WT40 AUDIO RESISTANCE MEASUREMENTS

AUDIO RESISTANCE MEASUREMENTS . . .

. . . to verify that components are correctly connected and properly soldered in place.

[[  Most recent update:  27 October 2012.  ]]

PREPARATION

IMPORTANT: Resistance measurements are to be taken BEFORE jumpers are installed – OR – with only one end of each jumper connected, as shown below.

Before doing resistance measurements, connect the two test leads on your DMM together and record whatever value you get with your meter set to its lowest resistance scale. This is the “Zero” Ohms value for your meter.

Your DMM may actually show zero ohms with test leads shorted together.

On meters I have used, shorted test leads show somewhere between 0.2 to 0.5 Ohm. Some meters have provisions for adjustment so that the meter will read zero for a dead short.

Unless specified otherwise, resistance measurements are taken from the POINT INDICATED to the GROUND.

Resistance values are specified in Ohms; k = X 1,000

Resistance Measurements (other than ZERO and OPEN, which should be “right on”) are OK if they are within plus or minus 15%. If measurements are off more than 20%, you have one or more wiring errors.

The readings shown below were taken using a EXTECH EX570 DMM, and have been rounded to the nearest value.

MEASUREMENTS TO GROUND

[] TP40 to Ground Buss . . . . Zero

NOTE: Now that you have verified that TP40 is connected to the Ground Buss, it can be used as your GROUND POINT for Resistance Measurements.

[] TP26 . . . . . . . . . . . Zero
[] TP27 . . . . . . . . . . . Open, No Continuity
[] TP28 . . . . . . . . . . . Open, No Continuity
[] TP29 . . . . . . . . . . . Zero
[] TP30 . . . . . . . . . . . No Measurement at this time
[] TP31 . . . . . . . . . . . Open, No Continuity
[] TP32 . . . . . . . . . . . Open, No Continuity
[] TP33 . . . . . . . . . . . Open, No Continuity
[] TP34 . . . . . . . . . . . Open, No Continuity
[] TP35 . . . . . . . . . . . Open, No Continuity
[] TP36 . . . . . . . . . . . Zero
[] TP37 . . . . . . . . . . . Open, No Continuity
[] TP39 . . . . . . . . . . . Zero

You have probably noticed on the Layout Drawing that TP28, which is the Entry Point into the Audio Amplifier module, does not connect to anything on the circuit board. This may seem rather strange at first glance, but a quick inspection of the Schematic Diagram shows what is going on. Before the signal enters the circuit board it passes through R17, the Audio Gain potentiometer, which will be mounted on the Control Panel of the Transceiver. (More about mounting potentiometers when we get to Building the Control Panel.)

TP28 provides a place to connect the signal wire from the Product Detector to the signal wire going to R17.

It is helpful to use color-coding for the signal wires, as illustrated in the photo below.

Rotary potentiometers are manufactured so that as the shaft is turned in a ClockWise direction, the wiper moves from the upper terminal (Green wire to TP26) in this photo toward the lower terminal in this photo (Orange wire to TP28). The Wiper terminal is in the middle (Yellow wire to TP27). The upper terminal in this photo (Green wire to TP26) would be connected to the Ground Buss via TP26. The colors used are arbitrary. More detail about this when we get to Current & Voltage Measurements and Initial Operational Test.

The photo also illustrates the use of heat-shrink tubing, which I added after cleaning the contacts. The heat-shrink tubing provides insulation on soldered joints, and strengthens the connection.

FYI: The potentiometer pictured here was salvaged from surplus military electronic gear.

R17 will not be connected into the circuit until we get to Operational Testing, but it does no harm to plan ahead.

Measurements on transistors are from the leg indicated to Ground.

Q5, 2N3904
[] Emitter . . . . . . . . . . . 220 Ohms
[] Base . . . . . . . . . . . 8.6k
[] Collector . . . . . . . . Open, No Continuity

Q6, 2N3053
[] Emitter . . . . . . . . . . . 148 Ohms
[] Base . . . . . . . . . . . Open, No Continuity
[] Collector . . . . . . . . . Open, No Continuity

POINT to POINT MEASUREMENTS

The three measurements, immediately below, are between two test points, NOT to Ground.

[] TP30 to TP32 . . . . . . 2.2k
[] TP30 to Q6 Collector . . . . . . Open, No Continuity
[] TP30 to Q6 Base . . . . . . Open, No Continuity

Correct any errors found during Resistance Measurements before proceeding to Current & Voltage Measurements and initial Operational Test.

– END of initial resistance measurements. –

 

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About w6bky

Retired 29 May 1987. Now do hobbies: blogging, ham radio, gardening, etc.
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