German “Ero” capacitors

[Whaam68]

I recently picked up a Harting valve tape recorder from another forum member for restoration. I cannot find any info on the unit But it is date stamped under the chassis as 1959. It contains a handful of German Ero capacitors at strategic points which I haven’t come across before, a paper outer with what looks like a hard resin core. Checking a few of these on an L.cR meter they test moderately high. For example a 33nf cap tests as 48.7 at 7.8 kHz.....and 54nf at 1khz (the tester only has 3 frequencies). Similarly their readings are a bit all over the place. As the deck has clearly been out of commission for a very long time my instinct is to Replace these as well as any obviously out of tolerance electrolytics before applying power through a lamp limiter. would a poly film cap be a suitable replacemen for these? (of the same rating). They are marked with a black band At one end but I’m assuming that denotes which end is connected to the foil rather than polarisation? Any wisdom On these would be appreciated.
Thanks
Mike

[Herald1360]

With those readings I'd say thet were well leaky!

Have you got megger to try on them?

[paulsherwin]

As a general rule of thumb, any old cap which reads significantly high on test is leaky. The DC path confuses the LCR analyser.

[Whaam68]

Thanks guys. No Megger but ...I do have an older bridge that tests leakage at working voltage....will have to dig it out of the cupboard and test. Can I substitute film caps for these?
Thanks
Mike

[Craig Sawyers]

ERO (Roderstein) existed as an independent capacitor maker from 1925 until Vishay bought it in the early 90's.

[vidjoman]

I’ve found most of these of that age are bad. They often look dry and faded. No point just testing them it’s easier to just replace them.

[Whaam68]

Ok thanks....to be fair physically they look ok.
Will replace using any old film....e.g. polyester/polyprop?
Mike

[DMcMahon]

Personally I would say Polypropylene metal film would be better, say 630 volts rating.

[Whaam68]

Thanks
Will search the stash & see what I have.

[Radio Wrangler]

Water has a very high dielectric constant, so once some moisture gets into a paper dielectric, the capacitance really does go high. Impure water is fairly conductive, so here comes leakage.

On top of that, some capacitance meters read quite high if there is leakage irrespective of any increase in C

David

[Lucien Nunes]

Polypropylene capacitors are superior to polyester in several respects, but in many positions in a circuit they would offer little or no advantage. Since they are larger and more expensive than polyester, I generally use the latter for non-critical applications.

Yes, the black band normally denotes the outer foil.

[Radio Wrangler]

The main thing to beware of with polyester capacitors is that they are prone to 'Dielectric Absorption'

If you use them in things like integrators and analogue to digital converters (particularly the integrator in a DVM) they will drive you up the wall. Discharge them completely fairly quickly, then remove the shunt and watch them charge themselves back up!

They can cause the oddest low frequency instabilities in feedback loops.

For most audio and radio purposes they're rarely a problem, but be careful in the feedback filter of phase locked loops.

If you've never run into this before, the first time comes as a bit of a surprise. Impossible things seem to be happening.

David

[Craig Sawyers]

If you search Farnell for Vishay polypropylene axial film caps you get

https://uk.farnell.com/w/c/passive-c...s=axial-leaded

Which are all axial lead Vishay Roederstein. If you want the restoration to look original, you can probably scoop out (or drill/dremel) the insides of the old ones and hide a new one inside.

Craig

[DMcMahon]

Originally Posted by Whaam68

I recently picked up a Harting valve tape recorder from another forum member for restoration. I cannot find any info on the unit But it is date stamped under the chassis as 1959. It contains a handful of German Ero capacitors at strategic points which I haven’t come across before, a paper outer with what looks like a hard resin core. Checking a few of these on an L.cR meter they test moderately high. For example a 33nf cap tests as 48.7 at 7.8 kHz.....and 54nf at 1khz (the tester only has 3 frequencies). Similarly their readings are a bit all over the place. As the deck has clearly been out of commission for a very long time my instinct is to Replace these as well as any obviously out of tolerance electrolytics before applying power through a lamp limiter. would a poly film cap be a suitable replacemen for these? (of the same rating). They are marked with a black band At one end but I’m assuming that denotes which end is connected to the foil rather than polarisation? Any wisdom On these would be appreciated.
Thanks
Mike

Have seen quite a lot of Ero paper capacitors in some of my very old German tape recorders. Many of the ones I have tested are very leaky/passing a lot of DC voltage and typically measure x 2, x 3 or more their stated capacitance value.

[Whaam68]

Thanks all
I’m not really bothered about the internal look...heretical I know. The parts stash has turned up some polyester (I think) 630 & 400v caps in the values I need. Examples attached. The nice looking Vishay polyprops linked above are quite pricey...I’m tempted to use what I have to get the thing going. I don’t think the foil is marked on these...I’m assuming that’s not going to cause an issue?
Thanks
Mike

[Radio Wrangler]

Probably mylar or polyester.

Mylar is a brand name for PET (poly ether tetra-phthalate like pop bottles) which has been physically stressed.

THe manufacturer of your set would have been perfectly happy to have used these.

David

[Maarten]

Polypropylene in an old radio is typically only necessary for snubber capacitors (boucherot circuit), capacitors across rectifiers or across inductions. Of course capacitors across the mains or from mains to ground need to be X2 or Y2 rated respectively, which may be polypropylene, polyester or ceramic.

[Herald1360]

Originally Posted by Radio Wrangler

The main thing to beware of with polyester capacitors is that they are prone to 'Dielectric Absorption'

If you use them in things like integrators and analogue to digital converters (particularly the integrator in a DVM) they will drive you up the wall. Discharge them completely fairly quickly, then remove the shunt and watch them charge themselves back up!

They can cause the oddest low frequency instabilities in feedback loops.

For most audio and radio purposes they're rarely a problem, but be careful in the feedback filter of phase locked loops.

If you've never run into this before, the first time comes as a bit of a surprise. Impossible things seem to be happening.

David

You can think of them as having an equivalent circuit consisting of a main capacitor in parallel with several other CR series units of low capacitance and high resistance.