Troubleshooting tips for a Tek 485 scope


Reed Dickinson
 

Hi Gang:

I want to pass on a few troubleshooting tips I have developed as an aid to keeping your 485 running perfectly. Follow these steps in sequence for best results if your 485 ceases to run properly.

1) Remove case and inspect all PCB's very carefully. Look for empty sockets or burned or charred components.
2) Before applying power measure the DC resistance, to ground, of each of the 13 low voltage power supplies in your 485. If you find one with low or 0 Ohms resistance start removing inter-board jumpers while watching the ohm meter. First remove the N and P jumper blocks. This isolated the A8, A10 and A11 PCB's. If the short persists then remove the X, Y and Z jumpers. If the short persists then remove connectors on P1500, P1580, P1590, P1560 and P1570. If the short still persists then the problem is on the A13, the power PCB. Before replacing the shorted cap you want to know if the short has taken any other components out. Find the number of the defective cap from the TANT_PCB.XXX file that I downloaded to the forum. Remove the associated capacitors while monitoring the DC resistance. When you find the shorted one check the schematic to see if any series component might be damaged. Don't overlook R1579, which may be on the back side of A13 if the +5.5V rail measures low or open. Once the first short is fixed re-install all the removed jumper blocks and repeat part 2 again. If no further shorts are found proceed to 3. If the short disappears when the N and P jumpers are pulled then leave N and P out, move your ohmmeter to the defective rail on the A10 PCB and pull jumper L to isolate the A8 PCB. pull jumpers E, F and K to isolate the A12 timing board and jumpers C, D and AA to isolate the A11 horizontal PCB. Go to the defective PCB, replace the defective cap and repeat part 2. Use similar logic to locate any defective caps on other boards.
3) Now apply power and is all is well the power should come on. If it does not then repeat part 2 again as another short has developed due to the shock of power being applied.
4) At this point you have determined that power should be on but it isn't. The +120, +25, +9, +5, -15, -9 and -5 Volt supplies are all monitored at pin 2 of U1910 on A13. This summing junction should be 0V for power to come up. With the power off jumper TP1951 at pin 2 of U1910 to ground. Put a Voltmeter on the +50 rail and momentarily apply power. If the +50 V rail does not come up replace U2042 and repeat 4. If the +9 rail is low replace U2052; U2062 if the +5 is low. U2072 if the -5 is low and, U2086 if the -9 is low. Do not leave the power applied any longer than necessary as a critical bias voltage might be missing.
5) If it still does not power up put a voltmeter on U1910 pin 13, if more than 0.5V is present when power is momentarily applied then something is drawing excessive current. Possible problems are shorted HV caps in the A11 PCB. With power off remove the brown wire shown on the lower left corner of schematic 13, be careful as there is -3KV on that wire, reapply power. If it still does not work check C1601, C1602, CR1601 or C1603. If these components are OK remove jumper U on A13, cut pin 3 free and re-install the U jumper with only 4 pins on it. This bypasses the X6 Voltage multiplier U1600, try power again. If none of these work check all the diodes and transistors on the A14 inverter PCB. If these are OK then you might have a bad transformer on the transformer PCB, replace the PCB.
6) I take out the A7 vertical PCB to replace all the 11 Tantalum high risk caps on it. To remove the PCB you must disconnect the high voltage lead going to the CRT. Do NOT, repeat NOT, pull the plug in the HV lead unless you have allowed at least 6 hours for the stored charge in the CRT to dissipate. The 18KV stored in the CRT will seek a path to ground and that path is through U660. If you pull the HV plug and there is a full charge on the CRT you will ruin U660, the vertical output IC. Please don't ask me how I found this out.

I have noted that the Tantalum caps that are in a hermetically sealed enclosure never fail while those in the dipped configuration, with a low Vc/Va ratio, frequently fail. So, I have concluded that the presence of contaminants has shortened the life of the dipped variety cap. It is either moisture entering via the exposed legs or components internal to the cap leaving via the exposed legs. I do not know which is the case but I do know it is happening. Tek used Tantalum capacitors in large quantity because they were thought reliable, cheap and plentiful. Over time Tek has found out that serious shortcomings exist that were not known early on. We are now paying the price for Tek's oversights.

Comments, corrections or complaints are welcome.
Reed Dickinson
reed714@...

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