Would a Part by Any Other Name Work as Well? Like Everything in Engineering, it Depends.
In the fast-paced world of electrical design, technology improves, manufacturing techniques change, and parts become obsolete at a rate unparalleled in most other fields of engineering. But there’s so much more to it than just IC marking lookup; yes, the first step is being able to identify circuit board components in the original board, but then the other considerations begin. Sometimes the same part is now being manufactured in a new way with slightly different specs, or a different kind of package. Sometimes that part now has a new name and a new manufacturer, following the merger or purchase of the original company by one of the larger companies consolidating power in the semiconductor industry. Sometimes there’s an entirely new type of device or type of circuit that is used today to achieve the function that the original part once did.
But let’s start at the beginning; identifying circuit board components. One of the tricky aspects of reverse engineering older electronic devices is that the original part datasheets can be hard to come by. Although, at least older electronics do have large components with full part numbers written on them. Newer boards almost all use small SMD components with scarcely more than a two-character code as a marking. For these, reference guides and SMD code lookup sites can prove invaluable.
In either case, the same basic advice holds true: the more information the better. Things like knowing the type of device, the package, the manufacturer, the ratings of other connected devices (like voltage or temperature), and the device functionality (such as power, logic, or circuit protection) can all go a long way to help finding full part names and original datasheets.
Sometimes no information is available, and reverse engineering becomes a more intensive process; that will be for another blog. Right now, let’s say that the full IC part name and datasheet are available online (congratulations). But after a few more searches it’s becoming clear that the exact part is now obsolete (bummer). Now what?
When it comes to ICs, a practical tip is to begin by looking at different part number variations. For example, ON Semiconductor has an obsolete IC which is part number 74F132SC (NAND gate, 4 channel, Schmitt trigger, 14-SOIC, TTL logic family). A reasonable substitute part available today from ON Semiconductor is the MM74HC132M logic IC, which has the same function and package as the original part and meets-or-exceeds all of the original specifications. Another avenue to try when reverse engineering parts is looking at a different manufacturer. For example, if the original part was made by General Semiconductor it may still be in production today with a different part number under the Vishay brand (which acquired General Semiconductor about 20 years ago). Or, say the original part made by Analog Devices is obsolete, an equivalent IC made by Texas Instruments may still be in production. Finally, there’s some crosses to be found when reverse engineering by exploiting environmental ratings. The quad comparators LM239 and LM339 are electrically identical, but have different operating temperature ranges: the LM239 works from –25°C to +85°C while the LM339 has a more limited range of 0°C to 70°C. In any case an LM239 can be used for an LM339. For a room temperature application, the LM339 could be substituted for the LM239 without any issues, but for an application like automotive use the temperature range could be insufficient, depending on whether the car owner lives in Ottawa (no chance) or Vancouver (lucky break).
The flip side of all these related part numbers is that it’s easy to miss when nearly-identical parts have important differences in their characteristics. Reverse engineering hardware requires such a deep understanding of the application to know which specs of different circuit board components matter. As an example, the Analog Devices ADA4000-4ARZ is an obsolete 4-circuit, JFET, 14-SOIC op-amp IC. There is also a newer Analog Devices ADA4004-4ARZ general purpose, 4-circuit, 14-SOIC op-amp in production today. Despite being only one digit off, the new piece might not be a suitable cross depending on the circuit and its application. The second, newer IC on the surface looks like a perfect replacement: it has the same package and pinout, the same operating voltage, it even exceeds the original 5MHz bandwidth with a GBP of 12MHz. But in specifications, the slew rate on the newer IC, 2.7V/us, is an order of magnitude lower than the slew rate of 20V/us that the original one had. The phase margin on the new IC, 48 degrees, is also lower than the original phase margin of 55 degrees. In a high-speed circuit, the lower slew rate can pose a problem if the IC is not able to keep up with the rate of switching required to convey the signals, and always seem to be stuck high or low. The lower phase margin could also pose a problem if now the circuit crosses over the threshold to turn an amplifier into an oscillator, or in more practical terms changing a control system from stable to unstable.
In the process of reverse engineering hardware, there are many traps and pitfalls when it comes to identifying circuit board components and finding suitable replacements. To avoid them requires a good understanding of device operation in circuit, knowledge of the evolution of electronic devices over time, and familiarity with all kinds of parts. There’s no shortcut to gaining that experience, but general tips like these can help to steer clear of the most common mistakes and provide advice for how to move forward.
Still stuck trying to identify and source parts to clone or repair an obsolete PCB? ENA specializes in reverse engineering and electronic repair, contact us directly to help with your project.