Some Very Good Mods For The HW-10x/SB-10x
The modifications below are mods to the HW-101 which any HW-101 will want to consider. Everyone will not want to make all these mods, depending on your operating preferences. These are categorized into 3 levels, simple, moderate, and major.
Simple Modifications
I recommend that everyone complete these two simple modifications. They are easily accomplished and they greatly improve the operation and convenience in using these nice old rigs.
Conversion to Low Impedance Stereo Headphones and Speaker Muting
The original audio circuit in the HW-101 had some very nagging features. First of all, high impedance headphones were required. These were produced in large numbers during World War II, but it is difficult to find them and the audio quality is not great. Low impedance headphones are the modern standard and available in many styles and at low cost. Secondly, the speaker remains active (at somewhat lower level) when headphones are plugged in. This is totally unacceptable particularly if your station is near other people who are doing other things and do not want to hear your QSO. Late at night when I am working CW (which I do 99% of the time) I do not want to disturb the sleeping XYL. The simple modification described here will fix both of these problems.
The circuit diagram for the modification is shown below.
The only parts required are a special 1/4 inch stereo phone jack with switching, two 100 ohm 1/4 watt resistors and about 3 feet of stranded wire. The special stereo jack may be ordered from the Order Page. The modification is accomplished in the following steps:
1) Disconnect and remove the old 1/4 inch mono phone jack.
2) Place the 2 100 ohm resistors and jumper wire on the new switched 1/4 inch stereo phone jack and solder all connections except the one where to two resistor come together.
3) Mount the stereo jack on the front panel of the HW-101.
4) Unsolder the green wire coming from the audio output transformer and one end of the 100 ohm resistor from the RCA (8 ohm speaker) jack on the right rear of the chassis and then reconnect the wire to the free end of the resistor.
5) Solder a 2 foot length of stranded wire to the RCA jack output and run this wire to the stereo jack and connect it to the pole of the unused switch.
6) Connect and solder a 2 foot length of wire to the junction of the green wire and the 100 ohm resistor and run this wire to the stereo jack and connect it to the normally closed side of the same switch.
7) Connect the White-Violet-Violet wire (which was disconnected from the old jack) to the stereo jack terminal where the two 100 ohm resistors come together.
Sidetone Volume Control
The original sidetone in the HW-101 is far too loud for anything short of a demonstration in a large auditorium. Even if you are strictly a SSB operator, you will want to add this modification for those occasions when you want to use the CW mode for tuneup. The only part required is a 200 K ohm audio potentiometer and a little wire. I purchase one with a knurled 1/4 inch shaft about 1/2 inches long. Mount the pot on the rear panel so that you can change the sidetone volume without opening up the cabinet. You could cut a hole, but I chose to remove the RCA jack labeled "SPARE" and mount it in that hole. The schematic for this mod is shown below.
The following steps will complete this modification:
1) Heat the end of R326 (1 Meg ohm) which is farther from V15 and pull that end of the resistor free from the audio circuit board.
2) Solder a 6 inch length of wire to the free end of the resistor and pass the free end of the wire through a nearby hole in the chassis.
3) Solder a 6 inch length of wire to the point from which the end of R326 was removed and pass the free end of the wire through a nearby hole in the chassis.
4) Solder a length of wire to some convenient ground location and connect the other end to the appropriate lug of the recently mounted pot .
5) Solder the other two wires to the appropriate lugs on the pot.
Moderate-Difficulty Modifications
Two modifications are proposed which are not simple, but can be accomplished by those with average or better skills in a relatively short period of time and with little change to the original HW-101. The first involves use of the 12.6 VAC filament supply and its uses. There are actually a number of variations in what one might decide to do in this regard. The first is replacing the incandescant lamps which illuminate the main tuning dial and the meter with LEDs. The second is the addition of a cooling fan. These two are accomplished as a single modificaton, which is necessary in order to keep the filament circuit balanced.
Understanding the Filament Circuit
Before doing any modifications to the circuit an understanding of how the circuit is designed and what must be done to keep it operating right. The filament supply is 12.6 Volts AC, but some of the tubes require 6.3 volts ans some required 12.6 volts. Those which require 12.6 volts are simply connected between the 12.6 supply line and ground. These are of no concern as far modifications to the circuit are concerned. The circuits also includes two #47 pilot lamps which operate from 6.3 volts. The circuit contains both tube filaments and these two lamps. The circuit is designed so that, in the event of failure of a tube filament or lamp, the voltage across any of the other filaments or lamps would not be greatly upset until the faulty ones are replaced. The diagram below shows the original filament/lamp circuit diagram, drawn in such a way that it is more easily understood.
In the circuit above, the filaments are shown as resistances but their values are shown as conductances, the inverse of resistance. The total conductance in a parallel circuit is the sum of the individual component conductances. Notice that there are two groups of components connected in parallel and the two groups are connected in series. When the circuit is properly balances, each of the two groups will have the same voltage(about 6.3 volts) across all its components. Another way of saying this is that the conductance of the two groups is equal and the two groups draw the same current. With this arrangement, one element burning out will not drastically upset this balance. Whatever change is made to this circuit, the conductances of these two groups should remain roughly the same.
LED Lighting and a Cooling Fan
The old rig with all those tubes inside can get really warm, especially if the air in the shack is not moving. A cooling fan can make a lot of difference and there is plenty of excess power availablein the filament supply to power a small fan. Fans that are readily available at good prices run on 12 Volts DC. You can buy them for $10 or so. They draw about 150 mA. If we take out the #47 lamps, we can replace must draw 300 mA, the amount drawn by the two #47's, from the filament supply in order to keep the circuit balanced. As it turns out we can build a doubler from the terminals where the #47's are removed and regulate it to 12 Volts DC. When this is applied to a fan drawing 150 mA, the current drawn from the 6.3 VAC at the input will be approximately 300 mA, just what we need. The new circuit is shown below. One to three birght white LEDs are also shown in the schematic. These will provide as much light as the #47's. They draw only about 25 mA, which is negligible. After this mod, my circuit was very well balanced with the top and bottom group voltages being 6.5 and 6.6 volts. So there you have it, a fan which keeps the rig cooler and cooler lighting. Select an 80 mm fan which draws 150 mA. Be sure that the noise level is less than 20 dBA. You can mount the 12 volt power supply with doubler on a small piece of veroboard and mount the fan by putting machine screws through the slots in the rear panel.
