Geo-TH,In
Well-known Member
Centex Farmal,
I thought about coming up with an inexpensive way for you to check leaky capacitors. Most caps used in old tube TV's and radios were leaky by design. However if too leaky they needed replaced. If I recall most big leaky caps were used half wave power supplies, around 155v dc. If capacitor leaked, the PS voltage wouldn't come up to the needed plate voltage necessary for the tube to work properly. I think some later models may have used a voltage doubler PS, for around 300vdc. Please don't hold me to it, my memory is old and it was a life time ago.
The start caps used on electric motors were also inheritanly leaky and they are used for just a few seconds on AC. You don't want to use do the test I posted yesterday on caps used in old TV and radios.
So, I came up with a very simple circuit using a ma meter, voltmeter, 2500 ohm resistor, a diode and a power cord. Cost me nothing, I'm a pack rat. Just had to locate the parts.
I ran a test on a 35 mfd run capacitor using the circuit I posted today. In theory it should take .43 seconds to reach 99+% of full charge, 5 time constants which it did. After full charge, the voltmeter would add a 1.5 ma drain. So if I removed the voltmeter, the 35 mic cap would remain charged for a long time. Electronics 101, a time constant is equal to the resistance times the capacitance, which is in farads not microfarads.
Using the 190-220 mfd cap, a start cap, it took forever for the charge to get to 80v. In theory a 200 mfd cap should only take 2.5 seconds to get fully charged, 161v dc. My start cap never get much past 80v and self discharge rapidly.
So you could play around with good capacitors and bad ones. Get a feel for their charge and discharge time using different resistors and different voltmeters. If you use the old school analog voltmeters, which are only 20k ohms/volt at best, the capacitor will discharge much faster than the digital meters. And you could also measure the power supply voltages inside a good radio and a bad one to get a feel for what the PS voltage is.
BTW, I'm guessing most of your defective caps are in the PS. Try it and post back.
If anyone has a better and inexpensive way to test for leaky caps please post your method.
Geo
I thought about coming up with an inexpensive way for you to check leaky capacitors. Most caps used in old tube TV's and radios were leaky by design. However if too leaky they needed replaced. If I recall most big leaky caps were used half wave power supplies, around 155v dc. If capacitor leaked, the PS voltage wouldn't come up to the needed plate voltage necessary for the tube to work properly. I think some later models may have used a voltage doubler PS, for around 300vdc. Please don't hold me to it, my memory is old and it was a life time ago.
The start caps used on electric motors were also inheritanly leaky and they are used for just a few seconds on AC. You don't want to use do the test I posted yesterday on caps used in old TV and radios.
So, I came up with a very simple circuit using a ma meter, voltmeter, 2500 ohm resistor, a diode and a power cord. Cost me nothing, I'm a pack rat. Just had to locate the parts.
I ran a test on a 35 mfd run capacitor using the circuit I posted today. In theory it should take .43 seconds to reach 99+% of full charge, 5 time constants which it did. After full charge, the voltmeter would add a 1.5 ma drain. So if I removed the voltmeter, the 35 mic cap would remain charged for a long time. Electronics 101, a time constant is equal to the resistance times the capacitance, which is in farads not microfarads.
Using the 190-220 mfd cap, a start cap, it took forever for the charge to get to 80v. In theory a 200 mfd cap should only take 2.5 seconds to get fully charged, 161v dc. My start cap never get much past 80v and self discharge rapidly.
So you could play around with good capacitors and bad ones. Get a feel for their charge and discharge time using different resistors and different voltmeters. If you use the old school analog voltmeters, which are only 20k ohms/volt at best, the capacitor will discharge much faster than the digital meters. And you could also measure the power supply voltages inside a good radio and a bad one to get a feel for what the PS voltage is.
BTW, I'm guessing most of your defective caps are in the PS. Try it and post back.
If anyone has a better and inexpensive way to test for leaky caps please post your method.
Geo
