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 sheet of of standard
weight copier or printer paper should do. Assemble the E laminations into
the core of the windings. If you can't get the last few pieces in, don't
worry, it will have little effect on performance.
Cut the gap spacer paper to the proper dimension and epoxy it
between the E and I lamination stacks. Insert the bolts and compress the
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laminations with a large vice or heavy weight.
This will force the epoxy to flow into all the crevices between the E and
I laminations. Thisstep must not be skipped or loud talk back will occur
when the transformer is put in use, resulting in feedback. If the work has
been done properly, you now have a modulation transformer!
In Figure 2, the final amplifier DC current does not flow through
the transformer but instead is handled by the inductance. This configuration
is known as modified-Heising modulation. The value of inductance can be
determined by a this rule of thumb: 10 Henrys for every 1 kOhm of finals
amplifier modulating impedance (final voltage divided by the final current).
A single choke or reactor is not required. The required value of inductance
can be obtained by connecting several chokes in series. Power supply filter
chokes are plentiful and can be used. Smoothing chokes (NOT swinging chokes)
must be used. You can get away with as little as 5 Henrys per 1000 Ohms
of final modulating impedance with some loss in bass response. Since there
is no DC current flowing through the secondary of the modulation transformer,
no modification of the transformer (as described above) is required. Low
frequency response is also better. Overall, this approach is superior to
that shown in Figure 1. The draw back is that room must be found or made
for the choke(s).
The table below shows the computed impedance ratios for voltages
commonly found in TV power transformers.
Table 1 - Available Ratios
* Values are AC rating with
115 volt primary.
* Z Ratio is full primary to secondary in modulation service.
| Secondary Voltage |
Impedance
Ratio |
| 1000 |
2.65:1 |
| 900 |
2.56:1 |
| 800 |
2.39:1 |
| 700 |
2.27:1 |
| 600 |
2.09:1 |
| 500 |
1.88:1 |
Here's how you would use the table if you wanted to plate modulate
a DX-60 with a pair of 807's. Assume the final amplifier is to be run at
700 DC plate voltage and 120 milliamps current. This yields as modulating
impedance of 5600 Ohms. The 807's want to see a plate-to-plate load impedance
of about 10 kOhms when run in class-AB1. With 600 volts on the 807 plates
about 45 watts of audio can be obtained. An impedance ratio of 2:1 is required.
Thus, a TV power transformer with a secondary voltage of 500 or 600 volts
is required. If the modified-Heising approach is used, a 10uF cap, several
filter chokes can be series together. Three typical values like 10 Henrys
at 150 mA, 16 Henrys at 150 to 200 mA, and 8 Henrys at 200 mA will add up
to 144 Henries, which is sufficient.
Once assembled, it is found that your modulator is performing
much better than you thought. The component that usually cost the most or
is the hardest to find, cost you nothing, and you had to look no further
than that junk TV in your garage!
In the case of replacing the modulation transformer in you DX-100
or Apache, where space is limited, you can solid state the power supplies
and move around components to make room for the additional Hiesing choke(s).
The extra space for the chokes is not required if you feel like performing
the gap modification. A TV power transformer was used as a modulation transformer
in the modified-Heising configuration in an Apache, recently.
In the case of a Viking Valiant, converting the HV supply to
solid state and moving the screen dropping resistor provides plenty of room
for two typical 10 Henry/300 mA chokes. An old power transformer can be
squeezed in place of the original modulation transformer.
You may be wondering what to do with any filaments on the TV
power transformer. They can be used for a variety of things. A 600 VCT unit,
with a 6.3 VAC filament winding, when used with a 5 kOhm plate-to-plate
load impedance, will present a load of 0.5 Ohms at the filament winding.
This makes an ideal pick-off point for monitor headphoniums. The voltage
step down ratio is such that enough level will be available at the filament
winding without blowing your ears off. You can also use the filament winding
for a loop a negative feedback into an earlier audio stage, in order to
improve the modulator frequency response and lower distortion. The winding
can also be used as an oscilloscope take off point to produce a trapezoidal
pattern for testing and monitoring modulation quality and level.
From The AM Press Exchange, August 1990.
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