Figure 6 Measuring Circuit of Heathkit IM-11.

Power Supply.

One prong of the AC line plug goes to the bottom of the power transformer primary. The other prong goes to the swinger of a 5 position simple switch. The five positions are off, AC, minus DC, plus DC, and Ohms. It is labeled S2eh. The switch is shown in the off position. The contact at the off position has nothing connected to it. The remaining 4 contacts for AC, minus DC, plus DC, and Ohms, are all tied together and go to the top of the primary of the power transformer. Connected across the primary is the series combination of a 220 k ohm resistor and a neon lamp. The transformer has two secondaries. One is a 6.3 volt winding for heaters and the other is a 125 volt winding for B plus. The two sides of the 6.3 volt winding are connected to the two heater pins of the 6AL5, 3 and 4. The two sides are also connected to the heaters of the 12AU7, pin 9 on one side and pins 4 and 5 on the other. One side of the 6.3 volt winding is connected to chassis ground. The top of the 125 volt secondary connects to the anode of a semiconductor diode. The cathode connects to the positive end of a 16 u f 150 volt capacitor. The negative end connects to the bottom of the secondary winding, not chassis. The positive side of the capacitor connects to one end of a 10 k ohm resistor. The other end of this resistor connects to the left end of a 10 k ohm pot labeled AC balance. The right end goes to chassis ground through a 100 ohm resistor. The wiper goes to circuitry that is almost identical to that found in the RCA VTVM. The right end of the pot also goes to one end of a 27 k ohm resistor. The other end of this resistor goes to the negative end of the 16 u f capacitor.

Meter amplifier and meter connections.

The meter amplifier consists of a dual triode, 12AU7. The cathode of the left hand triode goes through a 150 k ohm resistor to the negative end of the 16 u f capacitor. The cathode of the right hand triode goes through a 220 k ohm resistor to the negative of the 16 uf. A 15 k ohm pot connects between the two cathodes. The wiper of the pot goes through a 150 k ohm resistor to the negative of the 16 uf. Note, I suspect the 220 k ohm and the 150 k ohm to the pot wiper have been interchanged. End of note. The grid of the right hand triode goes through the parallel combination of a 10 meg ohm resistor and a 0.005 u f capacitor to chassis ground. The grid of the left hand triode goes through a 0.005 u f capacitor to common and to one end of a 3.3 meg ohm resistor. The other end of the 3.3 meg connects to circuitry which is almost identical to that found in the RCA VTVM. The two plates of the twin triode are connected together and to the positive side of the 16 u f capacitor. The positive terminal of the meter movement connects to the swinger of switch S2C and the negative terminal to the swinger of S2B. As you might have guessed these two are ganged with the switch that controls the power. Both switches have positions labeled off, AC, minus DC, plus DC, and Ohms. The off positions of S2B and S2C are tied together to place a short on the meter movement when power is turned off. The right hand cathode of the tube connects to the AC and minus DC positions of S2C. the right hand cathode also connects to the plus DC and OHMS position of S2B. The left hand cathode connects to three rheostats. One is a 15 k ohm and is labeled ohms adjust. The two remaining are 10 k ohm, one is labeled AC cal and the other is labeled DC cal. The other end of the ohms adjust rheostat connects to the ohms contact of S2C. The other end of the DC cal rheostat connects to the minus DC contact of S2B and the plus DC contact of S2C. The other end of the AC cal rheostat connects to the AC contact of s2B. This completes the description of the metering circuit.
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