Bush AC34 - "That Cap"

[gary_crutchley]

Hi all,

I have a Bush AC34 that works very well indeed. I have had it for two years and cleaned it thoroughly, used it regularly, but it seems to be in original condition. At some point I might consider a complete re-cap, but for now what I would like to do, having read the advice on this forum, is replace "that cap".

Can someone identify this capacitor for me please, and then suggest an appropriate replacement? I would also like to know where I can buy a replacement please?

Thanks in advance.

Gary

[AC/HL]

Manufacturer's data here: http://www.service-data.com/product.php/574/904/m10574
You need to replace C33, a .01uF 350v capacitor, nowadays called 10nF.

[petervk2mlg]

Hi Gary
Do you have the Trader sheet for the model AC34?
If you do the capacitor in question is C26. This is a 0.01uF capacitor. It's usual to replace with a 400V DC rated one.
Peter

[petervk2mlg]

Screen shot of the relevant part of the circuit

[gary_crutchley]

Originally Posted by petervk2mlg

Do you have the Trader sheet for the model AC34?

Hi Pete,

I do have the service sheet. Is it definitely C26? Someone above suggested C33?

Regards,

Gary

[gary_crutchley]

Would this be a suitable replacement? The voltage rating is higher, but I assume that doesn't matter?

http://www.ebay.co.uk/itm/10x-Capaci...item2ecc6db06c

[Herald1360]

Originally Posted by gary_crutchley

I do have the service sheet. Is it definitely C26? Someone above suggested C33?

C26 is Trader service sheet reference, manufacturer's reference is C33 for the same part. Not an unusual state of affairs- presumably The Trader may have reverse engineered sets where manufacturer's data was restricted to agents.

[paulsherwin]

Originally Posted by gary_crutchley

Would this be a suitable replacement? The voltage rating is higher, but I assume that doesn't matter?

http://www.ebay.co.uk/itm/10x-Capaci...item2ecc6db06c

That will be fine. Any modern cap with an adequate voltage rating will be OK, so you should just buy on price and cosmetic appearance.

[David G4EBT]

Originally Posted by gary_crutchley

Would this be a suitable replacement? The voltage rating is higher, but I assume that doesn't matter?

http://www.ebay.co.uk/itm/10x-Capaci...item2ecc6db06c

They're fine, and are widely used by restorers.

To clear up confusion about part numbering, Trader Sheet component numbering always differs from Manufacturers' data sheets.

On the Trader Sheet (No 1147) for the AC34, the audio coupling capacitor ('that cap') is C26, whereas on the Bush service data it's numbered C33. It's almost always preferable to use the maker's own data if it's available rather than the Trader Sheet, and from my experience this is particularly so in the case of Bush and Murphy sets. The Bush data on the AC34 runs to ten pages - the Trader Sheet just two. Both are available from the top RH. (Same goes for sets such as the DAC90A - Bush Data is eight pages - Trader Sheet is two).

The Bush Service data has a clear separate diagram of the coil switching, which hopefully you'll never have to refer to!

I think the AC34 is one of the nicer woodies - nice dial, and a nicely laid out underside of the chassis, with components easy to get at and not tightly packed together.

Good sound too, from the largish elliptical speaker.

Hope that helps Gary.

[petervk2mlg]

Hi. Sorry for being the cause of some confusion.
I crossed with Bill and by the time I refreshed my screen it was too late!
I wasn't aware of there being two sources of information.
Peter

[gary_crutchley]

Originally Posted by David G4EBT

To clear up confusion about part numbering, Trader Sheet component numbering always differs from Manufacturers' data sheets.

Hi David,

Thanks for the information, it really does help. It's the trader service sheet that I have, number 1147. I've just downloaded the manufacturer's data from the above link, I have this for my DAC90A and it is better as you say. What I like about it is that there are nice pictures of the component layout. Easier for people like me!

I am a bit of a fan of the AC34 actually, I use mine all of the time. It does have a good sound, especially at night when there isn't much interference. This particular example is in really good condition, the case looks great. The only thing I had to change were two dead scale lamps. I managed to find a box of original Philips MES bulbs on eBay, and so far I’m still on the original replacements.

I’ve attached a photo of the underside of the chassis, I presume the cap that I have indicated with the red arrow is the correct one?

Thanks again for your help.

Best regards,

Gary

[gary_crutchley]

Originally Posted by petervk2mlg

Hi. Sorry for being the cause of some confusion.

No problem Peter. It wasn't you that was confused, it was me! I just wanted to be sure, and of course you were both correct. Thanks for your help.

Gary

[gary_crutchley]

By the way, I presume the replacement axials I indicated above are non polar?

['LIVEWIRE?']

All capacitors of that type are non-polarized as ideed are most, if not all other than Electrolytic Capacitors & Tantalum bead types, most of which are produced in values of 1uf and upwards.

[David G4EBT]

Originally Posted by gary_crutchley

I’ve attached a photo of the underside of the chassis, I presume the cap that I have indicated with the red arrow is the correct one?

Yes Gary, that's the one. (It's non-polarised - either way round is fine).

(Don't try to unsolder the connections of the old one - just snip the wires off close to the turrets and wrap the wires of the new cap one turn around each turret and solder them).

Depends how you feel about the other waxy paper caps - 'routinely replace or not?' is a topic that tends to generate more heat than light.

They're not as crucial as 'that cap' if they're leaky in the electrical sense (that is they pass some DC as well as AC and may put DC where you don't want it), but by no means all waxy caps are leaky. However, given that the set is now over 60 years old, is used daily, and you intend to keep it, it might be worthwhile replacing the other waxies - there are several other waxy caps which are .01uF like the ones that you've ordered, which you might want to replace. Not all will have had a hard life - though they're rated at 350V, several will have had negligible voltage across them - C1 for example, on V1 cathode, where the voltage is 1.2V at 6ma. Just because a cap says 350V on it doesn't mean that's the voltage in each instance in a set's circuit. (The only way to properly test them for leakage is at their rated voltage, but it's pointless to test a cap at 350V that only has 1V across it in its daily life).

Another candidate worth considering to be pensioned off is C30 - 50uF 12V situated to the left of the volume control (VR1/S1), the V3 cathode decoupling capacitor. Really, the correct thing to do would be to check its ESR and capacitance, but if you don't have the test gear to do that, it's highly likely that it will have a high ESR and won't read 50 uF. Simple enough to change, and only a few pence. Admittedly a replacement won't 'look the part' as it will be very small in comparison. It's more likely that a modern equivalent will be 47 uF rather than 50uF, which is fine. As long as the voltage is 12V or higher, that's OK - eg, 25V or 50V.

Much depends on how far you want to go with the set.

If you just wish to take the precautionary measure of replacing 'that cap', then just do that and put the back on the set and continue to enjoy it. But given that most components are easy to get at, if you want to go a step further, you may wish to consider replacing those caps on the parts list marked 'PT' (paper tubular), and C30, the small electrolytic, and check the resistors to see if they've gone high. I usually leave any in place that are within 20% tolerance.

If you do decide to replace any other components, it's always a good idea to 'do a bit and test a bit' - that is to change parts one or two at a time, then turn the set on to make sure you haven't made an error. In my view, it's a set worth spending a little time and effort on - a much nicer set than the utilitarian DAC90A for example, with a higher component count, a tone control, AC only, and with a gram socket and external aerial socket.

Enjoy the set!

Just a final point - it's well worth considering buying Paul's DVD - only 12 downloads and it's paid for itself.

[paulsherwin]

It's very likely that all wax caps will be leaky by now in a domestic grade appliance like this, as they gradually absorb moisture over time. Some people including me do change them all as a matter of course, but it's true that slight DC leakage will only have significant effects in certain circuit positions, and conservative restorers like to replace as few components as possible to achieve a good result. It's just a matter of personal preference.

[Nickthedentist]

If you do change multiple caps, check the set still works after each one is replaced.

[gary_crutchley]

Hi David et al,

Thank you very much for taking the time to provide such great advice, and for confirming that I have the right component, I appreciate it.

Obviously I will follow the advice given here and take it one step at a time. I'll change "that cap" first and see how it goes for a while before tackling the others. It goes without saying that I'll be back for more advice shortly.

David, I agree, for me the AC34 is well worth the effort to keep it running, and in any case that's all part of the fun of owning these old sets. I also have a DAC90A, and whilst I really do like it, the set certainly doesn't match the sound quality of the AC34. Of all the sets I have the Bush AC34 has the best sound, only my Cossor Melody Master 523A comes close.

So I will change "that cap" first, then slowly work my way through David's list.

Thanks to you all.

Regards,

Gary

[Ian - G4JQT]

Originally Posted by gary_crutchley

By the way, I presume the replacement axials I indicated above are non polar?

Here's something to bear in mind regarding 'polarity'. Indeed these are not polarised capacitors, but you'll notice that many wax paper capacitors have a black band at one end. This can be seen on some of the wax-paper capacitors shown in post 11.

This indicates which capacitor lead is connected to the outer-most foil layer. Very often this is inconsequential, but occasionally this side should be at the earthy end - either DC or RF earths - to aid stability as it acts as a screen.

Most modern axial capacitors don't show this (probably because their construction means it's irrelevant) although the yellow Vishay polyester film types do have a faint line at one end that I'm told does indicate the outer foil.

Ian

[gary_crutchley]

Interesting, thanks Ian. It looks like the ones I ordered (see post above for link) don't have a line.

Gary