List of Materials:
Click here for a drawing of the parts.
- 12 volt AC wall transformer with 2.5 mm plug
- 2.5 mm coaxial socket (2)
- 1N4002 diode (2)
- 3,300 mfd electrolytic capacitor (2)
- LM7812 positive voltage regulator
- LM7912 negative voltage regulator
- 0.1 mfd capacitor (2)
- 1.00 K Ohm 1% resistor brown-black-black-brown--brown
- 9.09 K Ohm 1% resistor white-black-white-brown--brown
- 30 K Ohm 5% resistor orange-black-orange--gold
- 560 K Ohm 5% resistor green-blue-yellow--gold
- 0.1 mfd capacitor (2)
- 8 pin dual in line socket
- TL081 Operational Amplifier
- Double pole double throw switch (DPDT)
- 10 K Ohm panel mount linear potentiometer (2)
- 10 K Ohm trimmer potentiometer
- 12 Volt panel mount lamp
- Potentiometer knobs (2) - round shape, black plastic, with small screw on the side.
- Printed Circuit Board - plastic board with copper traces on one side.
- Metal enclosure - approximately 6 x 7 x 3 inches metal box: includes a black cover, gray chassis, black rubber feet and mounting screws.
- Double sided mounting tape
- Front panel transparency
- A Digital Volt Meter.
- Screwdriver
- Wire stripper
- Soldering Iron
- 60-40 Tin/Lead Solder
- 4 color hookup wire (green - red - black - white)
- 2 color hookup wire (black - white)
- 2 color bell wire (gray - white twisted)
Templates and drilling guide
PC board actual size (reversed)
Enclosure drilling guide, Front panel:
- Reference and Gain - holes for panel mount potentiometers (common sizes are 5/16 or 3/8 inch).
- mV-pH - hole for DPDT switch (1/4 inch)
- cell - hole for 2.5 mm socket (common sizes 1/8 or 3/16 inch)
- unmarked (about 1 inch above cell hole) - hole for lamp (1/4 inch)
Construction Procedure
1. Prepare the following wires (the drawing is not to scale):
a) Cut two pieces of 2 color hookup wire 6 inches long, separate the leads 1/2 inch at each end, and strip 1/8 inch of insulation from the end of each wire.
b) Cut one piece of 2 color hookup wire 7 inches long, separate the leads 1/2 inch at each end, and strip 1/8 inch of insulation from the end of each wire, then twist the wire 10 times, this will make a twisted pair about 6 inches long.
c) Cut one piece of 4 color hookup wire to make a wiring harness. At one end separate all four leads for a distance of 2 inches and strip 1/8 inch insulator from each end. At the other end separate the red/green pair from the white/black pair for a distance of two inches, then separate the read from the green for a distance of 1/2 inch, and the white from the black for a distance of 1/2 inch, strip 1/8 inch insulator from each end.
d) Cut a 2 foot long piece of gray/white bell wire and strip 1/8 inch insulator from each end. Put a piece of tape about 3 inches from each end to keep the wires from coming apart.
2. PC board assembly. The diagram shows the components as they are mounted from the side without copper. The copper traces are actually on the other side (solder side), and are shown as a guide. As each part is inserted, solder each lead and cut off any excess length. WEAR EYE PROTECTION WHILE SOLDERING AND WHEN CUTTING LEADS.
a) Insert, solder and cut the two 1N4002 diodes as shown, with the silver band (cathode) to your right. Verify that the leads are on the proper traces before soldering. You will be seeing a mirror image of the traces as they are shown by the figure.CAUTION: diodes are semiconductors and can be damaged by excessive heat, allow 30 seconds to cool down after soldering each lead, and don't heat for more than 10 seconds.
b) Insert and solder the 8 pin DIP socket as shown, CAUTION: the leads are easily bent. Some DIP sockets have a notch or a white dot to identify pin 1, if this is the case, it should face in the direction of the diodes. The drawing shows a socket with a round notch.
c) Bend each of the four resistors in the shape of a letter "U". Insert, solder and cut each resistor in the positions shown.Resistors are not polarized and can be inserted either way.
3. The drawing applies to the following steps: WEAR EYE PROTECTION WHILE SOLDERING AND WHEN CUTTING LEADS.
a) Insert, solder and cut the 7812 and 7912 voltage regulators as shown, with the metal tab facing away from the diodes, the 7812 to your right. Verify that the leads are on the proper traces before soldering. CAUTION: the regulators are semiconductors and can be damaged by excessive heat, allow 30 seconds to cool down after soldering each lead, and don't heat for more than 10 seconds.
b) Insert and solder the 10 K trimmer potentiometer as shown.
c) Insert, solder and cut the 0.1 mfd capacitors in the positions shown.These capacitors are not polarized and can be inserted either way.
d) Insert and solder the 3,300 mfd capacitors in the positions shown.The negative lead of each capacitor should be to your left.
4. PC board assembly - wiring. WEAR EYE PROTECTION WHILE SOLDERING AND WHEN CUTTING LEADS. Place the PC board as shown in the diagram, with the diodes to your left.
a) Insert and solder either end of one of the 6 inch long two color wires prepared in step 1 with the white wire between the two diodes and the black wire between the two large capacitors.
b) Insert and solder either end of the other 6 inch black/white wire in the two holes next to the 0.1 mfd capacitor that is closest to you (next to the 7912 regulator). The white wire closest to the edge of the board.
c) Insert and solder either end of the twisted pair as shown, the black wire next to the black wire soldered in step 4-b above, and the white wire in the hole adjacent to pin 3 of the 8 pin dip socket. (pin 3 is the third pin from the left in the row closest to you).
d) Insert and solder the end of the 4 color harness prepared in step 1 that has all four wires separated 2 inches in the positions shown: red next to the 30K resistor, black next to the 1.00 K resistor, green next to the 530K resistor and white next to pin 2 of the 8 pin dip socket.
e) Insert and solder the white wire from either end the gray/white bell wire prepared in step 1, in the position shown, between the 10 K trim potentiometer and the 560K resistor.
5. See diagram.Prepare a 6 inch long 3 color wire by removing the white wire from a 4 color wire. Separate the three wires 1 inch at one end. At the other end separate the green wire for a distance of 3 inches, and the red from the black for a distance of 1/2 inch. Remove 1/8 inch insulation from each end.
6. Enclosure assembly - diagram.
a) Align the "+" marks on the front panel transparency with the holes in the front panel of the enclosure. Cut the transparency to fit. With the transparency in place mark and cut openings the same size or slightly larger than the holes using cork borers. If borers are not available, the transparency can be melted with a hot soldering iron.CLEAN THE IRON IMMEDIATELY.
b) Insert the two 10K potentiometers, DPDT switch and 2.5mm plug, from the inside, and the lamp from the outside in the positions shown.NOTE: the potentiometers have a metal tab on one side, you must bend or break it off before it can be mounted.Leave the mounting nuts loose on all components EXCEPT THE LAMP. Insert the other 2.5 mm plug in the rear panel, leave the nut loose. 7. Rotate the 2.5 mm plug on the rear panel until the large metal tab is at the top. Solder the black/white wire from the PC board, next to the diodes in the solder eyelets shown, black to your right, and white at the bottom. (Seen from the inside of the enclosure.) NOTE: if the 2.5mm plug is not the same kind as shown in the figure, identify the tip contact, and the shaft contact. Solder the white wire to the tip contact, and black to the shaft contact.
8. Front panel wiring. See diagram. In this step you may find it easier to remove components from the front panel while soldering.
a) Cut a 1 inch long piece of hookup wire of any color and remove 1/8 inch insulation from each end. Place the 2.5 mm socket with the large metal tab towards the top of the enclosure as shown. Attach the 1 inch hookup wire as shown between the eyelets on the sides.SOLDER TO THE LEFT EYELET ONLY.
b) Solder the twisted pair from the PC board as shown, black wire to the right eyelet together with the 1 inch wire, and white to the bottom eyelet. NOTE: if the 2.5mm plug is not the same kind as shown in the figure, identify the shorting contact, the tip contact, and the shaft contact. Solder the white wire to the tip contact, black to the shaft contact, and the 1 inch wire from the shorting contact to the shaft contact.
c) Place the DPDT switch with the two rows of solder eyelets in a vertical direction as shown and place the potentiometers with the solder eyelets on top as shown.
d) Solder the end of the 3 color wire harness prepared in step 5 above that has all 3 wires separated 1 inch to the DPDT switch in the positions shown: black to upper left, red to lower left, and green to lower right.
e) Solder the other end of the green wire to the center eyelet of the "GAIN" (leftmost) potentiometer as shown.
f) Solder the other end of the red wire to the center eyelet of the "REFERENCE" (rightmost) potentiometer as shown.
g) Attach the black wire from the 3 color harness, one wire from the lamp (either wire), and the black wire from the two color wire from the PC board next to the 7912 regulator and 0.1 mfd capacitor to the left eyelet of the REFERENCE potentiometer. Solder all three wires to the eyelet.
h) Attach the other wire from the lamp, and the white wire from the PC board to the right eyelet. Solder both wires to the eyelet.
10. Front panel wiring-continued, see diagram.
a) Solder the gray wire from the gray/white bell wire to the left center eyelet of the DPDT switch. Use the same end of the wire which has the white wire soldered to the PC board.
b) Solder the 4 color wire from the PC board as shown: black to upper left eyelet of the DPDT switch, white to the right center eyelet of the DPDT switch, green to the left eyelet of the GAIN potentiometer and red to the right eyelet of the GAIN potentiometer.
c) Tighten the mounting nuts of all the front and rear panel components.
11. Final Assembly - drawing
a) Insert the TL081 operational amplifier in the socket.The small dot indicating pin 1 must be on the same side as the voltage regulators.
b) Mount the rubber feet of the enclosure using the screws and washers supplied with the enclosure.
c) Pass the gray/white wire through the openings on one side of the enclosure cover from the inside. Solder a black banana plug (or alligator clip) to the gray wire and a red plug (or clip) to the white wire.
d) Mount the knobs on the shafts of the potentiometers and tighten the setscrews until it holds the shaft lightly, then turn the knobs counter clockwise until the potentiometer stops, continue turning the knob counterclockwise (it will slip over the shaft if the screw is not too tight) until the pointer is at the "0" mark. Tighten the set screw completely.
e) Test and calibrate the meter as described in the Testing and Calibration section below. If everything is OK continue final assembly. Mount the PC board as shown using a piece of thick cardboard between two pieces of mounting tape. Close the cover and hold it with the screws provided.
Testing and Calibration
IMPORTANT: These procedures must be performed BEFORE the PC board is fixed to the enclosure and the enclosure is closed.
I. Power Supply Test
1. Check the circuit thoroughly, look for broken and loose connections, poorly soldered joints, and reversed components. Review the construction procedure testing each connection by measuring continuity between the soldered components. Ask a colleague to check your circuit also.
2. Connect the wall transformer to the pH meter by plugging the 2.5mm connector to the REAR 2.5mm socket. DO NOT PLUG IN THE TRANSFORMER YET.
3. Turn the PC board over, taking care not to break the wires. PLACE A PIECE OF CARDBOARD UNDER THE ELECTROLYTIC CAPACITORS AS AN INSULATOR-THE ENCLOSURE IS CONNECTED TO GROUND (0 VOLTS) VIA THE 2.5mm SOCKETS.
4. Set the digital meter to measure DC Volts. Identify the +12 volts, -12 volts and ground copper traces as shown in the figure. Plug in the transformer and measure voltage between the +12 trace and ground, and between the -12 trace and ground. NOTE: The voltages will not be exactly +12 and -12, they can vary by about half a volt.If this test fails, unplug the transformer, skip the rest of this section and continue with Troubleshooting.
5. Unplug the transformer.
II. Zero calibration and reference adjustment test.
1. Set the PC board upright again. PLACE A PIECE OF CARDBOARD UNDER THE BOARD AS AN INSULATOR.
2. Set the mode switch in the front panel to the mV position.
3. Connect the banana plugs to the digital voltmeter as shown: red to plus and black to minus. Set the digital voltmeter to the 20 VDC range.
4. Plug in the transformer.
5. Use a small screwdriver or other suitable tool to adjust the trim potentiometer until the voltmeter reads 0 volts. If you are unable to set zero, unplug the transformer, skip the rest of this section and continue with Troubleshooting.
6. Switch the voltmeter to the 2 VDC range and continue adjusting until the reading is between -2 and +2 milivolts. NOTE: It is very difficult to set the value to exactly zero.
7. Switch the voltmeter back to 20 VDC and switch the mode switch to the pH position. Turn the REFERENCE potentiometer fully to each end, the voltage should be zero volts counterclockwise and approximately 12 volts clockwise. If this test fails, unplug the transformer, skip the rest of this section and continue with Troubleshooting.
IV. Amplifier test.
1. Obtain two resistors with a ratio of 10 or more between them, and total resistance between 5,000 and 1,000,000 ohms. For example, the 560K and 30K values used in the pH meter.
2. Make a test cell by connecting two resistors in series across a 1.5 volt cell as shown (D size is more common but any other size is OK). Use a couple of wires and connect the ends of the smaller resistor to a 2.5mm plug.
3. Measure the voltage across the small resistor with the voltmeter. You will have to disconnect it from the pH meter. Calculate the approximate pH equivalent of this voltage by dividing it by 0.06. For example, if the voltage is 0.075 volts, then the pH equivalent is 0.075/0.06=1.25.
4. Connect the pH meter to the voltmeter again and set the mode switch to mV. Plug in the test cell in the 2.5 mm plug in the front. The voltage read by the voltmeter should be 10 times the voltage measured in step 3. above. If this test fails, unplug the transformer, skip the rest of this section and continue with Troubleshooting.
5. Set the mode switch to pH and turn the REFERENCE potentiometer fully counterclockwise. Turn the GAIN potentiometer fully to each end, the voltage read should vary at least ten percent above and below the pH equivalent calculated in step 3 above. For example, if the equivalent is 1.25, the voltage should vary at least between 1.125 and 1.375. If this test fails, unplug the transformer, skip the rest of this section and continue with continue with Troubleshooting, otherwise proceed with final assembly.
Troubleshooting
I. Power Supply Section Preliminary checks (Letters refer to the drawing above)
1. Check for loose, broken or poorly soldered wires.
2. Check for broken or shorted copper traces on the PC board.
3. Check for loose or poorly soldered components.
4. Verify polarity of devices: Diodes - from B to C, silver band to C and from D to B, silver band to B. Electrolytic capacitors - A to C, positive on C , D to A, positive on A. Regulators - Metal tab away from electrolytic capacitors, 7812 connected D-A-C, 7912 connected F-E-A.
Tests - power on
1. Measure AC voltage between A and B - Voltage should be above 12 VAC, up to about 22 VAC.
Symptom: Voltage too low or too high
Possible cause - Short or bad circuit, incorrect line voltage, or bad transformer.
a. Test - disconnect transformer and repeat measurement at transformer
Action voltage is now OK - continue with b.
voltage still low or high - measure line voltage CAUTION! (about 125 VAC), if line voltage OK replace transformer. If line voltage is bad, call electrician and/or power company.
b.1- Test - With power off and the transformer disconnected. Test the diodes individually by measuring between B and C, and between B and E. Replace any bad diodes.
b.2- Test - If b.1 is OK measure resistance between C and A, and between A and E, the common (black) lead of the meter connected to the second letter. The resistance should be low initially and increase gradually as the capacitors charge. If the test fails remove the capacitor and repeat the test. The resistance should be more than 20K. If the test fails again replace the voltage regulator, if the value is OK, replace the capacitor.
2. Measure the output voltages - Approximately 12 Volts between D and A, and minus 12 volts between F and A.
Symptom: voltage too high Probable cause - Ground pin of regulator open (the pin soldered to A in either case) or defective regulator.
Action: if ground is correct, replace the regulator.
Symptom: Voltage low (below 12 but more than 1).
Probable cause - Electrolytic capacitor disconnected or defective, defective regulator, or instrument circuit problem.
Action - a. Disconnect the wires to the REFERENCE potentiometer and remove the TL081 from the socket and repeat the test. If the power supply is OK now, test the instrument board. If the board is OK, replace the regulator(a defective regulator can read good voltage under no load conditions).
b. If the problem persists check the electrolytic capacitor contacts and resistance (see 1 b.2 above). Replace the capacitor if it fails, otherwise replace the regulator.
Symptom: voltage very low (below 1 volt)
Probable cause - shorted 0.1 mfd capacitor, defective regulator or instrument board problem.
Action - a. Disconnect the wires to the REFERENCE potentiometer and remove the TL081 from the socket, repeat the test. If the power supply is OK now, test the instrument board. If the board is OK, replace the regulator(a defective regulator can read good voltage under no load conditions).
b. If the problem persists remove the 0.1 mfd capacitor and repeat the test. Replace the capacitor if it is OK, otherwise replace the regulator.
II. Troubleshooting - pH Meter Section
Procedure
Preliminary checks
1. Check for loose, broken or poorly soldered wires.
2. Check for broken or shorted copper traces on the PC board.
3. Check for loose or poorly soldered components.
4. Verify polarity of the TL081
Tests - power off
1. Measure continuity between the center and outer connectors of the 3.5mm plug on the front panel(should be shorted), repeat with a 2.5mm plug inserted (should be open). If the test fails and the wiring is OK replace the socket.
2. Verify proper functioning of the DPDT switch by measuring continuity between the center and side connectors on both poles (one at a time, 4 tests in all). With the arm on the same side as the connector it should be open, with the arm away from the connector it should be shorted.
Tests - power on
Symptom - can't adjust zero in mV mode
Probable cause - defective or disconnected 10K trim potentiometer or bad TL081.
Action - power off, remove the TL081, verify continuity between the potentiometer leads and the 8 pin socket (left to pin 1, right to 5 and center to 4). Test between pins 5 and 1 (10K ohms) and between pins 4 and 5, and 1 and 4 (should vary between 0 and 10K as the wiper is turned in either case). If the test fails replace the potentiometer. If the potentiometer is OK, the TL081 is bad.
Symptom - Incorrect output in mV mode, pH OK.
Probable cause - defective 9.09K or 1.00K resistors.
Action - Remove one side of the 1.00 K resistor and measure both resistors (they will result in incorrect values if both are soldered), replace the bad resistor.
Symptom - Incorrect output in pH mode, mV OK.
Probable cause - defective 30K, 560K resistors or GAIN potentiometer.
Action - Remove one side of the 30 K resistor and measure both resistors and the potentiometer (they will result in incorrect values if all are soldered), replace the bad resistor or potentiometer.
Symptom - Incorrect output in both modes.
Probable cause - defective TL081. Action - Replace the TL081.
Symptom - can't adjust reference and/or gain in pH mode.
Probable cause - defective potentiometer.
Action - Remove the potentiometers and test outside the circuit, replace if bad.
Symptom - Output remains at a voltage above +10 or below -10 in either or both modes.
Probable cause -open input or bad feedback resistor.
Action- If it happens in both modes, the input section is open, check the twisted pair and its connections. If it happens in only one mode proceed as described above for "Incorrect output" to find the bad resistor.
Symptom - output remains at 0 for both modes
Probable cause - Defective TL081, shorted power supply, shorted output wires or shorted input wires.
Action - a) check the bell wire, the twisted pair and their connections. If all are OK , check the voltages at pins 7 and 4 of the TL081 (+12 and -12 respectively), If they are OK, replace the TL081.
b) If either voltage is bad, power off, remove the wires to the REFERENCE potentiometer, and test again, If the voltages are still bad, replace the TL081.
c) If the voltages test OK now, test the lamp and REFERENCE potentiometers out of circuit. Replace the bad component.
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