About caps reforming... 500VDC ?
- Thread starter hopkinWFG
- Start date
glip22
Well-known member
First remove all tubes except the rectifier tube.
Examine the capacitor. Most likely it will be a multi capacitor can. The capacitance and maximum voltage of each capacitor is marked on the side.
If some of the capacitors are rated at a lower voltage than the first one you should unsolder the resistor feeding the lower voltage units. Refer to the Figure above for the circuit of a typical power supply. Do not worry about capacitors that are not tied into the power supply filter by resistors. Sometimes a cathode bypass capacitor is part of the filter can.
Unsolder the wire which connects from the filament or cathode of the rectifier tube to the first filter capacitor. This is the dashed line in the figure above. Temporarily solder a 100 k ohm 1 watt resistor in place of the wire as shown.
Connect a voltmeter across the capacitor and turn on the power to the equipment. The voltage will come up very slowly.
When it gets to about half of the maximum capacitor voltage turn off the power and discharge ALL capacitors that are connected to the power supply.
Replace the 100 k ohm resistor with a 10 k ohm 2 watt resistor. Turn the power back on.
Measure the DC voltage across the 10 k ohm resistor. If it exceeds 150 volts turn off the power and substitute a 22 k ohm 2 watt resistor.
Continue to monitor the voltage across the 22 k ohm resistor and when it gets down to 100 volts or less, change back to the 10 k ohm 2 watt resistor.
Continue to monitor the voltage across the 10 k ohm resistor and when it gets below about 5 volts turn off the power, discharge the capacitors, and remove the resistor and solder the wire back the way it was.
Now turn your attention to the remaining capacitors which you have not unsoldered from the circuit. Their voltage should equal the voltage across the first capacitor within a few volts. Leave the power on for several hours to see if the voltage is coming up.
Test the resistor going to this capacitor to make sure it isn't open. 10% or even 20% above coded value doesn't count as open. You may need to connect a resistor in parallel with the one which is there to help the voltage come up. After these are up, unsolder any resistors you soldered into the circuit.
Once again turn off the power and discharge the capacitors. If you unsoldered some capacitors earlier resolder them.
Let's say the first three capacitors in the power supply are rated at 450 volts and the fourth one is a 250 volt unit. This is not uncommon. When all the tubes are plugged in their load current will keep the voltage across the last capacitor from exceeding 250 volts. Remember you have unplugged all the tubes.
Connect a voltmeter across this last capacitor and watch it while you turn on the power. If the voltage gets close to 250 volts quickly turn off the power. If it goes up to say 50 volts then you need to wait for this capacitor to reform. As before you may need to connect a resistor in parallel with the one which is there to speed things up a little. Don't forget to turn off the power and discharge the capacitors before soldering in another resistor. DO NOT LEAVE THE FORMING PROCESS UNATTENDED.
When the voltage reaches the rated voltage of this capacitor turn off the power, discharge all capacitors, and disconnect any resistors you may have temporarily installed.
Replace all tubes in their sockets. You did make a drawing showing where each tube goes didn't you?
Examine the capacitor. Most likely it will be a multi capacitor can. The capacitance and maximum voltage of each capacitor is marked on the side.
If some of the capacitors are rated at a lower voltage than the first one you should unsolder the resistor feeding the lower voltage units. Refer to the Figure above for the circuit of a typical power supply. Do not worry about capacitors that are not tied into the power supply filter by resistors. Sometimes a cathode bypass capacitor is part of the filter can.
Unsolder the wire which connects from the filament or cathode of the rectifier tube to the first filter capacitor. This is the dashed line in the figure above. Temporarily solder a 100 k ohm 1 watt resistor in place of the wire as shown.
Connect a voltmeter across the capacitor and turn on the power to the equipment. The voltage will come up very slowly.
When it gets to about half of the maximum capacitor voltage turn off the power and discharge ALL capacitors that are connected to the power supply.
Replace the 100 k ohm resistor with a 10 k ohm 2 watt resistor. Turn the power back on.
Measure the DC voltage across the 10 k ohm resistor. If it exceeds 150 volts turn off the power and substitute a 22 k ohm 2 watt resistor.
Continue to monitor the voltage across the 22 k ohm resistor and when it gets down to 100 volts or less, change back to the 10 k ohm 2 watt resistor.
Continue to monitor the voltage across the 10 k ohm resistor and when it gets below about 5 volts turn off the power, discharge the capacitors, and remove the resistor and solder the wire back the way it was.
Now turn your attention to the remaining capacitors which you have not unsoldered from the circuit. Their voltage should equal the voltage across the first capacitor within a few volts. Leave the power on for several hours to see if the voltage is coming up.
Test the resistor going to this capacitor to make sure it isn't open. 10% or even 20% above coded value doesn't count as open. You may need to connect a resistor in parallel with the one which is there to help the voltage come up. After these are up, unsolder any resistors you soldered into the circuit.
Once again turn off the power and discharge the capacitors. If you unsoldered some capacitors earlier resolder them.
Let's say the first three capacitors in the power supply are rated at 450 volts and the fourth one is a 250 volt unit. This is not uncommon. When all the tubes are plugged in their load current will keep the voltage across the last capacitor from exceeding 250 volts. Remember you have unplugged all the tubes.
Connect a voltmeter across this last capacitor and watch it while you turn on the power. If the voltage gets close to 250 volts quickly turn off the power. If it goes up to say 50 volts then you need to wait for this capacitor to reform. As before you may need to connect a resistor in parallel with the one which is there to speed things up a little. Don't forget to turn off the power and discharge the capacitors before soldering in another resistor. DO NOT LEAVE THE FORMING PROCESS UNATTENDED.
When the voltage reaches the rated voltage of this capacitor turn off the power, discharge all capacitors, and disconnect any resistors you may have temporarily installed.
Replace all tubes in their sockets. You did make a drawing showing where each tube goes didn't you?
glip22":2xh9iep8 said:
Howdy !! How are you ? Haha... thanks but a variac only fetches to certain voltage... i dont know now but am hitting at ARS electronics to sell me those rated caps i need new so i wont have a fear of the caps fail and the need to reform like these NOS sprague and mallory i got...