Channel Imbalance in DAC

Description: All resistors were tested, and it was noted that the readings of two resistors were inconsistent compared to the other channel (i.e., 1.2 kΩ vs. 2.2 kΩ). According to the color codes, the expected value should be 2.2 kΩ. The faulty resistor, measured at 1.2 kΩ, was replaced with a new 2.2 kΩ resistor, but the issue persisted. The new resistors still read 1.2 kΩ, while the desoldered ones appeared to be functioning correctly at 2.2 kΩ. This indicates that the resistors seem to have approximately halved their value when mounted on the board. If it had been stated that the resistor measured incorrectly, it might have been assumed that the individual lacked understanding of how a resistor on a board may not reflect its printed value due to parallel components. However, the correct measurements from the other channel indicate a definite issue. The problem must lie in the connections to the two resistor terminals, either at the pads or along the tracks leading to them. There should be an open circuit between the pads for the problematic resistor when it is removed, but a resistance of approximately 2.6 kΩ is present, suggesting a potential issue. The most likely cause is a solder bridge, possibly introduced during capacitor replacement, but it could also be a small piece of loose solder, a cut component lead, or a defective capacitor. An unseen short due to a copper string embedded in the PCB is unlikely, as the DAC presumably functioned correctly in the past. Initial troubleshooting should focus on the capacitors, particularly a 220 pF capacitor in series with an unreadable resistor, both in parallel with the problematic resistor. All capacitors should ideally show open circuit readings (megohms, unless they are electrolytic, which should read in the hundreds of kΩ). Any capacitor that does not meet these criteria should be replaced. Voltage comparisons to ground between channels should be conducted with the DAC idle. If discrepancies are found, the components associated with those voltages should be examined. Active components can be removed and tested individually, and a meter with a transistor tester (hFE) would be beneficial. The presence of output from the channel suggests that no transistors are completely non-functional. Transistors can be tested as back-to-back diodes, showing a voltage of approximately 0.6 to 1.0 V from the base to both the collector and emitter (one direction should show continuity, while the other should be open circuit). Identification of base, collector, and emitter can be achieved by consulting the datasheet for the specific transistor part number or by testing randomly to deduce their configuration.

The circuit in question appears to involve a differential amplifier configuration where the resistors play a critical role in setting gain and impedance. The inconsistent readings of the resistors indicate potential issues with the PCB layout or soldering quality, which can lead to unexpected resistance values due to parasitic effects. The presence of parallel components, such as capacitors, can significantly influence the measured resistance when the circuit is powered on, leading to erroneous readings.

In such scenarios, it is essential to examine the PCB for solder bridges, especially in densely packed areas where components are replaced frequently. A visual inspection under magnification may reveal hidden shorts or solder splashes. The use of a continuity tester can help confirm the integrity of the connections between the resistor pads and the traces leading to the components.

When testing capacitors, it is advisable to utilize an LCR meter to obtain more accurate readings, as standard multimeters may not provide sufficient insight into the capacitor's behavior under operational conditions. Any capacitors found to exhibit leakage or low insulation resistance should be replaced to ensure optimal circuit performance.

Furthermore, when diagnosing the active components, it is prudent to check the power supply voltages and ground references, as variations can indicate issues upstream in the circuit. A systematic approach to troubleshooting will help isolate the faulty component or connection, restoring the circuit to its intended operational state.Tested all the resistors and noted 2 resistors` readings were out compared to the other channel (i. e.1. 2Kohm vs 2. 2 kohm). Based on the color codes, should be 2. 2kohm. THought I spotted the problem, repl the "1. 2kohms" with new 2. 2 kohms but the problem persist. The new resistors still reads 1. 2koms and the desoldered ones seems ok reading at 2. 2. i. e. the resistors seem to have almost halved the value when mounted on the board ! Tested all the resistors and noted 2 resistors` readings were out compared to the other channel (i. e. 1. 2Kohm vs 2. 2 kohm). Based on the color codes, should be 2. 2kohm. THought I spotted the problem, repl the "1. 2kohms" with new 2. 2 kohms but the problem persist. The new resistors still reads 1. 2koms and the desoldered ones seems ok reading at 2. 2. i. e. the resistors seem to have almost halved the value when mounted on the board ! If you`d just said that the resistor measured wrong, I`d have thought that you were a novice who didn`t appreciate that a resistor on the board may not read it`s printed value because there are other components in shunt (parallel) with it. The fact that the other channel measures correctly, however, means that there is a definite problem. The problem MUST be in something that is connected to the 2 resistor terminals, either at the pads or somewhere on the tracks that connect to them.

That could be a chain of components, or a single component. There should be be an open circuit between the pads for the problem resistor (when it`s removed), but instead there is a resistance of ~2k6 (by calculation). The most likely cause is a solder string somewhere, probably introduced when you replaced the caps, but it could be a tiny piece of loose solder or cut component lead, or a faulty cap.

I`ve even seen (detected) an invisible short due to a string of copper embedded in a PCB, but this is unlikely in your case, since I guess the DAC worked OK at some stage in the past. First check the caps that are obviously potentially a problem, there`s a 220pF close at hand in series with a resistor whose value I can`t read, the both in parallel with the problem resistor.

All caps should read open circuit (megohms unless electrolytic, in which case certainly hundreds of k). If you can find one that isn`t, that`s a good one to replace. You can check voltages to ground comparing channels with the DAC idling. If you can find a clearer version of the diagram, write the quiescent voltages on it and post it. If you can find voltages at odds with the good channel then the components on whose pins you find them become suspect.

You can pull the active components and check them one-by-one. If you have a meter with a transistor tester (hfe), so much the better. The fact that you`re getting something out of the channel tends to indicate that none of the transistors is dead though. Transistors read as back-to-back diodes, so you should see 0. 6 ~ 1. 0V with a diode tester from the base to both the collector and emitter (in one direction, open circuit in the other).

You can identify the base, collector and emitter by googling the transistor part number and looking at the datasheet, or just test them at random and figure it out.