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Author Topic: Help with Valiant mod transformer  (Read 5578 times)
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stevef
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« on: July 03, 2005, 06:37:08 PM »

I am new to the forum, so please forgive me if this is a repeat topic.

I am fixing up a Valiant and am trying to get the mod transformer apart to reconfigure the laminations.  I got the "I" piece knocked out, but I cannot get the winding "donut" off of the "E" piece.  Do I need to heat it up or something?

Also, when I put the transformer back together, should I put lacquer between the laminations to hold them together?  or do I simply lacquer the outside of the whole block of laminations once it's completed?

Thanks for any help.

Steve
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The Slab Bacon
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« Reply #1 on: July 05, 2005, 04:36:18 PM »

Steve,
          Gently heating the core and bobboi with a good high powered hair dreyer should loosen up the wax just enough to free the bobbin from the core.
        However why would you want to cross laminate a mod transformer?
Even if you use modified heising you will still have a dc bias on the primary of the transformer?  cross laminating it will cause it to saturate at a much lower dc current.
        I have dissasembled and repaired many transformers over the years. Usually to replace brittle lead out wires, or preserve deteriorating fish paper seperators etc. Normally, (with the end covers removed) I will heat the transformer (core and bobbin assembled) to 180 - 190 degrees in a table top toaster oven and bake it for 6 hours to expel any internal moisture. Then while it is still hot I will drop it into a bucket of OIL BASED varnish to cool. this way as it cools it will draw the varnish in as it contracts. After it cools for a few hours pull it out and let it "drip dry".
Dunk it a few more times at room temp and let it dry in between to form a nice glaze on everything. Once it is good and dry reinstall the end covers and this will give a new second chance to a somewhat questionable transformer.
                                                            de KB3AHE
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k4kyv
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Don
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« Reply #2 on: July 05, 2005, 10:01:15 PM »

Quote from: The Slab Bacon
 However why would you want to cross laminate a mod transformer?  Even if you use modified heising you will still have a dc bias on the primary of the transformer?  cross laminating it will cause it to saturate at a much lower dc current.    


The primary is midtapped for pushpull, so assuming the modulator tubes are relatively balanced, the average unbalanced DC to each tube flows in the opposite direction from the midtap, so that the magnetic bias cancels out.  Most broadcast transmitter mod transformers, designed for use with a reactor, are cross laminated just like a power transformer.  Getting rid of the gap in the laminations effectively increases the amount of iron in the core, and thus the low frequency response, and phase shift distortion is reduced.  

Of course, the modified transformer must be used with a reactor, since any significant unbalanced DC through the secondary will cause saturation.

The old UTC catalogues even specified the maximum DC unbalance in the primary of pushpull stages using specific transformers.

If it turns out to be totally impossible to remove the winding from the E part of the core without damaging the bobbin, you can remove the fibre spacer between the E and I laminations, and replace it with a thin layer of rag paper or wax paper (to reduce accoustic rattling) and shove the E and I parts together.  This will still increase the inductance of the coils, even though not quite so much as actually cross laminating, and will allow more unbalanced DC prior to saturating.

Another trick I have tried that works is to use a small hardwood dowell rod, driven with a hammer, to knock out some of the laminations in the middle first.  Once  the first few laminations are removed, the rest usually fall right out.  Heating the coil with a hairdryer or heat gun to soften the wax should help.
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stevef
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« Reply #3 on: July 09, 2005, 07:20:03 AM »

Thanks for the tips.  I am following some of the tips on improving Valiant performance noted elsewhere on the AM Forum.

Quote
the modified transformer must be used with a reactor


What do you mean by a reactor?
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Ed/KB1HYS
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« Reply #4 on: July 09, 2005, 09:44:42 AM »

A reactor = Choke, in parallel with the secondary of the mod xformer, and theres a cap in series too.
IF I remember right, a good size choke is required (several Henries) and and equally stout cap also as these are really working at the audio frequencies.
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« Reply #5 on: July 09, 2005, 10:36:13 AM »

Quote from: Ed/KB1HYS
A reactor = Choke, in parallel with the secondary of the mod xformer, and theres a cap in series too.
IF I remember right, a good size choke is required (several Henries) and and equally stout cap also as these are really working at the audio frequencies.


A couple of mFd max at typical R.F. final impedances will work. Too much will cause overshoot according to several sources.
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Bacon, WA3WDR
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« Reply #6 on: July 09, 2005, 11:37:32 AM »

The "gap" in a unbalanced-DC-handling inductor or transformer is really a compromise.  It substantially reduces the effectiveness of the core, but it is the only way to get the core to work at all in the presence of significant unbalanced DC.  (The balanced DC of a push-pull stage does not count, because its magnetic effect balances out to zero.)

You get a whole lot more core effectiveness if you cross-laminate the core, without any more leakage inductance or stray capacitance.  Cross-laminating the core will not hurt the top end response, but it will greatly extend the bottom end response - IF you keep unbalanced DC off the transformer windings.  This is the function of the modulation inductor and the coupling capacitor.

The RF PA DC goes through the modulation inductor, and a lot of audio voltage appears across it - so this inductor needs to be fairly large (typically 10 to 50 henries), and it needs to be designed with a gap, because it has to work with heavy unbalanced DC on it.  You couple the modulation transformer secondary through a coupling capacitor, because otherwise there would still be a little DC on it because of the resistance of the modulation inductor winding (small DC drop = DC).

The coupling capacitor can be huge (I used 30 uF and 50 henries way back when), but you may get a bonus if you use a strategically smaller value.  The capacitor works with the modulation inductor to make a high pass filter.  The right L and C values can give you a small peak just before cutoff.  With the right values, you can get this peak to work with the rolloff to give extended flat response down to a lower frequency, before it rolls off.  You can actually get flat response down to a lower frequency this way, than if you had a much larger capacitor there!  Because of this effect, you may be able to use a value as low as 2 uF where you might have expected to need 20 or 30 uF.  Note, however, that the rolloff below that peak is pretty fast, and a lot of low frequency AC voltage can appear across the coupling capacitor.  If you use a small capacitor, its voltage rating should be one or two times your RF B+.

Your mileage may vary; experimentation is one of the purposes of amateur radio.  Don't be too afraid to try lower L and C values and see what happens.  Be careful with small C values if you want to modulate with 30 Hz furnace rumble, though; the small peak is followed by a faster cutoff.

So why do we go to all of this trouble with a modulation transformer and a coupling capacitor and a modulation inductor, when the modulation inductor still needs to have a gap?  Because the net result can be better audio quality at a reasonable cost.

A modulation transformer is a bulky mass of wire windings, and there is a lot of stray capacitance and leakage inductance that screws things up at the higher audio frequencies.  By using a cross-laminated core, we can reduce the amount of windings, and still get good low frequency response from a transformer that is smaller and has better high frequency response.  And we can use better core materials that are not well suited to unbalanced-DC applications.  The modulation inductor still needs a big core and a lot of windings, but the winding is simpler (no secondary), and leakage inductance is less of a problem.

In amateur equipment, we are stuck with the modulation transformer windings and core materials that we have, but we can make better use of them by cross laminating the core and adding the modulation choke and coupling capacitor.
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« Reply #7 on: July 15, 2005, 12:44:25 PM »

Your probably reading my notes on this conversion. It's a lot of work but with about 20-25 H. of Heising choke the performance is good. Would I do it again? No, it was a real pain in the ass. I think I would just look for a better transformer to replace the stock one before I went through all that work again. The best sounding Valiants around use an external amplifer to drive a reverse connected 100 watt Hannond output transformer and a heising setup. This does require disconnecting the audio system in the radio and the expense of an external amp. You will also have problems with the band switch arcing & will need to modify it if you go with external high power audio. That's the breaks, they built the Valiant with Ranger parts!
Keith
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