Murphy A4 with A24 chassis.

[PerdioPal]

I did one of those for a Phillips trf set and took the opportunity to fill the space with bigger/more caps.

[Geordie McBoyne]

The capacitor block is in it's metal box and refitted on the chassis. Also, the 4microfarad HT reservoir capacitor as been rebuilt and will be fitted in the chassis when the Araldite has fully hardened. The 8microfarad HT smoothing capacitor is already in position.
The reservoir capacitor must be isolated from the chassis so a SRBP disc will be made for that purpose.

Geordie McBoyne

[Radio Fixer]

The electrolytic looks pretty much sealed up. I have come to re-stuffing mine leaving a vent hole. This is in case the caps ever decide to blow up and avoids pressure build up.

I believe the caps themselves are made with a top that fails if this should happen?

Any views on this?

Gary

[boater sam]

I think that the scored cross on the end of the can is to provide a controlled bursting rather than the old bang and spread of electrolyte everywhere.

[joebog1]

I think Sam has it!!!. He collects 64,000 dollars and passes go.
Check any motherboard from 10 to 20 years ago.
They ALL have pregnant capacitors.

Just my two bobs worth.

Joe

[DrStrangelove]

I have fond memories of a demo to Esteemed Customers (first year students) of what happens to a reverse biased electrolytic.

It was impressive.

Fortunately I'd stuck an empty CD plastic container over the top, because, when it went it cracked the plastic.

It was a small 25V 100uF cap IIRC, on a psu with the current limit turned up.

The X on the end didn't rupture.

The kiddies were impressed.

And so was I.

[ppppenguin]

Back in the 1970s a colleague was looking at a problem in a stabilised PSU. I forget the make. What I won't forget is the mixed dielectric cap that exploded in his face. It was across the mains, before Class X had been invented. Fortunately he wore glasses and no harm was done except to nerves. It took out the glass fuse in the kit, the fuse in the 13A plug , the fuse in the plug that connected the bench to the wall and the 30A MCB in the lab.

But we're wandering a long way off topic.

Closer to topic, when replacing old, low value electrolytics would you use modern plastic film caps? If they will fit, is there a downside?

[Mike Watterson]

I'd check ripple current rating if there is any significant AC current (coupling) or hum to be decoupled.

You can test with a suitable source and load and see does it warm. Likely if no rise it's fine.

[Geordie McBoyne]

The A24 chassis restoration is close to completion.
In the A24 and A24C the three position wavechange switch also actuates the on-off switch. The A24 on-off switch operates in a different manner to the switch in the A24C chassis.
In the A24 the fully clockwise position switches the set off, in the A24C the off position is in the conventional anti-clockwise position.
Other differences between the two chassis are: no audio frequencies filter unit in the A24C and in the A24 the gram jack socket is not present in this chassis. No indication that there was was one fitted.

Geordie McBoyne.

[Geordie McBoyne]

The A24 chassis is working and giving a good account of itself. Excellent sensitivity, especially at the HF end of the medium waveband. Not so good on the long waveband.
Selectivity is excellent as one would expect with a receiver having a low IF, 117Kc/s.
Thanks to the thorough HT smoothing the hum level from the loudspeaker is very low.
The original mains transformer had terminals for mains voltage selection and the loudspeaker field coil. The replacement Elstone transformer had no such items. A new terminal panel has been made, the brackets glued into position using Evostik "Serious Glue" it gives a very strong bond between the brackets and the transformer frame.
The HT rectifier is a Marconi U12 It would seem Murphy preferred to employ M.O. rectifiers instead of Mazda, at least in the early models.
The chassis is now installed in the A24 cabinet.

Geordie McBoyne.