Repair of electronic parts and circuit

Now, that the vernier was working, came the test.

The converter's transformer provides only 2.5 V for the filaments of the 24-A and the 27.No B+ voltage is provided!! To provide the B+ I made a half wave power supply with a small transformer and a Pi filter. For this filter I used two 33Mf/160v capacitors with a 4.2 Kohms/1 W resistor in between both capacitors.

This provided a B+ of 92.2 volts and a plate voltage for the 27 type oscillator of 73.2 V. Just perfect! The location of the transformer and associated parts see below:(Click images to enlarge)


For this test, the converter was coupled to the Lincoln radio that is used with the 738 Silver Marshall Converter.
It worked with good sensitivity, but it showed a major problem: The band change toggle switches were making false contact, and the variable condenser was very noisy.

The variable condenser was fixed by removing both brushes, and cleaning them with an ink eraser rubber. Contact cleaner was aplied to the brushes and ball bearings. Finally a drop of thin oil was poured into the ball bearings at both ends of the shaft, and the body of the condenser was strapped to the chassis. See the photo above to the left.

The band change toggle switches are located to the left of the ANTENNA COIL.



An attempt to restore these switches was made. Both were completely taken apart throughly cleaned and oiled.



Unfortunately they worked well only a couple of month. Then, another solution was adopted.

Before addressing this problem, I must say that the 2 Megaohm grid leak bias resistor was found altered, and to bring it down to its correct value of 2 Megaohms, a 2.7 Megaohm resistor was connected in parallel with it.
The solution of the Band Change Problem was accomplished by strapping together three microswitches. To this bank of switches a foot was added in order to secure it to the chassis.

This photo shows that the Band Change Switch connects and disconnects three wires from ground. This is accomplished by a lever operating such a switch.



In my solution, this lever effect, is obtained by wrapping around the same shaft a green rubber lace, and tighting it well.
This photo shows that the three microswitches are connected in parallel with the three wires of the original switch, which was left in place and still is operative. This preserves the originality of the Band Change Switch to a certain degree, while correcting efficiently its intermittent operation.