Capacitor help

[HoraceBatchelor]

ive done one or two cap replacements that's all, but never a multi change. But iv'e bit the bullet and decided it as to be done. Ive chosen what to me looks like a relative easy job, all caps look to be available to reach and replace without much fuss. But my trouble is, im at a loss with what caps i will need.

The radio ive chosen is a 1947/8 Defiant MSH248. the one radio that doesn't seem to have any info written about it (service sheet etc). Im not a repairer so i don't have a drawer full of bits and pieces to choose from. I haven't a clue as to what the cap values are until i can get them out and be able to read them. and i cant take them out until i have replacements.. So what i'm looking for is someone to tell me the most likely caps i will need. There's maybe around about ten waxies that I will change. For me no little job.

If i have to buy twice as many to get a few of the ten correct, then so be it.

[TrevorG3VLF]

The capacitors should be marked.
You may be able to find a set with the same valves to get an idea of the correct vaues. Most capacitors are not critical except tone control and frequency determiniing capacitors.

[ms660]

According to Google pics the Ultra U506 chassis looks to be the same:

https://www.vintage-radio.net/forum/...d.php?t=113253

https://www.radiomuseum.org/r/ultra_u506u_50.html

https://www.service-data.com/section.php/8365/1/u506

Lawrence.

[paulsherwin]

Many values aren't very critical. You can use 0.1uF for all the decouplers, and usually for the couplers too, though that would be considered bad practice. The tone correction cap across the output transformer needs to be 1kV or better.

If you buy some 0.1uF and 0.01uF caps you will probably be OK.

[David G4EBT]

I've got an MSH 248, which was given to me by a neighbour in 1959 when in my late teens. So long ago when I restored it, that it would probably benefit from a further fettling up! A nice looking little set which performs well on a few metres of wire for an aerial. Yes - it's basically an Ultra 506, but in a wooden rather than Bakelite cabinet. If you download the datasheet, the Ultra 8-page data is a better bet than the 2-page Trader Sheet. The underside of the chassis isn't too cramped, so should be easy to work on.

You'll see that there are seven paper capacitors which would benefit from being change, the most pressing ones being C12 -(maker's data sheet), the audio coupling capacitor, and as Paul points out - the tone corrector capacitor, C13 across the primary of the output transformer.

Four of the caps are .01 uF, (1,000V), two are .05 uF 500V, (use .047 uF nowadays), and one is .005 1,000V (Use .0047uF). I guess you might as well use 1,000V ones throughout. Makes sense to do them one at a time - 'do a bit, test a bit', then you can see step-by-step if you've made an error. Always wise to take some pics before you disturb anything.

Worth checking if the resistors are within say 20% of their stated value and consider changing any which are much above that. Hopefully the smoothing/reservoir caps will reform OK, but if you get excessive hum, you might need to consider replacing/re-stuffing that, but it's early days to jump to conclusions.
.
Bear in mind it's a live chassis set, so make sure you have the mains polarity correct, and take care with any live testing.

Hope that helps a bit.

Good luck with the restoration.

[flyingtech55]

Just in case you weren't aware...remember to fit the capacitors the right way round or you may end up with hum.


HTH


TimR

[Graham G3ZVT]

Originally Posted by flyingtech55

Just in case you weren't aware...remember to fit the capacitors the right way round or you may end up with hum.


HTH


TimR

While that is theoretically true, the outer foil is unlikely to be marked on the OPs new capacitors, and while this can be ascertained in various ways, an old radio is going to have a native amount of hum anyway which will swamp any tiny improvement you are likely to make.

In fact one could argue that putting a cap in the wrong way may very well "buck" some hum created elsewhere.

In short, it's probably not worth the effort in my humble opinion.

[Catkins]

Originally Posted by David G4EBT

Bear in mind it's a live chassis set, so make sure you have the mains polarity correct, and take care with any live testing.

Always best to use a 1:1 isolating transformer here if you have one.

[HoraceBatchelor]

Much obliged for all your advice, its a great help, but there's a "but". I'm having difficulty with buying the caps. I'm looking on ebay and i'm struggling with the drop down boxes you choose whatever you want from. I just can't to seem to fully understand what, and how many i'm buying.

The 0.01uf seem to be straight forward to order, with drop down boxes asking for type, amount, and the price. But the 0.1uf isn't so clear, and the price is so much more expensive making me doubt if i'm reading the order form correctly.The capacitors are of the yellow kind with the 0.01uf coming in packs of ten priced at about four pounds. But the 0.1uf seems to be sold single at almost the same price for one, and it is this difference in price that makes me doubt my purchasing skills.

Am i right in thinking the ceramic type or less expensive, and can they be recommended. All these questions may seem so trivial, but i trust you to put me on the right path. Once again thanks for all the help.

[Graham G3ZVT]

It's more usual these days to specify caps under 1uF in nanofarrads
So your 0.1uF becomes 100nF

https://www.unitconverters.net/elect...-nanofarad.htm

[David G4EBT]

Originally Posted by HoraceBatchelor

Much obliged for all your advice, its a great help, but there's a "but". I'm having difficulty with buying the caps. I'm looking on ebay and i'm struggling with the drop down boxes you choose whatever you want from. I just can't to seem to fully understand what, and how many i'm buying.

The 0.01uf seem to be straight forward to order, with drop down boxes asking for type, amount, and the price. But the 0.1uf isn't so clear, and the price is so much more expensive making me doubt if i'm reading the order form correctly.The capacitors are of the yellow kind with the 0.01uf coming in packs of ten priced at about four pounds. But the 0.1uf seems to be sold single at almost the same price for one, and it is this difference in price that makes me doubt my purchasing skills.

Am i right in thinking the ceramic type or less expensive, and can they be recommended. All these questions may seem so trivial, but i trust you to put me on the right path. Once again thanks for all the help.

As has been said, values are often quoted in nF (nanofarads) rather than uF (microfarads).

To convert uF to nF, just move the decimal point 3 places to the right or vice versa. Thus:

0.1 uF - same as 100nF (100,000pF).
0.01 uF - same as 10nF (10,000pF).
0.001 uF - same as 1nF (1,000pF)
0.047 uF - same as 47nF (47,000pF).
0.0047 uF - same as 4.7nF 4,700pF).

Most restorers, myself included, tend to replace tubular waxy paper capacitors with yellow tubular polypropylene ones. How much you pay for them depends on where you buy them from and how they are described. Expect to pay five to ten times as much for anything which is described as 'audio' or 'guitar quality'. Some people only use major companies such as RS, CPC/Farnell, Rapid Electronics, or long established trusted smaller companies such as Cricklewood.

Cricklewood are here:

https://www.ebay.co.uk/itm/Polyester...r=563884556733

I've tended to use Dunkselectricshop and Switch Electronics, both UK suppliers, with many transactions and excellent feedback.

Both supply packs of ten caps in the values that you need for under £5.00:

Dunkselectricshop gets capacitors made to his own spec. You can read the write-up at this link:

https://www.ebay.co.uk/itm/10x-Capac...MAAOSwrP9Z1PaW

Switch electronics are here:

https://www.ebay.co.uk/itm/Axial-Pol...53.m1438.l2649

Almost all suppliers supply 630V ones, (including the BVWS who sell capacitors to members), though your service data specifies 1,000V for some of yours. If you want to stick to 1,000V ones Cricklewood can possibly help, at a price. I think most of us take the view that modern 630V ones are superior to 60 year old 1,000V waxy paper ones.

I've no connection with any of those suppliers except as a satisfied customer. I'm simply trying to answer your request for information.

Forum rules sensibly preclude discussions about the merits or otherwise of ebay suppliers.

Hope that helps.

[crackle]

David didn't mention that you need a 0.1uf capacitor, where does this go in the circuit?
Mike

[David G4EBT]

Originally Posted by crackle

David didn't mention that you need a 0.1uf capacitor, where does this go in the circuit?
Mike

Looking at the Makers data (not the Trader Sheet), it shows only the following paper capacitors:

C1: .005
C9 & C10: .05
C11, 12, 13, 14: all .01.

Seven in total.

Unless I've missed something?

The Trader Sheet (885) shows the same caps, but gives them different numbers. Though the maker's data is on the whole more comprehensive and runs to eight sheets, the Trader sheet has the merit that of describing the role of each component, which is useful, and through reading that I realised that C14 . (makers data), 01uF, which is denoted as C22 on the Trader Sheet, though not immediately obvious, is actually an RF bypass capacitor across the mains, so should rightly be an X class capacitor. (Or just discarded).

I guess 0.1uF X class might be easier to find than .01uF and would I think be fine. EG, these:

https://www.amazon.co.uk/Safety-Supp...TS1D83F8QQW0AZ

[Herald1360]

Just to stir the pot a little...... the HT voltage for the set is only around 250V dc so with the exceptions of the capacitor across the output TX primary and the RF bypass capacitor from rectifier anode to chassis, 400V rated parts will be fine for waxy replacements.

The TX primary capacitor can usefully be 1000V rated but its s/c failure is unlikely to cause any damage and would be very obvious (lack of output). The RF bypass should be an X-Class mains suppression type though a 1000V dc rated polypropylene type would still be better than the wax paper original!

Edit: Some crossover with David's summary but no conflicting ideas. Phew!

[HoraceBatchelor]

Thanks Chaps, really do appreciate your help, and with a special mention to David. Whoops! I had better stop there as it could begin to sound like i have just won a Bafta.