Okay, this was something I was doing on and off and someone asked me specifically for reforming help. That is when I realized there is no proper write up or video on the whole process, specially covering scopes, where we have up to a 8-10 electrolytic caps.
So here it is. I put a video covering the high level process, including Heathkit IT-11, Eico 950B, and Sprague TO-5.
Now, video says a bunch of things. Lets see what we can do above it.
Assume you want to reform in a much more “measurable manner” using the leak tester. Here is how to do it.
There are two points which we miss with the simple leak tester method. Voltage across the capacitor and current through the capacitor.
So here is how to make this 100% professional 🙂
So if you add two DMMs to the setup, you can monitor the actual voltage across the capacitor and measure the exact current. The top multi-meter measures the voltage across the cap, where the right side one can tell you the exact current through the capacitor, as it sits in series with the tester and cap. You can use any capacitance leak tester used in the video to do this.
Second – How do you know if the cap is bad –
Here is a reference link – which shows the leakage current for vintage and modern caps.
Dont try to match the leakage current to the modern caps, as these instruments and caps were not designed for it. They do can handle extra leakage current as it was designed at the time.
Vintage caps are more flexible, they can take higher than rated voltages (10%) for sometime and wont hurt. And these caps are not used in solid state gear, so dont worry, the tubes knows the caps. 😉
Another point specially applicable for Tektronix scopes and other gear too. Bleed resistors. Here is one in Tek 547
Remove or disconnect one end of the bleed resistor before reforming. Else ? The bleed resistor will bypass the current and give you a closed (sleeping) eye. These resistors are typically mounted on the capacitor in most cases, so you will have to remove one end off it.
=== Schematic – Already covered in video ===
Capacitor Leak testers- Voltage on negative lead.
Entire test voltage is on negative test terminal. Highly dangerous if you are reforming a capacitor mounted in a chassis with negative to chassis.
Sprague TO-5 (Potentially other TO-X too)
~30% of test voltage on -ve test terminal. Potentially dangerous.
Heathkit IT-11 (28 too, I guess, didn’t check schematic, lazy)
Least dangerous of all. Only grid bias voltage on negative test terminal.