Troubleshooting Hints

A few people have had trouble making the board work. By and large, poor soldering technique is the biggest single problem. Some voltage measurements and hints about what they mean are also presented here.

Desoldering Parts

I've seen boards that were pretty well butchered by builders trying to desolder a component, and being totally thwarted by the plated-thru-holes. As it says in the manual, unless you have a powered desoldering tool, trying to desolder a part where there is plating in the hole can be very, very difficult.
  • The best thing you can do is to be fourple damn sure before you solder, so you don't have to desolder.
  • If you have to desolder a resistor, put the board in a vise, grab the lead with a pair of long nose pliers, apply tension, heat the joint from the other side of the board. Once the solder melts, the lead should pull free. Repeat for the other end.
  • Anything with more than 2 leads can be very difficult.
  • Consider wiping out the part (destroy it, cut the body off) so you can remove the leads one at a time.
  • Once you get the part clear of the hole, then you can clear the hole with a round toothpick, or something straight and pointy that doesn't take solder. We use a dental pick.

Replacement Parts

You can get replacement parts from Uneeda Audio. You can get up to 6 replacement parts at no charge, except for the LM394 which is $24, because they are now unobtanium ($22 for the part, $2 for shipping. hint: don't screw it up! ). There is a limit to our generosity, and you are advised to not test it. (This offer can be withdrawn at a moment's notice, at our whim.) The circuit board is not part of this policy; if you damage it during construction, you must purchase a new one.

Contacting Us

If you decide to contact Uneeda Audio, please include the voltages measured (make a table using the same steps, in the same order as that presented below), and the results of any other tests. Please state the configuration that you built (including if you installed the LED), what other test equipment you have or donít have (DMM, OíScope, Signal Generator), and your electronics experience level (rank novice, veteran kit builder, EE, etc.)

Troubleshooting The Board

Once you get the thing assembled, then you try it, and it doesn't work... Where to start?

The first thing to do is to disconnect the microphone, plug the thing into a p48 input and listen to it. You may need to turn the gain up. You should just hear hiss. No crackling, no hum, etc. Then take a small screwdriver, and with your finger touching the blade, input gain up, and your other hand either holding the board by the edges, or not touching anything else (so that your body is not grounded), touch the input terminal on the circuit board. These are the terminals where the PZM plate connects. If you don't know which one is input, try all three. One of them should produce an audible hum. If it does, then look at the microphone connections.

Remember that for Configuration 1, neither of the microphone wires is grounded.

FOR THE 2003 BOARD (no LED)

All voltage measurements made with a DMM, with respect to ground, unless otherwise noted, with the microphone capsule connected.
Schematic (pdf)

Step What to do Expected Reading Remarks
1. What is the voltage at the junction of r7, r8, and r9 to ground? 18-22vdc This is the supply voltage to the circuit.
2. voltage ACROSS R11. 0.6 to 0.7vdc. It is limited by the Vbe voltage of Q5. If zero, then Q5 is likely toast, or there is something wrong in the R9, R10 chain.
3. The voltage at the 'top' of R2 and R6 (referring to the schematic) should be about 1/2 of that measured in step 1 minus about 0.6v. About 1/2 of what was measured in Step 1 If the voltage is high, then Q3 is probably toast. If it is low, then there's something wrong with R9 or R10. Correct value stuffed?
4. If the voltage across the capsule matches (or nearly matches) the voltage in step 3, then the capsule isn't drawing current. That could be a bad capsule, bad cable, or bad connections between the board and the cable. 3 to 6v, possibly higher, but not 10v or more. If the voltage is that high, and you are using configuration 2, then check Q2 and its circuit. If you are using configuration 1, then check current source Q1 and its circuit.
5. The voltages at the junctions of R14 and R7, and R8 and R15, should be equal, or very, very nearly so (within millivolts). 18 to 22v ref to ground check voltage in steps 6 and 7; these should also be very, very nearly equal.
6. The voltages at the bases of Q8 should also be very, very nearly equal, if not equal (equal is ideal) slightly less than what was measured in step 3. component values (R2, R6) or capacitors C1, C3.
7. The voltages at the emitters of Q8 should also be equal, or very, very nearly that. about 0.6v less than that measured in step 6. Q8 may have been maimed, especially if the collector voltages aren't very nearly equal.
8. There should be substantially no voltage drop across R2 and R6. or stated another way, the voltages at the BOTTOM (referring to the schematic) of R2 and R6 should be very nearly the same as that measured in step 3. zero, 0. If there is voltage drop, then Q8 may have been maimed, or C1 or C3 are defective (more likely C3). C3 may be soldered in backwards (white polarity stripe towards OUTSIDE of the board).

For the 2006 board (with provisions for LED)

All voltage measurements made with a DMM, with respect to ground, unless otherwise noted, with the microphone capsule connected.
Schematic (pdf)

Step What to do Expected reading Remarks
1. what is the voltage at the junction of r7, r8, and r10 to ground? 18-22VDC This is the supply voltage to the circuit.
2. The voltage ACROSS R16. No more than 0.7v. It is limited by the Vbe voltage of Q5 and the two series diodes D1 and D2. This is an across voltage because the presence or absence of the LED is not known to me. Doing an across measurement eliminates the LED from the picture. So, if this voltage is OK, AND the LED is lit (if present), then this part of the circuit is OK.
3. The voltage at the 'top' of R3 and R9 (referring to the schematic) should be about 1/2 of that measured in step 1 minus about 0.6v (the Vbe drop of Q3). About 1/2 of what was measured in Step 1 If the voltage is high, then Q3 is probably toast. If it is low, then there's something wrong with R9 or R10. Correct value stuffed?
4. If the voltage across the capsule matches (or nearly matches) the voltage in step 3, then the capsule isn't drawing current. That could be a bad capsule, bad cable, or bad connections between the board and the cable. This is irregardless of configuration. 3 to 6v, possibly higher, but not 10v or more. If the voltage is that high, and you are using configuration 2, then check Q2 and its circuit. If you are using configuration 1, then check current source Q1 and its circuit.
5. The voltages at the junctions of L1 and R7, and R8 and L2, should be equal, or very, very nearly so (within millivolts). 18 to 22v ref to ground check voltage in steps 6 and 7; these should also be very, very nearly equal.
6. The voltages at the bases of Q2 should also be very, very nearly equal, if not equal (equal is ideal) slightly less than what was measured in step 3. component values (R3, R9) or capacitors C12, C5.
7. The voltages at the emitters of Q2 should also be equal, or very, very nearly that. about 0.6v less than that measured in step 6. Q2 may have been maimed, especially if the collector voltages aren't very nearly equal.
8. There should be substantially no voltage drop across R3 and R9. or stated another way, the voltages at the BOTTOM (referring to the schematic) of R3 and R9 should be very nearly the same as that measured in step 3. zero, 0. If there is voltage drop, then Q2 may have been maimed, or C12 or C5 are defective (more likely C5). C5 may be soldered in backwards (white polarity stripe towards OUTSIDE of the board).

For the 2011 board (with provisions for LED)

All voltage measurements made with a DMM, with respect to ground, unless otherwise noted, with the microphone capsule connected.
Schematic (pdf)

For the 2011 board (with provisions for LED)

All voltage measurements made with a DMM, with respect to ground, unless otherwise noted, with the microphone capsule connected.
Schematic (pdf)

Step What to do Expected reading Remarks
1. With the capsule disconnected, and the preamp connected to a working microphone input with P48 supplied and the gain turned up, touch the input connection with your finger. You should hear hum when you touch the input. The preamp is (apparently) working. Reconnect the microphone capsule and continue.
2. if the LED is installed, is it illuminated? If it is not installed, there must be a wire jumper installed in its place. LED is lit. Checks the current source, Q5 and for current flow from the P48 source.
3. what is the voltage at the junction of R7, R8, and R10 to ground? 18-22VDC This is the supply voltage to the circuit.
4. The voltage ACROSS R16. No more than 0.7v. It is limited by the Vbe voltage of Q5 and the two series diodes D1 and D2. This is an across voltage because the presence or absence of the LED is not known. Doing an across measurement eliminates the LED from the picture. So, if this voltage is OK, AND the LED is lit (if present), then this part of the circuit is OK.
5. The voltage at the 'top' of R3 and R9 (referring to the schematic) should be about 1/2 of that measured in step 1 minus about 0.6v (the Vbe drop of Q3). About 1/2 of what was measured in Step 1 If the voltage is high, then Q3 is probably toast. If it is low, then there's something wrong with R10 or R12. Correct value stuffed?
6. If the voltage across the capsule matches (or nearly matches) the voltage in step 3, then the capsule isn't drawing current. That could be a bad capsule, bad cable, or bad connections between the board and the cable. This is irregardless of configuration. 3 to 6v, possibly higher, but not 10v or more. If the voltage is that high, and you are using configuration 2, then check Q2 and its circuit. If you are using configuration 1, then check current source Q1 and its circuit.
7. The voltages at the junctions of L1 and R7, and R8 and L2, should be equal, or very, very nearly so (within millivolts). 18 to 22v ref to ground check voltage in steps 6 and 7; these should also be very, very nearly equal. These are the XLR connections.
8. The voltages at the bases of Q2 should also be very, very nearly equal, if not equal (equal is ideal) slightly less than what was measured in step 3. component values (R3, R9) or capacitors C12, C5.
9. The voltages at the emitters of Q2 should also be equal, or very, very nearly that. about 0.6v less than that measured in step 6. Q2 may have been maimed, especially if the collector voltages aren't very nearly equal.
10. There should be substantially no voltage drop across R3 and R9. or stated another way, the voltages at the BOTTOM (referring to the schematic) of R3 and R9 should be very nearly the same as that measured in step 3. zero, 0. If there is voltage drop, then Q2 may have been maimed, or C12 or C5 are defective (more likely C5). C5 may be soldered in backwards (white polarity stripe towards OUTSIDE of the board).

Return to PZM Pages

Last modified 09/22/2017.