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TA-12 Modulator PSU




 
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WU2D
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CW is just a narrower version of AM


« on: December 29, 2012, 09:58:56 PM »

I have a Bendix TA-12D that I have restored to working order that I picked up from Brown NZR last year. It had been modified by some conversion artist and I had a lot of tracing and rewiring to do to get it opeartional. It also had a busted roller inductor and needed a lot of mechanical work and lubrication. But It now works nicely on 75M.

 AL WA1VTP and Larry NE1S both also have TA-12D's that they are playing with as well and they were lucky enough to find original nodulators. I went as far as to design and build a power supply modulator last winter. It is basically a pair of 807s and I simply used an old TCS transformer. This transformer is a 1:1 ratio. No dice - she no modulate. Now I may still have an issue in the wiring but I am pretty sure that I need a 2:1 or 3:1 transformer for a pair of 807's modulating a pair of 807's as RF amplifiers in parallel. Tim HLR suggested that I simply try an ART-13 transformer. I happened to have a spare so I stripped it, baked it and measured the resistances and moiunted it in place of the TCS iron. Obviously this will require some insulation!

My question is what should I do with the unused screen winding that the ART-13 iron has?

Mike WU2D


* Bendix_WU2D_2.jpg (705.03 KB, 3330x2490 - viewed 673 times.)

* ART13_TA12_Modulator.jpg (579.35 KB, 3072x2439 - viewed 695 times.)
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N8ETQ
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Mort


« Reply #1 on: December 30, 2012, 08:19:21 AM »



   Hey Mike,

      I would strap 5 or 10 K across the
winding just to keep the AF voltage down.

  It may also be an easy way to set up
for a "Trapazoid" pattern on the scope.

GL

/Dan
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W3RSW
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Rick & "Roosevelt"


« Reply #2 on: December 30, 2012, 10:58:24 AM »

I like your 807 wooden column protectors.   Grin

Perhaps the feedback winding could be used for just that with some R/C network  from it to an earlier audio stage.

Very nice rig and bet you'll have fun with it.
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WU2D
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CW is just a narrower version of AM


« Reply #3 on: December 30, 2012, 05:15:52 PM »

Nice clean sine wave tip to tail. The ART-13 transformer is arcing over internally at just over 175 VAC RMS into 4000 Ohms. Makes a nasty sound indeed. I tried the 10K ternimating the tertiary winding. Methinks I need yet another piece of Mod iron. Fail #2.

The wooden column protectors protect the 807's and level the beast when it is upsidedown make servicing easier! And this thing needs servicing.

Mike
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WU2D
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CW is just a narrower version of AM


« Reply #4 on: December 30, 2012, 08:49:30 PM »

I also am seeing that the driver transformer (from an Apache or DX-100) is pretty weak in the drive department and haywired in a small 10 Watt modulation transformer. I also tied in a TCS transformer in in place of the ART-13 transformer.

This produced 250 V RMS into 4000 Ohms before saturation and no arc over. She did sing some when I pushed her. So this could do 15 Watts and probably higher on peaks. Al said that a TCS transformer had been used in somebody elses homebrew modulator and now I believe it. This would modulate the TA-12 but it would be borderline. At least it proves the system will work. So tomorrow I will try an Apache mod transformer with this setup. I hope that pull is good!

Mike 
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WU2D
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CW is just a narrower version of AM


« Reply #5 on: December 31, 2012, 10:14:04 AM »

Third try a charm...

After figuring out the Apache mod iron (which is not something easy from the manual) I came up with the following:

Blue Primary Plate 1
Brown Primary Plate 2
Red Center Tap Primary
Slate  (short wire) Ground
Black - Secondary (B+}
Yellow - Secondary To Modulated Stages
Green - Secondary 600 Ohm Tap

It works! I easily was driving 300 VRMS into 4000 Ohms which is plenty to modulate the TA-12. I will have to set up a voltage divider to measure higher. No singing.

Now to mount the beast.
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WU2D
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CW is just a narrower version of AM


« Reply #6 on: December 31, 2012, 05:36:39 PM »

It works and needs to be set up for proper bias etc.. before putting it on the air - but no smoke and I even made a contact on 3885 with it. I will post a schematic after I translate my henscratch markup. Basically is is a power supply for the transmitter - +28VDC and +550 VDC and -60V Bias and 300V, 6.3VAC and -25V for the modulators and then the modulator itself which is a 6SJ7 speech amp, 6J5 which takes the feedback, 6F6 triode connected with feedback, the driver transformer , the pair of 807 Modulators and the Apache modulation transformer. This is not unlike the original modulator out of the conversion manual but it is more substantial and has separate HV and MV and Bias power supplies.

Mike


* FInal_Modulator Setup.jpg (372.39 KB, 2568x2094 - viewed 673 times.)
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« Reply #7 on: January 01, 2013, 09:48:10 PM »

Al confirmed that I have a problem with distortion for about 20 seconds and then it settles down to good audio. This was traced to bad regulation on the bias of the 807 modulators. It was going from -32 VDC to -41 VDC on TX and then ending up around -35 VDC. A zener fixed this problem and now the bias is steady.

Next we will get down to the basics like am I in phase? and the quality of the waveform.  I am using an Electrovoice 638 Classic mic. The frequency response is good according to Al, not military sounding at all.

I have attached the schematic from the CQ Surplus Conversion manual that mine is based on. My power supply is solid stated and I have adjustable bias and parasitic caps but this is the basic idea.

Mike


* TA12Modulator_ConversionManual.jpg (144.76 KB, 944x649 - viewed 666 times.)
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