mount with no shake. Some shots took up to a full second to shutter. It is much more controllable and what you see if what you get.  I believe the F-stop and / or shutter speed is automatically controlled. The camera stays on depending upon the light available.

The first pics are standard size while the last ones are resized larger and show the beginnings of graininess.  Check out the closeup of the Commando brass plate. There was no way I could get a closeup like that before due to flash glare and unfocused light. With the tripod it's a cinch to get good quality. I will continue to experiment and hopefully my pics are much better in the future.

The  light balance across the whole picture allows better detail and also eliminates the harsh imperfections that are not visible in normal room light conditions.

T

Pic #1:  Notice how easy it is now to get all the gold/brass to come out and shine!

Pic #3:  "YAZ'  is a single 813 series modulated by a pair of 813s.  No audio transformers. All have Plexiglas cases. There is little glare on the Plexi now. It was brutal before.   All rigs use the GFZ MOSFET audio driver board.

*** NEW DISCOVERY***   Click on the pic below and get a 1/4 page sized pic.  But click on the blue fie name at the bottom of each pic and get a full page pic that can be enlarged using your "+" icon or mouse roller. You must click on the file icon that appears at the bottom of your screen to activate the full screen feature..  Use the mouse roller to enlarge.  I never knew about that feature. The enlarged pics are really something else.



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« Reply #180 on: April 20, 2022, 10:19:19 PM »

Pic #1 is an enlarged pic of the brass plate. Impossible to get before... I tried many times.


Pic #3 has reasonably balanced color and light.. about what I see in person.  One of the best front-panel Commando pictures taken to date.


*** NEW DISCOVERY***   Click on the pic below and get a 1/4 page sized pic.  But click on the blue fie name at the bottom of each pic and get a full page pic that can be enlarged using your "+" icon or mouse roller. You must click on the file icon that appears at the bottom of your screen to activate the full screen feature..  Use the mouse roller to enlarge.  I never knew about that feature. The enlarged pics are really something else.


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« Reply #181 on: April 20, 2022, 10:21:46 PM »

Check out pic #3:    With the flash before, the details were very harsh and beat up looking. This is more what it looks like in natural light with the human eye to me.  I was relying on a flash to make a fast shutter speed to prevent camera shake.  Now the light is controlled before the shot and the longer exposure is steady in the tripod..

T


Pic #1:  813s modulated by a pair of 813s. The best shot I ever made of it.  

Pic #2:  Single 3-500Z linear with 1500V - super clean linear, -45 dB 3rd.  Great driver.

These older rigs are now getting the pre-tune label mark ups for each band.



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« Reply #182 on: April 20, 2022, 10:28:58 PM »

This should be enuff pics to see the difference.  

This pic has been enlarged a bit. Notice the graininess.  Just experimenting.

Live and learn.

T


* DSCF0038.JPG (393.31 KB, 900x675 - viewed 53 times.)
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« Reply #183 on: April 21, 2022, 09:23:39 AM »

Tom,
Those pictures look excellent and the rigs are sweet looking.  yes, tripod in natural room light is the ticket. stability makes all the difference in sharpness.   

I did an online comparison of xenon flash vs nature light and xenon apparently is absent of orange, yellow and some red portions of the light spectrum so blue, green and even UV are the main color components of a white xenon flash where natural light has very little loss of color spectra.  I figured xenon covered all the colors giving it its whiteness and brilliance but apparently not.  And camera sensors are sensitive to UV too. All of which might explain that harsh look.

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« Reply #184 on: April 21, 2022, 12:10:19 PM »

Tom,
Those pictures look excellent and the rigs are sweet looking.  yes, tripod in natural room light is the ticket. stability makes all the difference in sharpness.  

I did an online comparison of xenon flash vs nature light and xenon apparently is absent of orange, yellow and some red portions of the light spectrum so blue, green and even UV are the main color components of a white xenon flash where natural light has very little loss of color spectra.  I figured xenon covered all the colors giving it its whiteness and brilliance but apparently not.  And camera sensors are sensitive to UV too. All of which might explain that harsh look.


Hi Bob,

FB on the lighting. It's hard to beat an incandescent lightbulb.


I found out two more things:  Most of the images I posted in the last group are only 640 X 480.  So they cannot enlarge much without graininess.  But the one Commando brass plate pic is 1920 X 1440.  I'm not sure why the others are not too.  

Also, if you click on the photo .jpg picture link and then right click on the pic, there is a selection called "file information."   It shows the pic size and the shutter speed, f-stop and mm size.  For example one pic shows:  640 X 480     1/2 sec   f/2.8   6mm.    That 1/2 sec and f/2.8 are indicative of a close up shot.  I just need to get the camera to do 1920 X 1440 next time to allow exquisite enlargements.


You axed about the black on white labels for white panels use... they worked out very well.  The white background blended in FB. I'm working on them now. I need to fire up every rig in the shack on every band to find the best presets. Lots of work but valuable info for jumping on a rig and getting tuned up at random.

T




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« Reply #185 on: April 21, 2022, 02:47:19 PM »

Tom,
the only thing I see when I right click is image size and file size.  I can see all those parameters in Photoshop though if I download the file.  must be something unique to your system.  Not sure if you see this in AMFone, but the file size and image size is displayed next to the filename.  What is interesting is that your image size numbers are indeed different.  Back on page 7 image size is 1280x960 for most and on page 8 they vary to something smaller as well as the 640x480 size.  I can't think of a reason why a camera would change that on the fly but maybe some cameras do that to speed up write time. Just a guess but it would explain graininess when enlarging.  I don't think my Nikon does that. Of course it has so many features, functions, menus and buttons it is impossible to know and keep straight.  But at least you nailed getting pix the way you want them to look or should look.  
 

Glad the black on white labels match up nice. I have black on clear now too.  

Different subject.  Your plexiglass cabinets, how do you get nice bends for the corners on such a large piece of plexi and have everything line up?
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« Reply #186 on: April 21, 2022, 03:46:14 PM »

Yo Bob,

The Plexiglass bending is all done on a table edge using a regular propane plumber's torch.   First I measure the width, length and height of the proposed cover area.  I add about 1/2" for each bend to curve properly and use up some material.   So if the rig panel is 19" and I need the Plexiglas to fit around it, I will add 1/2" plus another  1/2"  for the two bends making the cut 20".    I then use the edge of a table to let the Plexiglass hang over exactly on the + / -   19.5" distance for each bend.    I make the Plexi sheet  a big 'U'  -  with no covering in the front or rear of the cover.

To bend with the propane heat, always keep the torch moving on the bend line. Heat BOTH sides evenly.  Too much heat and it will make internal bubbles and a white opaque.  If you mess up, you can always tape it off and spray aluminum colored paint to simulate 1" aluminum rails.

The Plexi will heat up after 1 minute or so of heat and naturally fall (with a slight push from you) and make a nice 90 degree angle bend. The table edge will dictate where it bends.  Before it cools, eyeball it to be sure it cools at exactly 90 degrees and hold it steady for a couple minutes.  Be sure to apply torch heat evenly and keep moving.. watch out for the opaque look of overheating at all times.  The edges are very prone to this overheating.

Another method is to use 1" aluminum angle  with 6-32 nuts and bolts  to hold it together. They both look good if done right.

I like the Plexi for my smaller rigs just to look at them every day I walk by. When in a rack it's easy to forget your work inside.  For the bigger rigs, I use metal cabinets or racks for RFI, no Plexi anymore.

I usually let the rear of the Plexi cover overhang a few inches to better protect the parts in the back of the rig.  I like clear Plexi - the tinted stuff may look good but is harder to see thru.


T
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« Reply #187 on: April 21, 2022, 04:28:58 PM »

FB on the plexi.  I'll keep that in mind for the future.  I've wondered about how to do big sheets and get the sides to line up.  

I'm going to do some soldering tonight. Got a bunch of parts in this afternoon for the GFZ PS board.  Some of what I ordered before didn't quite fit correctly.  These should finish the job.  then it's time to plan fitting things into the 813 rig. Got some heat sink tapping to do too.  Baby steps.  

Thanks.
rkw
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« Reply #188 on: April 25, 2022, 01:05:53 AM »

Update 4-25-2022:

The TDA7293 dual chip kit boards were a success. No more parasitics.  I had my doubts that I would be able to successfully build this tiny board but it works!  I used Mouser  chips and the dual board from China.  On the bench the new board was putting out a measured  200 watts into a power resistor at +/- 45 volts.    I also have the rig making 200%+  modulation peaks using tones.  The heatsink gets warm but not too bad unless I do a sustained tone at heavy modulation for 1-2 minutes.

Now here's the problem...   I have another rig using a 4D32 in class C plate modulated by the same model transformer, 1642SE.   It runs very quiet with no talk back even with high mic gain levels.  But this Hammond transformer in Commando and the twin TDA7293s  sings like a bird at about 2 KHz. The same thing happened with the last two amplifiers. Acoustic feedback between the mic and 1642SE audio transformer.

It does a sustained howl at about 2KHz with as low as 30% mic modulation level.  If I unplug the mic it goes away completely. If I push on the chassis around where the transformer sits, it comes and goes in resonance, as if it's using the chassis as a sounding box.  So looks like I will need to mount it on rubber. But what else can I do?  How about a soaking it in tar or taking it apart for a treatment of some kind?  I already tightened the four bolts with no chage in sound level.

I tried wrapping it in foam sound insulation stuff, but it barely had an effect.  It will be located about 7' from the mic when in operation. At 4' away it is out of control.  The mic is directional, but does not help; the sound seems to be bouncing off the ceilings and walls.

I might have to call Hammond and see what they have to say. The point that I have another identical transformer working well in basically the same job is a strong point. At this point the rig works really well, but impossible to use because of the feedback.

I've had various rigs with talk back in the past that were curable. Most fed back at 90 to 100% mic level, which can be handled. But this feedback is so strong and fixed at low mic levels, it's going to require something clever to fix it.

Any ideas?

T

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« Reply #189 on: April 25, 2022, 04:40:56 PM »

kool news ! so no RC circuit between the bridged output and the 8ohm winding of the mod xfmr ? just a fuse on the negative lead ?

Peter
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« Reply #190 on: April 25, 2022, 06:18:10 PM »

Kudos on successful high-level modulation with the new amplifier module!

If the transformer laminations are tight  (as you confirmed) there must be something else loose that is allowed to vibrate and cause the talk-back.  My first guess would be the bobbin may not be securely affixed to the EI laminations.  Perhaps it might help if something non-metallic is wedged between the core and the bobbin.  Wedging it so that it cannot move side-to-side should be easy, but it may be loose between the bobbin and the inside section of the EI core, and this would be a bit more difficult to secure with the laminations in place.  This might be a question for the manufacturer to answer.  (Can I hear a free replacement coming in the mail?)

You are experiencing the talk-back with the plate and screen current of the trio of 6146s through what you are using as a secondary of the modulation transformer.  Perhaps this DC bias on the winding is exacerbating the talk-back problem.  To verify or exclude this theory, would it be possible to place a fixed resistor load on the secondary, and run the modulator into this load with no plate voltage on the final amplifier?  If the talk-back persists, then the DC bias is not the culprit, but if this test reduces or eliminates the talk-back, then a modulation reactor might help.  But unfortunately that device would be BIG and HEAVY, there is no room for it in Commando.

Regarding Peter's comment (KC2ZFA) there should be no need for any DC blocking capacitor, or other RC network between the output of the amplifier and the low-impedance primary of the modulation transformer.  The PC  board includes a high-frequency RC filter at the output terminals.  The outputs of the two amplifiers are not bridged, they are paralleled in what is called "modular" configuration.  With the DC and AC feedback into the amplifiers, the output should not have any significant DC offset.  The amplifier current consumption at idle should be about the same whether or not the transformer is connected.  If the current rises, or the resting power supply voltage sags when the transformer is connected, then suspect a DC offset, and look at the DC feedback components for resolution.  Leakage of the capacitor in the feedback loop could cause a DC offset.  This is highly unlikely though.
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« Reply #191 on: April 25, 2022, 07:34:41 PM »

Hi Tom,

One power supply company I worked at many years ago could pull the top of the lacquer container into a vacuum. This pulled the air out of the transformer pretty quick.  Yes, talk to the mfr.  Worst case you have to remove the transformer, send it back, and they do the super-duper dunk and vacuum pull and real good bake.

GL OM.
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« Reply #192 on: April 25, 2022, 07:39:08 PM »

Very good info, Rick - thanks!

I pulled the Hammond  out today and will run the tests you described with clip leads. I also want to sit it on foam rubber and see if the talk back is reduced - showing if there is some sound box acoustic amplification going on or not.

I will try the DC offset test and transformer current test you described.

The good news:  I called Hammond  today for some advice. They are shipping me a replacement, free of charge incl shipping. What a great company.  A friend suggested that to fix it might require an elaborate procedure including stripping out the old varnish and using a vacuum process. Tough to get at stuff once potted. So they took the easier way out.
Hammond has always been a first class company. I remember I used to talk with Mr. Hammond on 75M. He was a generous guy and helped out his ham buddies. One night I called CQ on my 4-1000A plate modulated rig and he came back using a broadcast rig. More tall ships broke in. He was a humble guy and you would not know who he was.

I will compare the new xfmr when it arrives using clip leads.  BTW, I tried Summer Breeze again last night (single 4D32 class C plate modulated with same Hammond model) and it was quiet as a church mouse, even at really high mic levels.  

Commando now has much higher  audio peaks [tones] than SB as a result of the new TDA 7293 dual amplifier. That thing really plays. Again, no parasitics or anomalies as before with the "7294."    I may put the spare dual TDA into Summer Breeze and make it self contained.

One odd thing I noticed... when the TDA sits in idle (and rig unkeyed) it starts to get warmer than it does when in operation. I have to keep it below +/- 35V or so. At 40-45V it runs FB but during idle the heatsink and whole area gets too warm. I'm wondering if the disable function might be needed with a PTT relay.  I ran the voltage up to  +/- 50V and it really played. But I want to keep it at about 35-40V max at all times in the future. There appears to be plenty of headroom for the audio peaks at 125W carrier output.

Tom: We crossed posts... FB of the vacuum process.  It wud be nice to salvage it eventually. We will have to see if the new one solves the problem first. Worst case is the bad one gets used in a sound proof compartment.

T

** BTW, spent part of the day outside building up a pair of stacked wire dipoles for 40M.  Wanna get on that band with the new AM rigs.  Stacked at 130' and 65' high, in phase / out of phase switched for high angle local or low angle USA DX.  Gotta climb up there to put in the ropes and pulleys between towers. Also need to fix my damaged 75M delta loops.  Antenna work is fun but unnerving to climb.

** Peter:  Yes, that's correct... I'm going directly into the 8 ohm  winding. No network needed so far.  Just a fuse, 8A, is the protection for the 8 ohm winding.   5A popped under normal use, so 8A  it is.   If it gets unstable, I will try the 16 ohms instead. But so far FB.     Bridging and 4 ohms has always given me trouble, so I stay clear.   There is plenty of audio power available now.  The parallel TDA technique seems better.  There is a four parallel TDA board available for those of you more adventurous to experiment with bigger iron..
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« Reply #193 on: April 26, 2022, 05:20:06 PM »

Rick,

I ran the tests you suggested:

The audio  +/- power supply current does not change when the module gets connected to the 8 ohms winding.  So the DC offset is very low.

The transformer is just as loud sitting on the table with foam rubber underneath.  So, the sound box amplification is invalid. A noisy xfmr itself.

 I connected a 5K load across the xfmr 5K winding and keyed it without 6146 HV.  The talk back at 2KHz was almost as loud. I could see the stunted audio on the scope as I talked  due to the audio HV on the tubes.  So the DC current is not the major problem here.  The load resistor got quite warm to the touch in 30 seconds.

The talk back microphonic barely changed in frequency in all the tests, all about 2KHz.

So it appears to be a bad xfmr; something loose inside.  I peeked into one of the xfmr holes and can see a black looking, solidified, shiny urethane material. Looks like it is potted and would be very difficult to pull apart without destroying the windings, etc.  Maybe a heating in the oven for a few hours would help. I might axe Hammond their opinion later on.   No sense wasting a potentially good xfmr.  I also wonder if put in a soundproof box, would the loud squealing / vibration adversely bring on audio distortion in a sonic manner?

An experienced xfmr friend commented:  "The problem with your transformer is the center laminations of the E stack. They are free floating inside the winding bobbin. The bolts are too far away to make a difference. Jamming wedges in the windings can damage the transformer if you are not careful and go too tight. It is likely there is an air pocket in the stack so the laminations can vibrate. Pulling a vacuum on the transformer may not work because the void is likely sealed from the outside world so new varnish can't get into the void even under vacuum. This is the reason it will likely need to come apart."


I'll post again after the replacement is tested.

T
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« Reply #194 on: May 03, 2022, 12:14:41 AM »

Update 5-2-2022:

From Commando Headquarters:

The replacement 1642SE Hammond transformer arrived.  I installed it and the talk back was much less.  I'm able to run the audio gain up to about 90% of normal before it takes off. Once in the cabinet and relocated, it should be fine.  So the original xfmr WAS faulty.

Rick, W8KHK has been giving me his calculations and ideas about getting the TDA7293 and the general rig working well. He's been a great help. I finally settled on two 6146s in the final and have optimized the screen dropping, grid leak and general parameters. It is doing about 100 watts  carrier output at about 140% positive peak modulation headroom according to the REA mod monitor.  Between 75 to 100 watts is where I will probably run it barefoot.

I ran into problems with heat from the TDA7293 module. I blew out my first module. It just stopped working when I first turned it on after a test session the previous day. Even with voice programming, the heatsink and two chips were getting hot.  I ended up with the best solution: I added a 2.5" X 3" X 3/8" polished copper plate as a heat spreader.  The TDA7293 is white pasted and bolted with tapped 6-32 steel screws directly to the copper plate. The copper is white thermal pasted and bolted with SIX balanced tapped 6-32 steel screws directly to the aluminum heatsink.  The whole structure is floated on Plexiglas because the chip tab and now the whole heatsink sits at -40V.   No sil pads, shoulder washers or other gimmicks. What a big difference in chip heat transfer this made!  I think it will survive this time. The chips now stay lukewarm with the heatsinks and dissipates the heat quickly. I am adding a super quiet 3" 120VAC fan to blow air thru the fins in case of old buzzard transmissions.

At 120 watts carrier output, I am up against the limits of the dual chip module to make 120% positive.  Even the +/- 40V supply sags a little. I may need to do something about that. Maybe a bigger bleeder resistor to pull it down from peak voltage.

The Hammond transformer is handling it well at about 220 mA plate current.  The grid is 5K GL, -95V total GL and fixed bias, screen 15 Ma at 145V using 30K  25 watt dropping resistor,  HV usually run at  600 - 750V on a Variac.

But all in all, when I run Commando at 75-100 watts, there is ample headroom and a very robust sound in the monitor. Very clean with audio response from 20 Hz to 20 KHz no problem.

If I were to do it again I would probably get the three parallel TDA module for a little extra thermal conservative safety and peak audio headroom. It's really about one chip per 6146 required. (at least)   This chip needs special heatsinking care to keep the heat from destroying it. Even short transmissions could kill it when not using a copper spreader and pushing it to its specification limits..

I got the presets on the panel all set for rapid band changing.  There's not a lot more to do but install the fan when it arrives and get on the air with Commando!  I might test it into the 4X1s linear amp soon too.  At 30 watts carrier drive the audio headroom will rock the socks off Rico Suave.

I'll take some pics of the new (and greatly improved)  TDA7293 heatsinking array soon.

T

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« Reply #195 on: May 06, 2022, 11:02:07 PM »

Update 5-6-2022:

Commando  is finally working as planned. Tested and beat on.  I took some pictures of the TDA 7293 audio module, polished copper spreader, heatsink and fan assembly. The fan is so quiet you wouldn't know it's on - but it made all the difference in cooling!  I can make an old buzzard series of atomic yallos and really slam the audio -  and the heatsink / chips don't even get luke warm now.  The fan blows air thru the aluminum fins and its speed is controlled by the same Variac that controls the audio +/- 46V supply. IE, the higher I raise up the audio HV, the faster the fan spins.

The biggest cooling difference was made by using the 3/8" thick copper spreader and the fan.   Before that the chips didn't last for more than a day or so. One OB xmission and it was all over when using just the aluminum heatsink alone.

The audio is very flat and clean from ~10 Hz to 20 KHz ++.   There is no cross over or peak distortion that I can see.   The SDR spectrum scope looks clean and the harmonic distortion is a little better than most of my all-tube rigs.

I tried some things to get the audio HV regulation improved, but nothing really worked. I tried bigger caps and bleeders, NG.  The only solution may be normal regulation MOSFETs or a capacitor multiplier. Also, there are some +/-  45V /50V  8A switched power supplies on eBay for $25 I might try too.  But I see no signs of distortion or flat topping under normal use or testing.

I plan to seek out some on air contacts soon, so give me a call if ya hear me on.

T

Pic #1:  Notice all surfaces use only white thermal paste for direct contact. No sil pads, mica, shoulder washers, etc.  Everything is easily accessible. I can change out a TDA module in a few minutes.

Pic #2:  The fan pushes air directly into the fins. There is no way out except forced air out thru the top of the fins.  Decent breeze.

Pic #3: Everything in the entire rig will fit under the Plexiglas enclosure, yet to be made.  The front panel sets the height of everything in the rig.


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« Reply #196 on: May 06, 2022, 11:03:21 PM »

Pic #2 shows the TDA7293 module, copper spreader and heatsink all floating on Plexiglas. The chip tabs and all are sitting at -46V.

I still need to neaten up the TDA installation. It is still in the testing / optimization mode right now.

The rig has been very reliable; other than a TDA module failure last week.   Easy 100 watt carrier; 150% modulation peaks headroom.  DC to light audio. Who could axe for more?

T


Pic #1:  It's getting mighty crowded in here!   Oh, I've been asked... what is that white thing holding the coil?  That's the 160M coil add-on supported by a white ceramic post / pillar.

Pic #2 is a good look at the TDA7293; in this case a DIY kit using two chips for 140W of audio.  It will modulate the heck out of the two 6146s.  Three 6146s are just too much for the Hammond xfmr (150 W carrier) and would require a 3 or 4 chip module to fully modulate up to the audio peaks that I require.  But with two tubes, the Hammond 1642SE xfmr shows no signs of warmth and handles the additional 220 mA of DC plate current.  Possibly the main risk here is the 5K winding of the Hammond. There can be as much as 2KV on it during heavy modulation peaks. It is hi-pot tested at 3500V.  Yes, we ARE on the edge of wetness as usual.  :-)


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« Reply #197 on: May 17, 2022, 11:29:50 AM »

Update 5-17-2022:

I blew up a second TDA7293 audio module.  Working fine until I dropped it by accident into the Variac hot leads and bam!  Rick sold me his two TDA kit modules and I built up two more. It's all working again with one module for backup just in case.  My bad.

** The TDA module continues to run cool to the touch after normal voice transmissions. I'm actually surprised. I think it is a very efficient way of modulation. In contrast, even running class C, the two 6146s throw off a lot more heat than the modulator. But all said, Commando is a very cool running rig for 100 watts of carrier and 150% audio positive peaks. It is a great daily driver, especially in the hot summertime.

Commando was performing perfectly EXCEPT for a persistent 60Hz AC hum on the carrier. I tried many things to fix it with no luck.

I finally located the problem and I must admit it was thru trial and error. In hindsight it was an obvious problem, as always.

The TDA7293 audio module and heatsink assembly were sitting directly on top of the +/- 46V HV power transformer which powers the audio. I was running out of room!  I relocated the TDA 1 1/2" ABOVE the xfmr on plexiglas and the hum is completely gone. Now below ambient room noise and birds chirping outside.  Evidently 60 Hz magnetic coupling will go thru aluminum and even steel.  The square function of moving it away just 1 1/2" did the trick.  I should have known better but I had to try all the other stuff first, I guess.

So Commando is about ready to be moved into its place of honor within the harem soon.


***  So for the last few weeks I've switched into the NEC antenna modeling and tower climbing mode; fixed the 160M dipole at 190', adding a new pair of stacked dipoles at 130'/65'  for 40M, rebuilding the 75M delta loops at 190' using #10 copper wire openwire tuning stubs and feedline  for full tuning ability from the ground...  No OWL spacers, just tightly stretched between tower and ground.  

And, this is exciting: Inspired by the possibility of a good solar cycle, I'm adding a driven, broadband, open wire fed, 10-20M pair of aluminum tubing stacked lazy H's at  25'/ 50'/ 75'/ 100'.  (effectively eight 1/2 wave elements on 10M)   Lots of new ants to play with soon.  I mean WTF - might as well have some fun.

T
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« Reply #198 on: May 17, 2022, 04:47:35 PM »

Evidently 60 Hz magnetic coupling will go thru aluminum and even steel.  The square function of moving it away just 1 1/2" did the trick.  I should have known better but I had to try all the other stuff first, I guess.


mu-metal? 
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« Reply #199 on: May 17, 2022, 06:35:10 PM »


mu-metal?  



That sounds like a great idea, Bob!   It would further reduce the tiny residual hum that probably still exists.

I did some reading and see it really does work, needs no grounding but is crazy expensive. I could use a 4" X 6" piece, 0.5mm thick or better.  

Krell metal also works, but is about $1 trillion/ gram and forbidden, so I'll stick with mu metal.

I see small beat up plates going for $40+ on eBay. Amazing.

Supposedly some old scopes have a mu metal plate shielding the rear of the CRT.

Anyone have some scrap, please let me know.

T
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There's nothing like an old dog.
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