The repair industry has grown tremendously in the last five to seven years as companies convert their maintenance departments into electronic repair shops. To manage a profitable repair depot, you need a good understanding of the diagnostic capability of the tools available and the resources to develop appropriate testing procedures. The tools and resources must be factored into a plan to show an effective return on investment.
In-house repair has its own issues, such as:
o Limited technical capability.
o Limited or no documentation.
o Test systems that don’t perform repeatable tests.
o Test systems that don’t provide good diagnostics of a malfunctioning board.
Unfortunately, the process of troubleshooting electronic circuitry is not straightforward. There are no entirely automatic systems that will probe a product and provide a test program to your repair department. Some work must be invested since every board type has its own idiosyncrasies.
One solution is a board diagnostic tool which uses in-circuit test techniques but doesn’t require expensive fixturing or an involved program development effort. It can assemble tests for shorts, opens, voltages and functionality of components, all while in-circuit, and without the need of an intimate understanding of the functions of the board.
In-circuit test eliminates unnecessary components removal and associated potential for board damage. Where multiple devices are involved on a bus, test is made easy by a controlled disabling or guarding of adjacent devices.
Couple that with the capability to develop a schematic of the board and you have a practical solution for board repair. To demonstrate this capability, consider the circuit shown in Figure 1.
A controller elsewhere in the circuit dictates whether devices Z39 and Z43 or Z38 and Z42 are on the bus sending or receiving data. In normal operation, this would be the CPU or a control chip.
The primary control line on these devices is either a Chip Select line or the output Enable line. The controller will select a low condition on the pair of devices it needs to communicate with and hold the other control lines high. Data transmission works well until a device fails somewhere on the board. Then all sorts of things may occur.
Troubleshooting requires the capability to control the enable/disable lines of other devices as well as to test the device in question. The Pro-Line PL5000 Digital Repair Station from B&K Precision uses programmable guards to disable activity on parallel devices in much the same way as a microprocessor controls normal operation.
While testing Z39 and Z43, the data flow onto the data bus is disabled by driving a high condition on Pin 22 of Z42. Pin 22 of device Z43 is held similarly when testing Z38 and Z42. This process allows effective exercising of all four devices. The tester can electronically isolate each in-circuit component while the circuit is functioning normally. For example, Z39 will be addressed, written to and read from.
The tester also checks surrounding circuitry for shorts, opens and proper voltages. Additionally, and perhaps more importantly, the tester will evaluate the nature of the surrounding pins and adjust the functional test accordingly.
Now imagine the difficulty facing technicians when schematics or associated board documentation are missing. A natural fallout of test development using the Pro-Line PL5000 is: o It determines the devices in a circuit.
o It learns the interconnections to each of the pins.
Using this information, a circuit schematic can be developed. The resulting node map will also identify tristatable devices as potential disabling candidates.
The tester uses clips and probes to learn and save interconnects on a board and develops an interconnectivity table (nets list). It maps the interconnections by guided probing of all devices.
The resulting net list is read by a schematic package. An automatic routine routes the wire connections, minimizing time and potential errors for the user. The resulting schematic can now be used to define a viable test procedure.
One Company’s Experience
Many companies use in-house repair to gain control of maintenance costs. The results can be significant. In eight months, one company realized a savings equivalent to two-thirds of the purchase price of the tester. In previous months, repair costs per board averaged about $100 plus extended down time. The average repairs now are $25 to $35 and the turnaround significantly faster.
Additional benefits included increased control over repair quality and first-hand system understanding. The company systemized the repair process. As a result, development of in-house maintenance proved to be both reliable and profitable.
About the Author
Dave Austin is the Pro-Line Product Specialist at Maxtec International Corp. Before joining Maxtec, he was affiliated with Tektronix, GenRad and Gandalf Data and Teltrend Inc. Mr. Austin was educated at DeVry Institute of Technology. Maxtec International Corp., 6470 W. Cortland St., Chicago, IL 60635, (312) 889-1448.
Copyright 1995 Nelson Publishing Inc.
February 1995