Which terminal of this old FROLYT electrolytic capacitor is which? - Page 1
Forum section: electronics / projects

author: Whales
author_rank: Super Contributor
timestamp: December 25, 2015, 11:15:27 pm
content: I've been trying to repair one of my father's old tube radios. One of its problems is a 50hz hum, which I would presume is caused by the main filter capacitor:I don't have a clue which end is negative and which is positive. There are the numbers '1' and '2' on the bottom terminals of the cap and the (German?) writing translates to something approximately like 'negative pole is the case'.I'm having difficulty identifying the polarity from the circuit: in the ratsnest I think it uses valve rectifiers. For all I know this may be a double electrolytic/non polarised capCan anyone point me in the right direction?

author: mikerj
author_rank: Super Contributor
timestamp: December 25, 2015, 11:24:20 pm
content: The way the capacitance is stated gives the answer (50+50uF); it contains two 50uF caps with a common negative connection to the case.

author: wraper
author_rank: Unknown
timestamp: December 25, 2015, 11:25:35 pm
content: This is dual cap. Both terminals in the center are "+" for each of them, case is common "-" for both of them.

author: Whales
author_rank: Super Contributor
timestamp: December 25, 2015, 11:32:55 pm
content: Thanks wraper and mikerj. This explains why they went to the effort of a large spring washer for mounting it to the chassis: the case needed to be connected.

author: MickM
author_rank: Regular Contributor
timestamp: December 25, 2015, 11:56:37 pm
content: If you want to keep the same look, no big open holes, you can "re-stuff" that can.Open it up and put two new caps in there.In Bob's video the cap is not the same style as yours, but it might be close.I subscribe to his channel, mostly 1930+ radios and 40s-50s TV sets.Mick M

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2015, 08:21:17 am
content: Replace with 2 47uF 450V electrolytics, which probably will be 105c versions as that is the most common, and they will be radial. You probably will be able to fit them under the chassis, and because they are low ESR you want to connect a 10R 1W resistor in series with the positive leads, so that you do not blow up the valve rectifier from the current pulses, as the old ones would have an ESR in that range in any case.

author: wraper
author_rank: Unknown
timestamp: December 26, 2015, 02:22:53 pm
content: Quote from: SeanB on December 26, 2015, 08:21:17 amDoubt this is necessary, Most likely there are 100+ ohm series resistors anyway if valve rectifiers are used.

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2015, 03:08:44 pm
content: 10R resistors if direct from the rectifier, just to take off that sharp current spike which heats up the anodes so much. Second stage not really needed, as there will be either a choke or a dropper resistor there. Kind of belts and braces only.

author: wraper
author_rank: Unknown
timestamp: December 26, 2015, 03:28:13 pm
content: Quote from: SeanB on December 26, 2015, 03:08:44 pmI mean there already should be a resistor between rectifier and cap. If there is not, circuit is crap.

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2015, 03:58:42 pm
content: Many did rely on the resistance of the transformer secondary and the ESR of the capacitor to limit anode current. Replace the rectifier with a silicon diode and you need the resistors, otherwise you risk blowing up capacitors and the transformer from those new higher peaks.

author: Whales
author_rank: Super Contributor
timestamp: December 28, 2015, 10:10:47 am
content: I can't find any resistors, but I will add a couple of 10R when replacing the caps. I've heard about this before and was surprised not to find any.