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Forum Home > Troubleshoot Info for Mods & Tonmann Guides > Troubleshooting Opamp Circuits

Barry
Site Owner
Posts: 8793

Tonmann Lecture:


A bit of background information that should help you and others to find problems with op amp circuits. If you can remember this you should find troubleshooting a lot easier.

Almost all op amp circuits, or at least the audio parts of the circuit, are designed so that the DC (bias) voltages on all pins except the power supply pins measure half the voltage of the +ve power supply.

There are a very few circuits where the bias voltage is deliberately set slightly higher or lower than half the power supply voltage and there are some non-audio parts of the circuits e.g. low frequency oscillators (LFO) where you will have a constantly varying voltage.


You will sometimes find that the pin voltages are not exactly half the power supply voltage and there are two reasons why this is so:

 

If you are using a cheap DMM (Digital Multimeter) to measure voltages on any circuit that contain high value resistors (100kΩ and over) you will get false readings because the DMM input resistance is too low - this doesn't mean the circuit isn't functioning properly it just means you have to take into account the effect the DMM has on the circuit when reading voltages.

I would always suggest to people that are going to be building circuits on a regular basis to buy a decent DMM; these don't have to be the expensive models but you should check the technical specifications to make sure that the input resistance in voltage mode is at least 10MΩ - if you can't find the technical specifications for a DMM don't buy it !!!!

Don't use an analog multimeter to measure voltages - it's input resistance is much to low to be of any use.


Op amp circuits are designed with the "perfect" op amp in mind, imperfections during the manufacturing process may cause less than "perfect" predicted voltages on the pins. The audio circuits that we build are generally non-critical i.e the voltages don't have to be exact to make the circuit function - a few hundred millivolts either side of half the power supply aren't going to matter.

Get two 1MΩ resistors (1% metal film resistors guarantee a good match) and solder them in series, that is end to end. Connect one lead to +ve power supply and the other end to ground. Measure the power supply voltage and then measure the voltage where the two resistors join.

If both resistors are exactly the same value your second voltage measurement should be exactly half the first measurement e.g. if the power supply is 9V the voltage where the resistors join should be 4.5V.

 

  • Because your DMM adds some resistance in parallel to the lower resistance it's total resistance is less than 1MΩ so the actual voltage you measure will be less than 4.5V.
  • The lower the input resistance of your DMM the further away from 4.5V your measurement will be, here's an example to illustrate the point:
  • If your power supply is 9V, the two resistors are 1MΩ and the input resistance of your DMM is 10MΩ (typical for a good DMM) your voltage measurement at the junction of the two resistors will be 4.29V

 

If your power supply is 9V, the two resistors are 1MΩ and the input resistance of your DMM is 1MΩ (typical for a cheap DMM) your voltage measurement at the junction of the two resistors will be 3V

 Obviously a DMM with a higher input resistance will give a more accurate voltage reading.

 

 

We can check the voltages with the IC out of the circuit. Remove the IC, apply power to the circuit and grab a schematic of the circuit. Remember that all voltage readings are referenced to ground unless otherwise stated.

 

First check the power supply voltage to the board (+9V pad) and then check the power supply voltage to the IC, you should do this by measuring the voltage between pins 4 & 11 both readings should be the same.

 

Any pin that is connected to the bias voltage VB either directly or through a resistor should yield a voltage reading the same as VB, if your measurement is 0V there must be a break between VB and the pin.

 

 

Check the part of the circuit that supplies the bias voltage VB

When measuring the voltages on the above pins you might get a "kick" in your readings due to capacitors charging / discharging so keep the probes attached until you get a steady reading. Any reading other than 0V would suggest a short circuit (bridge) somewhere in the circuit.

 

 

Voltage measurements will tell you where the problem is or isn't, unless you are good at electronics they won't tell you which component is causing the problem. The second part of fault finding is measuring resistances to find short circuits, open circuits and correct resistor values.

 

There are a few points to bear in mind before you start to measure resistances:

 

Physically remove the power supply / battery from the circuit and wait a few minutes to allow any large capacitors to drain to ground. Never measure resistances in the circuit with power applied, you will more than likely destroy your DMM at worst or have to replace a fuse at best.

 

 

Remove all active components (ICs, transistors and diodes) before you start measuring. Your DMM uses an internal voltage when measuring resistances, this voltage could turn on transistors and diodes which will give you a false resistance reading.

 

 

Knowing what resistance value to expect between two points in a circuit is not very difficult, the hardest part is if you have more than one path running between two points in the circuit - instead of simple resistances in series we now have resistances in parallel which takes a small amount of maths to calculate.

 

 

An easy method to get things right is one I have used ever since I started electronics:

 

Print a copy of the schematic

Erase the active component symbols from the schematic (you have removed the components from the board)

Erase all capacitors from the circuit - DC voltage won't "pass" through capacitors so for all intents and purposes they act as a break (open circuit) in the path.

Look for multiple paths between two points in the circuit - use a pencil to mark the paths between two points.





Here's my "resistance" schematic for the ParaMix.  As all circuits are different there is no actual one method to follow so I will just run through how I would measure the resistances for this circuit.


Check VB supply:  Measure from the top of R13 to the bottom of R14 - which is ground (I can do this on pins 4 & 11 of the IC).

Expected value is R13 + R14

Notice that I have to remove the paths through R17 / D2 & SW2B to ground and the path through R16 / D1 and the bypass switch to ground otherwise I would have three parallel paths to ground which will throw off the expected result.  This is especially  true if the LEDs start to conduct when using the DMM in resistance mode.  The easiest way to disconnect the paths is to remove the ground wires at the foot switches.


Pin 10 to VB - I can use any VB on the circuit; the top (layout) lead of R13, pin 3 or pin 5.

Expected value is R2


Pin 9 to Pin 8 - connected together.

Expected value is 0Ω


Pin 10 to Pins 9 / 8 & 11 - checking for solder bridge (short circuit)

Expected value is infinite Ohms (open circuit) for pins 9 /8 and R2 + R14 for pin 11


Pins 3 & 5 to VB - between the top (layout) of R13 and both pins.

Expected value is 0Ω


Pin 2 to Pin 1 - depends on rotation of the pot

Expected value is either R5 + P1 or just R5


Pin 3 to Pins 2 & 4 - checking for solder bridge

Expected value is infinite Ohms for pin 2 and R13 for pin 4


Pin 5 to Pins 4 & 6 - checking for solder bridge

Expected value is R13 for pin 4 and infinite Ohms for pin 6


Pin 6 to Pin 7

Expected value is R11


Pin 7 to Pin 8 - checking for solder bridge

Expected value is infinite Ohms


Pin 12 to VB - depends on the setting of SW1

Expected value is R6 in up position and 0Ω in down position


Pin 12 to Pin 13 - depends on the setting of SW1

 

Expected value is R7 in up position and R6 + R7 in down position


Pin 12 to Pin 11 - checking for solder bridge

 

Expected value is R6 + R14 with SW1 in up position and R14 with SW1 in down postion


Pin 13 to Pin 14

 

Expected value is R8


I hope I have covered all possible resistance measurements.



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August 13, 2014 at 3:27 PM Flag Quote & Reply

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