Well I finally completed my cap reformer and it works well. Pix below.
Notes: I designed the thing around the pile of octal gas regulator tubes I had sitting around. These (or the 7-pin versions) are reasonable choices for the thing because the idea was to seriously limit charging current so that if any given cap exploded, it would be a small conflagration versus a life (or room) changing event.
It had been a fair while since I had built anything, so in my case, I wanted a project that didn't have to be good looking.
The idea was to:
1: provide a means of gradually hitting a to-be-reformed/checked cap with small, then larger voltages.
2: limit charging current to 5 mils.
3: Since this is a thing you want to leave on for several hours before reusing a cap in something, provide a bleeder resistor on every leg to pull about 1 ma so that charged caps do not sit around dangerously charged up.
4: meter the output so that once charged, one can observe the leakage current. Should be nearly zero.
5: Not dedicate a variac (which would otherwise be a perfectly viable means of doing the same thing)
All parts were junkbox parts except for the filter caps and alligator clips. The color coded clips 'n boots came from a package of cheap test leads, cut one end off. Yes, I could have used binding posts. Not that cheap to buy in the 21st century.
There's nothing particularly magical about it.
It's possible to change voltages by shifting the tubes around because they all have the same base pinout. I wanted a low "first hit" level (75 volts) thus the 0A3, then to work the applied volts up about 100 volts at a time to rated volts.
The 0A3, 0B3, 0C3, 0D3 tubes want to be current limited to about 25 mils, 30 mils, absolute tops. They are OK running 10-15 mils, too. The important thing is that current limiting MUST be provided. I only use one alligator clip at a time, two could probably be used without overtaxing the regulator tubes. The topmost tube gets only a bit warm after several hours in use.
It would probably be intelligent to place a diode in the meter feed so that when the power is shut off and the cap discharges, the meter isn't driven backwards, possibly bending the needle. But that current is also limited to 1 ma, so the meter isn't slammed badly.
The resistor values on the schematic are calculated values. Only need to get close.
Originally I mounted a pilot lamp holder like Fender amps use in the middle of the "control" panel (ha!)
After all, you definitely want to know when the thing is on and throwing 420 volts out into the world.
But there's no real need for a pilot light with that nice orange 0A3 lit up.
There's a piece of 1/8" plexiglass super-glued top the panel to give the alligator clips a place to live when not in use.
When powering up caps, I place them in a cut-off plastic bottle in case they decide to blow up.
The meter was originally a 1 ma full scale meter, I had to figure out a shunt to convert it to a 10 ma meter.
I also did not want to employ a rotary switch for 300-400 volts, which might have allowed only one set of output leads. The color-coded leads are (to me) a little more foolproof.
Note: The +620 volts on the output of the rectifier is a completely unloaded reading, without even the reg tubes. When the regs are lit up, that drops to 600 even, and if you add up the reg tube voltages, to 570 volts, the current through the stack of reg tubes is 30/1200 = 25 mils.
Topic: Cap Reformer (Read 3749 times)