Powering up some very old high powered tube amplifiers

Actually that's what I suspected they were called and I have found references to that term online. But using google (images), most appeared to be larger caps. Apparently these were the very early 'lytics used on the very old radios and other stuff.

They looked almost identical to the aluminum can 'lytics, except the tops had slight indented circles (like a target) and that tiny hole in the center.

Anyone have a schematic for a simple variable DC power supply I can make from salvaged tv parts, to be used for reforming old 'lytic capacitors from tube equipment? I have plenty of parts including transformers from old tube type tv sets. Solid state Rectifiers preferred.

Thanks for all help.

Get an old power transformer that can give you about 400V after rectification and a low power variable transformer (Variac). Add some rectifiers to the HV secondary. Wire the Variac between the AC line and the transformer primary. Add an On-Off switch and a fuse between the AC line and the Variac.

Please view in a fixed-width font such as Courier.

Pwr Variac Xfmr FWB Fuse +---+ +---+ +---+ / +---+ | | | | | | ---/ ----| |--| | -----| |--------|~ +|---------(+) output +---+ | | | | | | | | | | | | | | | | | | |300V | | AC Line | |

Please remember that the electrolyte in electrolytic capacitors is rather conductive, not an oil at all.

?-)

This is where it may get tricky. If you reformed only to 70% of rated they might still go bang if roughly subjected to rated voltage. I = suggest reforming to 125% of rated, slowly. After all, they were formed to above rated so that they would be reliable at rated.

?-)

Simple, rectify the mains or transform it to give your dc V, and just attach a series resistor. Job done. The voltage on the output will be determined by the cap and what it can handle, and itll rise on its own as it reforms. Keep i very low to avoid risk of damage and make the output touch safe when not isolated.

NT

Your drawing came thru well. Thanks

Where might I get a variac? I'm still trying to figure out what FWB means? I know that's the rectifier, just dont understand those initials.

One other thing, to use this as a reformer, are filter caps put after the rectifier on this device, or do I just feed the TEST cap directly?

Aside from the variac, I should have all the parts.

Variacs are spendy, and IMHO, unnecessary. Just make a DC supply that'll provide the voltage that the caps need to be reformed at, and put a honkin' big resistor in there. You probably only want to allow a few milliamps into the caps, maybe even 10mA (Google for cap reforming). Size the resistance and power dissipation to match.

Older ARRL handbooks covered the process in reasonable detail.

Dont have any of those books, even though I am familiar with them. Might they have the older ones online on a website or PDF file?

Thanks

Yes, variacs are a bit pricey when compared to non-variable transformers, but certainly not a bank breaker. In this case, a variac would certainly be the best solution, since the OP wanted a simple circuit for reforming unknown capacitors. His capacitors might be rated at 450VDC or 50VDC. It wouldn't be such a great idea to connect a 50volt capacitor to a 400VDC source, even with a honkin' big resistor between them. The only other solution would be to build several DC supplies for different voltage capacitors. He needs a variable voltage source that can be adjusted for the appropriate reforming voltage.

To the OP's questions... FWB means Full Wave Bridge, and yes, it's the rectifier. Get one having about 1000V PIV (Peak Inverse Volts) rating. Variacs are available on Ebay almost all the time. You don't really need a filter cap in the reformer; it's OK to hook the capacitor under test directly to the output of the reformer. Be sure to add a resistor of about 10K at 5 or 10 watts in the output of the reformer. See the new diagram.

Please view in a fixed-width font such as Courier.

Pwr Variac Xfmr FWB Fuse +---+ +---+ +---+ / +---+ | | | | | | +----+ ---/ ----| |--| | -----| |--------|~ +|--| |-(+) output +---+ | | | | | | | +----+ | | | | | | | | | | | |300V | | 10K/10W AC Line | |

RL

Another way, if he doesn't have a variac, is voltage dividing the output to get reduction he needs to the max voltage of the cap. Something like 10 5W resistors in series from V+ to gnd, 10K each. Then he can pick the "max tap" that gives him the closest voltage under the cap's v rating as the max, and reform in steps using the lowest voltage tap first and progressing up to the "max tap". With his 350V transformer, that would be good for 50V up to 500V caps.

Ed