Bipolar electrolytic in CRT

[McBryce]

Hi all,
      I'm in the process of recapping an old CRT from a Bally Gamemaker Arcade machine. It's never been recapped and the picture has started to drift with temperature, so a recap is required. One of the capacitors is a bipolar electrolytic cap. The value on the schematic is 4µ7/50V (Marked in Schematic below) , but the part on the board is a 5µ6/50V (which I don't have at the moment). As I'm not really a CRT expert and the capacitor in question feeds directly into the flyback transformer, I'd like to know how critical this value really is. Can I use a 4µ7 or even a 6µ8 (of which I have many) instead, or is the part somehow part of a tuned circuit and must be exact?

McBryce.

[vk6zgo]

If you could quote the component number it would be helpful.

I remember replacing a large bipolar electrolytic in the "Boost HT" circuit of a monitor once, with a much smaller modern "equivalent".

I marvelled at how technology had reduced the size, until it started to "steam".
It turns out the old one was only large because it had a much lower ESR.

The old one was dead, & none were available locally, so I made a "Christmas Tree" of "Greencap" polyesters.
Not pretty, but it worked for years!

If the 5u6  has the right characteristics, it was probably a "near enough" replacement from years ago.
Just make sure that it isn't a "special type", like mine was.
If not, you can probably use the 4u7/50 you already have.

[McBryce]

There's no other markings on the capacitor other than 5.6µf 50V and the schematic above doesn't give any further information either.

McBryce.

[amyk]

An electrolytic would not have been used if its value was critical to operation. The typical +/-20% tolerance means that a 4u7 at the high end of its range may actually be closer to 5u6. A 4u7 should be fine, although after replacement you would probably need to readjust the trimpots anyway.

[McBryce]

That was my thought too. I've managed to find a 5.6µf bipolar on ebay now anyway and have ordered it, just in case the trimpot doesn't have the range.

McBryce.

[floobydust]

The part likely experiences high ripple currents which is why there are some oddities in the circuit, such as it being bi-polar and a bit larger in value. But I have not seen how big L1 is.

Small electrolytic capacitors have a big variation, over 2:1 20:1 in their ripple current rating (between manufacturers). It can be from 25-500mA at high frequency. So the part you select check that spec. or it will have a short lifetime. BP caps are higher ripple-current rated like Nichicon UDB series.

I'd just use a film capacitor to replace it, like Kemet R82, Wima MKS2. 4.7uF 63V.

edit: you can add a 1uF in parallel to get 5.7uF if the value is critical.
Tolerance on electrolytics is ±20%, so a 5.6uF could be 4.5uF min.

[james_s]

Yes the ripple current is going to be a factor here, quite a few CRT monitors and TVs have that arrangement with a bipolar cap in the horizontal circuit. Typically the value is not super critical, as was already mentioned electrolytic capacitors have rather wide tolerances.

It's common to find capacitors in old arcade monitors that don't match what is on the schematic. It is a combination of manufacturing variations and techs over the years replacing parts with whatever they had on hand. During the heyday of the arcade golden age demand was huge and manufactures were pumping out as many monitors as they could to meet demand. In the field, there was a lot of pressure to fix broken games quickly to get them back to earning, an out of order game doesn't bring in any quarters. You see a lot of hacks done just to get things up and running as quickly as possible.

Also worth noting, a lot of the "cap kits" on the market use really cheap parts. Instead of buying the kits I usually assemble my own using quality parts like Nichicon, Rubycon or Panasonic.

[madires]

Reichelt: JAM NKR4R7M2AF11 (4.7µF, 100V, bipolar, Jamicon's NK series)

[floobydust]

Jamicon NK series 4.7uF 50V BP is only 44mA, 100V is 50mA ripple current at 120Hz, that's not enough.
Realistically, the part does not need to be bi-polar due to diode D8.

[Bashstreet]

Yeah there is not really much trouble replacing the cap with ether 4.7µF or 6.8µF  as tolerances in electrolytic are not that tight any case  (+-20 like other contributors have pointed out) Both would likely be on trim pot range at least one of them.

Any case might as well use the one you ordered although i rather use a good quality cap as eBay is full of fakes so maybe keep that in mind next time.
I am not expert on ripple effects but as some contributors have said if a capacitor is "victim" to high ripple it's lifetime will be shorter from the heat it gets exposed to.

It might be sensible consider a poly but we are getting to the area of too much fuzz over nothing.

Replace it with good quality electrolytic with suitable specks and it will most likely last next 30 years even a wan hung lo one most likely 20.

[james_s]

I can't speak for other monitors, but last year I worked on a 27" CRT in an arcade game that someone had worked on previously. They replaced a bipolar electrolytic in that same circuit location with a new part of the same value and rated voltage but it was physically much smaller. When I looked at it the new capacitor was bad already and it was getting so hot that the wrapper had turned from blue to dark brown.

[David Hess]

The capacitance there is not critical; it provides a low high frequency AC impedance to ground.

Use a higher voltage part to get an increase in ripple current rating; try to aim for the same physical size or larger.  Maybe also use 2 bipolar capacitors of twice the capacitance each in series to double the ripple current rating if necessary.

[McBryce]

The part I ordered is NOS, so it's unlikely a fake and also not smaller than the original part. It's also rated for 63V whereas the original is 50V. If it fails in a few months that also isn't really an issue as the machine is my own and I can just swap it again to something else (no peeved customer asking why the repair didn't last).

McBryce.

[PKTKS]

Just for the record... 

i have just overlooked the output stage....

It seems that the cap is just part of the damping H pulse

An old trick of old schools discrete TV techs was to
replace those "bipolar" caps with 2 normal ELCOS
placed in series WITH PROPER POLARITY
(opposite each other)  and of course
 with proper resulting final value

you will hardly find those nowadays
I do have some old ones but I doubt I would use them
I would rather use the trick above with 2 brand new ones

Paul

[McBryce]

Yes, I've considered that solution too. The seller of the NOS caps I've ordered has stated that they have been tested to be good and well within the original specs, but if they end up failing I'll probably use the back-to-back solution.

McBryce.

[PKTKS]

I have been seeing lately a new brand of ceramic CAPs
of high capacitance and very good voltage ratings.

They are made from modern material fabs and
considering the relative low value in this case...

I would stress the search trying to find a ceramic
CAP suited for this replacement.

If I got the picture right the cap is part of the damping
pulse and should not be a problem which type it is.

Mostly the spike surge - which ceramic are remarkably good at  -
and the proper value - once it will change the damping RC waveform

I would try that first before the obsolete bipolar ELCOs

Paul

[Twoflower]

You can still order bipolar caps for cheap. Usually they're used in speakers for the low/high pass circuit. A German shop starting with R has radial, 4.7µF, 100V bipolar cap for blasting 0.11€.

[andy2000]

Has anyone else noticed that the cap in question is shown as polarized in the schematic?  Maybe someone replaced it with a bipolar cap, or it was all the manufacturer had on hand.

Either way, I wouldn't go with the cheapest cap I could find, and new old stock isn't ideal for an electrolytic.  Since it's only 4.7uF, a film cap might be the best replacement. 

[McBryce]

Has anyone else noticed that the cap in question is shown as polarized in the schematic?  Maybe someone replaced it with a bipolar cap, or it was all the manufacturer had on hand.

Either way, I wouldn't go with the cheapest cap I could find, and new old stock isn't ideal for an electrolytic.  Since it's only 4.7uF, a film cap might be the best replacement.

Yes, the symbol is wrong, but take note of the "BP" mentioned in the value.

@Twoflower: I did see those caps from a German shop that ends in eichelt But I only order from them when I need lots of stuff. Their postage prices make a single capacitor extremely expensive (more than I paid for 3x NOS caps including postage).

McBryce.

[Twoflower]

Either way, I wouldn't go with the cheapest cap I could find, and new old stock isn't ideal for an electrolytic.  Since it's only 4.7uF, a film cap might be the best replacement.

#

What do you think would a decent cap cost? It is just an electrolytic cap that has a special design feature. I haven't looked into the schematic, but using a film-cap might introduce a far to low ESR and cause trouble as well.

A NOS electrolytic cap with more than 1000h shelf-time might not necessarily better than the one that is in there. If the sealing isn't perfect the NOS could be dried out. And the cap might need to be re-formed as well (not sure how this is done with Bipolar caps). I'ts probably the best to test the NOS cap close to nominal voltage with at 1kOhm series resistor to check if the oxide layer is intact. Do this in both directions as the bipolar caps has two oxide layers. Probably measure the current while doing this to see if they have a too high leak current.

[David Hess]

Has anyone else noticed that the cap in question is shown as polarized in the schematic?  Maybe someone replaced it with a bipolar cap, or it was all the manufacturer had on hand.

Yes, the symbol is wrong, but take note of the "BP" mentioned in the value.

@Twoflower: I did see

I noticed the same thing but the BP notation is more significant than the symbol.

[McBryce]

A NOS electrolytic cap with more than 1000h shelf-time might not necessarily better than the one that is in there. If the sealing isn't perfect the NOS could be dried out. And the cap might need to be re-formed as well (not sure how this is done with Bipolar caps). I'ts probably the best to test the NOS cap close to nominal voltage with at 1kOhm series resistor to check if the oxide layer is intact. Do this in both directions as the bipolar caps has two oxide layers. Probably measure the current while doing this to see if they have a too high leak current.

That's a good plan. Will test it like that before I install them. The seller told me they were tested good for value and ESR, but didn't mention what method they used.

McBryce.

[Bashstreet]

A NOS electrolytic cap with more than 1000h shelf-time might not necessarily better than the one that is in there. If the sealing isn't perfect the NOS could be dried out. And the cap might need to be re-formed as well (not sure how this is done with Bipolar caps). I'ts probably the best to test the NOS cap close to nominal voltage with at 1kOhm series resistor to check if the oxide layer is intact. Do this in both directions as the bipolar caps has two oxide layers. Probably measure the current while doing this to see if they have a too high leak current.

That's a good plan. Will test it like that before I install them. The seller told me they were tested good for value and ESR, but didn't mention what method they used.

McBryce.

Well i would not really buy old caps from eBay (unless it would be the only choice.) of course if nothing else has them... well
But i would think there are quite good component provides there in Germany ?

At least in U.K 4.7μF 50V bipolar Panasonic caps are 3.80£ for 25 delivered.

[McBryce]

Well the NOS caps arrived and they are well within spec. Replacing the old part somewhat improved the situation, but I haven't replaced all the electrolytics yet so the issue has not been fully resolved yet.

McBryce.

[Bashstreet]

Well the NOS caps arrived and they are well within spec. Replacing the old part somewhat improved the situation, but I haven't replaced all the electrolytics yet so the issue has not been fully resolved yet.

McBryce.

Right.

Did you find good place to get rest of the caps in Germany with reasonable price ?

[McBryce]

All the other caps are standard electrolytics (4.7µF to 1000µF), nothing exotic, so I'll just order them from my usual supplier (The one that starts with "R")

McBryce.

[Bashstreet]

Right no worries then