A friend of mine just purchased an old Tek 570 Vacuum Tube Curve Tracer that hadn't been used in decades. He is a competent technical guy, but wasn't too familiar with old tube scopes. I made up a list of things to change and check, so I thought I would pass it on to the group.
Note starts here >>>
I've owned a Tek 570 for over thirty years and have tested thousands of tube on it. It is from around 1958 and has rounded corners and the black and white plastic meter. Here are my recommendations for initial bring-up and potential weak spots to address:
1. Before turning on, either: a) carefully reform the electrolytic capacitors or b) plan on replacing all electrolytic with new ones. This can be done by wiring them under the chassis and leaving the old cans in place. For reforming the capacitors, I recommend using a high-voltage adjustable power supply and slowly increasing the voltage across each capacitor until it draws 10 mA. Only after this decreases can the voltage be increased - eventually reaching the maximum rated voltage. A series resistor from the power supply can also be used to limit current.
2. Change all tubular paper capacitors. This is especially important if they are the black color-striped capacitors - these are notoriously bad. Regular mylar caps are a good replacement. Make sure to change the high-voltage capacitors in the CRT power supply, unless they are ceramic.
3. Check all the tubes on a tube tester and replace any weak tubes. Luckily all types are common. In later models the 6BQ7s were replaced with 6DJ8s. You could go back to using the 6BQ7, or you can use the Soviet 6N23P, equivalent to the 6DJ8/ECC88.
4. Spot check the resistance of the carbon-composition resistors. In some years they had a tendency to drift with age, always upwards in value. Replace any that are out of tolerance. About 1/5 of all my resistors had drifted out of spec.
5. Make sure to use silver-bearing solder on the ceramic terminal strips.
6. Lubricate the switches and pots with a drop of oil on the shaft and grease on the detents. I've found that military rifle grease works well here. A brief spray of DeOxit will clean the switches and pots. Use lightly. Also lubricate the fan.
7. I don't recommend changing the tube rectifiers to silicon - this will stress the rest of the system. Plus, the tube rectifiers used are common and cheap. If you want to replace the 5642 HV rectifiers with silicon, use only high-speed rectifiers.
8. The CRT is the same type as used in the Tek 575, so if there is a problem with the CRT, find a 575 parts unit. They are infinitely more available than the 570. The CRTs seem to be quite reliable, though.
9. In my 570, the readings were erratic at low current settings of the VERTICAL MA/DIV switch. It turns out that the current-sensing resistor string on this switch is grounded to the frame of the switch. If there is any corrosion where the switch is mounted to the front panel an erratic voltage drop is created. Running a wire from the switch frame to a ground point under the chassis solved this problem.
10. The step-generator circuits use some very high impedances. You may have to try several different 6AN8 tubes in the V8 and V38 sockets to get linear steps. It is helpful to use an oscilloscope on the step generator output to check for linearity.
Manuals and factory calibration procedure are available at:
end of note.
- John Atwood
