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AM Amazing
Transformation
by
Tim Smith, WA1HLR
There comes a time when the modulation transformer in your DX-100, Apache, B&W 5100, 32V, Viking I or II, Valiant or other 50-200 watt power class transmitter zorches out. Maybe you are interested in building a low bucks transmitter or modulator for your CW rig. The biggest stumbling block is getting a modulation transformer. Well, fear not! The TV power transformer has come to the rescue.
There are a number of methods in using these beasts as shown in Figure 1 and Figure 2. These are two variations of a similar configuration. The original 120 volt primary is connected to one leg of the high voltage secondary (600-800 volts). The windings have to be in phase or it won't modulate worth a damn.
A test voltage can be applied to the center-tap and one end of the secondary, as shown in Figure 3. The test voltage can be from 6.3-120 volts AC. The total voltage obtained from the combination of the primary winding and one-half of the secondary should be greater than the test voltage applied. If not, the windings are out of phase. In this case, just reverse the lead on the primary or connect the test voltage to the other end of the secondary.
In tests conducted on an 800 volt, center-tapped unit, with a 115 volt primary, the impedance ratio was approximately 2:1. This is a perfect ratio for the usual setup with a common high voltage (HV) power supply, and typical 6164's modulated by a pair of 807's, or other lash-ups where the HV does not exceed 800 volts DC. The limiting factor is the high voltage break down rating of the transformer insulation. Most are designed for 1500-2500 volts breakdown. In some designs the primary is the first winding on the core. In others, the primary may be wound on the inside and is more susceptible to crap out. The transformer may also be mounted or insulated above ground in order to accommodate higher working voltages.
You may ask yourself, "How does a power transformer behave at audio frequencies?" Amazingly enough, most will have very good response characteristics. There are so many variables that it would be difficult to accurately predict what it will do. The power handling capability of the usual TV power transformer is in the order of 150-300 watts depending on the size of the core. It can generally be assumed that one pound of core material will handle 10 watts of audio for good bass response and 20 watts per pound with higher impedances (i.e. PP 811's modulating an 813 final amp).
In Figure 1, the final amplifier DC current flows through the transformer. This uni-directional current flow can cause the core to saturate. Remember, the secondary is center-tapped. In its original use, current flowed in both directions through the secondary, thus the magnetic flux canceled, and no core saturation. Therefore, the transformer must be modified to handle the uni-directional current flow, or for short, unbalanced DC.
If you are observant, you will notice that typical modulation transformers, filter reactors and audio output transformers for single-ended operation have a core constructed with "E" shaped laminations stacked together next to "I" shaped laminations. The E and I shaped stacks are separated by a heavy paper spacer. This arrangement enables the transformer to handle (without core saturation) a certain amount of unbalanced DC. There are drawback, however. The inductance of the core and windings falls off rapidly, as the spacing or gap between the laminations is increased. This causes a marked drop in the low frequency response of the transformer. In the case of the TV power transformer, a gapped unit will work okay over the frequency range of 70-5000 Hz.
In order to modify the transformer, the whole unit must be disassembled. With careful use of a hammer and several sizes of screwdrivers, it is possible to do the job within an hour. Remove the bolts holding the the cover bells in place. It may be necessary to give the core stack a couple of well-placed whacks with the hammer to loosen up the laminations. Some transformers come apart easily and other take more patience. Exercise extreme caution so as to not damage the windings.
Once the laminations are apart and are clean of foreign matter, group the E and I laminations in separate stacks. An educated guess will have to be made on the desired thickness of the paper for the gap. For plate currents of 200-300 milliamps, try a piece of paperback book cover (or paper of similar thickness). For currents of 75-150 milliamps, a
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