Advice on the condenser bank in a Murphy A30 please.

[MurphyNut]

I’m currently restoring my first pre-war radio a rather nice Murphy A30 very kindly given to me.
At this time Murphy used a “condenser bank” to house the electrolytic capacitors.
This has been leaking wax quite badly so I’m going to re-cap it. I’ve got the circuit diagram showing how it’s wired up and underneath I’ve drawn how I think it should go.
I’m just a little confused with the red wire with regards to the polarity of C28 and C29, have I got it right?
I hope someone can help point me in the right direction if I haven't, thanks.

[Leon Crampin]

Clive, you have the polarity of C29 reversed in your diagram.

The positive terminals of C28 and C29 are connected together, whilst the negative end of C29 terminates in a blue wire, which is connected to the HT winding centre tap and one side of the 'speaker field. It's point 45 on the circuit. All the other capacitors have their negative terminals connected to chassis (10).

These are really good radios for their component count. If you have trouble aligning the band pass aerial coils, look for degraded rubber insulation inside the braiding screen to the AC/TP grid cap. It took me a while...

Leon.

[ms660]

Originally Posted by MurphyNut

I’m just a little confused with the red wire with regards to the polarity of C28 and C29, have I got it right?
I hope someone can help point me in the right direction if I haven't, thanks.

From what I can figure out the polarity that you've shown for C29 should be reversed, ie: C28 +ve and C29 +ve should be connected together, the -ve end of C29 should connect to the filter choke and the center tap of the mains transformer secondary.

Whoops... post crossed.

Lawrence.

[turretslug]

It would make more sense for C29 to be the other way round, i.e. with positive towards red and negative towards blue. This is taking red as HT positive, black as chassis negative and blue as transformer HT secondary centre tap- it wasn't unusual, particularly with pre-war practice, to derive negative bias point(s) by passing return current through resistor(s) or loudspeaker field coil/smoothing choke connected between chassis and HT negative. C29 thus would be "reservoir" and C28 main "smoother", in conventional terminology.

Are you able to post any more of the PSU circuit- it would likely help to settle the question,

Colin.

Edit: Crossed with the others while I dithered!

[MurphyNut]

Many thanks for the remarkably fast replies literally pointing me in the right direction, looking forward to getting this one up and running.

[Edward Huggins]

Seeing the "Cap-Bank" her, I've always been intrigued by the low value of smoothing/reservioir caps in pre-war sets at around 8 + 8uf, sometimes 8 + 16uf. Post war, 16 + 32uf or 32 + 32uf, became the norm and with 50 + 50uf in Hi-Fi. Why were such low values used, was this partly due to the use of Choke smoothing and/or Humdinger circuitry? Edward

[electrogram]

Maybe cost and size were an issue, also what would rectifier cathodes stand ?

[ms660]

Yes usually low values of C when a CLC filter was used, in general, increasing C above 16uf would only mean a marginal increase in regulation.

Another limiting factor as has been mentioned is the rectifier specs.

A good read for radio power supplies can be found in RDH4 Chapter 30:

http://www.paleoelectronics.com/RDH4/CHAPTR30.PDF

Lawrence.

If you want to 'restuff' the capacitor bank modern non polarised capacitors are so small these days (the yellow polyester jobs) you will be able to fit enough in to do it. And they will not (for ages and ages) degrade.