The purpose of this article is to provide you with common errors I see during repairs. In general, most repair issues are related to poor soldering and misplaced components. Taking care and time during the build process will avoid 90% of repairs. If you're uncomfortable with troubleshooting, consider sending in the unit for flat rate repair.
Note about DMMs and AC signal
Digital Multi-Meters are a must have for DIY audio however, they have limitations when measuring AC signal. If you look at the specifications of your meter, many cheap meters will only accurately measure up to 200-500Hz. That said, even if the measurement error is very high (10%) any cheap meter should still give you an accurate enough measurement with a 1KHz test tone. The real limitation is unlike a oscilloscope, you do not get to see the signal. So you don't know if you are measuring AC signal, noise, or a bunch of oscillation. As a general rule, if the AC signal measurement is steady and constant at a test point, it's probably good AC signal. If it's erratically changing and drastically off what you expect, you're probably just measuring noise or an oscillating signal.
Many builds are poorly soldered, have "cold" soldered joints or even missed jointed altogether. If your solder joints are large blobs, try removing some of the solder with a de-solder braid or sucker and apply new solder to the joint. If you isolate your issue to an area and the solder joints look good, you can re-flow the joints by reheating them and adding a touch more solder. Also confirm every joint is soldered and none have been missed.
Damaged Pad or Trace
Poor solder technique can damage pads or traces on the PCB. Often a lifted pad may not be visible under a soldered joint. You can use your DDM continuity tester to make sure appropriate connections are being made. Check the schematic for the area having problems and confirm which components should be connected to each other. Then locate the components on the PCB and you'll see the traces on the top or bottom making the connections indicated on the schematic. Now you can probe those pads and confirm all the trace connections are good in that area of the circuit. If you find any damaged connections, they can be repaired by connecting the pads with a short piece of wire.
Transformer Wires in the Pad Joint
When soldering wired leads, be careful not to melt the wire coating into the pad joint. Melting wire coating into the joint can prevent proper connections. You should be able to pull the coating back from the pad a little. If you can't, the wire coating may be melted into the joint and you'll need to de-solder and re-solder the connection.
Confirm all of your components are the correct value and inserted with correct polarity. Multiplier mixups like 270Ω vs 270KΩ are very common. It's important to understand that measuring resistors in circuit is not always straight forward. If the resistor is in parallel with another component it will read much lower than its value. To measure with a DMM you'd have to calculate those effects on the value or de-solder and lift one side of the resistor. Since bad resistors are pretty rare, start by confirming the color codes match the value. Most importantly check for multiplier mix ups. For example, with 5 color band resistors the multiplier (4th band) would be brown for 1K-9.9K resistors and orange for 100K-999K resistors.
Also, confirm all capacitor values are correct including the tiny ceramics. Note 27pF is marked 270 and 270pF is marked 271, with the last number being the multiplier. Confirm polarized capacitors, ICs, and diodes are installed with correct polarity.
If your product has a discrete op-amp and you have others available, replace the op-amp with a known working op-amp. This is a very easy way to confirm is the issue is with the op-amp or main PCB.
It's Probably NOT a Transformer
For whatever reason, new builders experiencing issues immediately become suspicious of a bad transformer. I'm here to tell you there is a 99.9% chance it's NOT the transformer. Transformers are, simply put, a large hunk of metal with a few wire windings wrapped around it. They are all tested when they leave the factory and really the only thing that can go wrong is a wire break and that rarely happens.
If you are building your second FET/RACK and having problems that seem to isolate to the ratio board, trying the ratio board from your working build and confirm the problem area.