Tektronix TDS460 400 series Oscilloscope Power Supply Repair

>> YOUR LINK HERE: ___ http://youtube.com/watch?v=9lmAQUjs_cE

Power supply had a problem of cycling ON OFF, but would not stay ON. • Download the Service Manual for the LeCroy 9354A-M-L-TM o-scope, it uses a nearly identical AC-DC Power Supply. • Google search for manual: • LeCroy_9354A-M-L-TM_Service_Manual_Complete.pdf • Direct link I found: • http://www.ko4bb.com/manuals/73.3.51.... • This video describes repairing my TDS460 o-scope that was refusing to Power-ON . It would continually blink the front panel lights ON OFF at the rate of about once per second. • • Another symptom is blinking of the 7-segment LED display (located on the CPU Board) ON OFF... but never staying ON. • • In mine, the problem was found to be in the Low Voltage Power Supply board (main AC-to-DC power supply). • STEPS: • 1) Remove the Power Supply • 2) Using compressed air, blow all dust off of the board • 3) Give it a GOOD visual inspection for bad solder joints • • There is a very common problem with the main J1 output connector being poorly soldered. It’s probably best to just resolder the entire connector. This will not fix the Pulsing ON/OFF problem, but it is something that needs serious attention on these ‘scope models. • • (This will take a soldering iron with enough power to heat the pins/pads quickly, as there are many HEAVY traces, with a lot of copper to heat. I suggest at least a 50W soldering iron.) • • The actual problem that mine had was in the SD (Shut-Down ) detection circuits in the Power Supply. (see schematic, pg 425) • • There are several Comparator sections (mainly U502, LM339) with their outputs wired together in an Open Collector Wired-OR configuration. In this way, if any one comparator section Output goes LOW, it causes the SD signal (/ShutDown) to turn the Power Supply OFF. The power supply will then wait a short time, and try to turn ON again. Also included in the wired-OR is an LM339 on the main board, U701. This tests for Over-Voltage and Under-Voltage aka Power-Fail . • • Before continuing, check all of the DC Outputs of the Power Supply to be sure that each regulator is actually performing correctly . Use a Peak Hold DC voltmeter, or another o-scope to determine if each Output is reaching the correct Voltage (during the ON/OFF cycles). • Determine WHICH of the comparators is causing the problem means isolating each output from the wired-OR bus, until the power supply operates normal. • On the daughter board near J1, the LM339 (U502) compares for Over-Current protection of each of the four output voltages. I found my problem was at U502, the section with pins 4, 5, and 2. This section tests the 4th power supply output, which is -15V DC in the Tektronix. • (NOTE: The LeCroy schematic has some errors related to both of the LM339 IC's... their Power Ground pins are labeled wrong: +V is really pin 3; -V (Gnd) is pin 12 (per package). • BTW, the +V supply for these IC's is about 19V, but all inputs of U502 are very nearly 0V. These comparators allow sensing down to the Ground rail, so that's allowable. • I determined that the (+) input (pin 5) of this section was not behaving (during power-up) like the other 3 sections of the same IC... when they all share identical circuits. Pin 5 was ‘drooping’ lower than pin 4, causing a false over-current detection. • My main problem ended up being a bad 1M resistor, R413. • I see this as a flawed design... high impedance circuits like this should be coated, so that humidity doesn't cause problems on old/dusty boards. I noticed that sometimes it would work fine, but if I breathed on it, and then power was applied, it would start blipping again. • Once I got it running (I jump-started it by connecting another 1M resistor from the 2.5V Vref to U502 pin 5) it would start and run fine... which verified that there really was no actual over-current problem. • I just soldered a 1M resistor across the original one (again, it was R413). • This fixed it. • Additionally: • When I removed the daughter board, I found that some of the main output capacitors seemed to have begun leaking (R401 current sense wire looked corroded). They were Nichicon's, so I had a hard time convincing myself it could be true, but replaced them anyway (use 105°C caps). • I had some 100uF MLC capacitors that are GREAT for switching power supply filters, so I added one to each output. This will relieve most of the hard work the large caps are asked to perform. • I also found other design flaws... R502 and R504 should have both been 1/2W, not 1/4W. • R502 is on the daughter board, drops the 19V down to the 2.5V Vref TL431 regulator. That's 270mW dissipation. • R504 is on the main board, AUX supply, series dropping from the same 19V supply, down to a 5.1V Zener - VR502. (it dissipates 200mW). • I suggest swapping both for 1/2W parts. • Have fun!

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