A friend has an early model British car that is at the moment negative earth. He is going to put it back to the original positive earth (goodness knows why) and also wants to change the radio over. Is it just a matter of changing the vibrator to positive earth type?

Hi Airzone,
First time that I have heard of some-one converting back to Positive earth......
Don't know about the Vibrator/radio but your friend will also have to consider repolarizing the Generator (Alternators are only negative earth) and also look carefully at the Tacho, if fitted.
I have 67 MG, converted to negative earth. If you need some 'How To' type articles let me know.
Ian

Unless it is a an electronic type replacement vibrator, the polarity is irrelavant.

The heaters do not care which way the polarity is they still heat & the rectifier sorts out B+

Normally there are no electrolytic caps in the vibrator circuit & I would expect the primary of the aerial to be isolated from any polarised circuitry.

Might be a good idea to check for old paper caps & electrolytics in it.


Marc

I would have thought that some car radios might have "stacked" the vibrator circuit's rectified output atop the +12VDC feed from the car. To get "free" 12V in addition to get higher B+. You'd connect the vibrator transformer's secondary's centertap to the 12VDC line instead of ground to do this. Not a huge increase, but it costs you nothing. With a positive ground system, you would lose this boost (obviously you'd connect the above secondary centertap to ground and not the negative 12V supply!). Or 6V. Though you might be able to use the negative supply voltage to get some negative bias for the output tube's control grid, and have the output tube's cathode tied to ground. Some audio amps do this. Something to think of if your friend converts his car's electrical system to a positive ground.

The Car radio is an AWA Pressmatic

On the case it has writen:- 927F - 12V
8 - 18 M67533
Does anyone have a circuit, then I can see what to do?

Airzone,

I have seen several old car radios with a + / - polarity switch on the underside. Worth checking!

Cheers,
Ian

G'day Air zone, there is a 927 listed on http://www.kevinchant.com/awa1.html, can'tview it on my tablet for some reason to see it resembles what you describ, but worth a look. Cheers Paul

The circuit will open with adobe reader, you may have to download?

Typical Pressmatic Two 6BA6 6AV6 6BE6 6AV6 6AQ5.

This has electrolytics on the vibrator side and "appears" to be floating. which would be a requirement if it was bi-directional.

It shows 6 & 12V vibrators as there were plenty of old 6V Holdens & Volkswagens about when it was made. The Holden Aircheif also had 6 &12V power supplies for one particular basic radio, which also went into Vauxhall's.

The aerial may need investigation as to the shield earth otherwise, there is no DC path in the core.

Marc