Bush AC11 earth worthwhile?

[merlin1]

Good afternoon all. I have 3 of the above. They are all working at least on MW and cabinets etc in fairly good nick so they seem good for further restoration.
They all have 2 wire mains cables and the + is broken by the on off switch. I understand that the presence of an autotransformer means that the chassis is live during operation as it is used for the HT negative line.
What has thrown me slightly is that during my initial safety checks my neon tester lights when I touch the chassis. Also the earth input to the radio lights the neon.
Does all this sound normal to you much greater experienced types? Also, is there a real benefit in fitting a new 3 core cable to the radios and attaching the earth wire to the chassis? Any guidance greatfully received. Chris.

[Refugee]

In short you will trip the RCCD if you fit an earth to the chassis on and set that does not have a full isolating transformer.
If there is an earth terminal it will have a capacitor fitted that will light a neon tester.
The capacitor would need to be tested with DC from something like a Megger.

[Boater Sam]

If you want to use the earth terminal, change the isolating capacitor to a Y type.
Neon screwdrivers tell lies, OK for a quick check but better to test properly with a meter.

[boxdoctor]

Just a sniff of current (microamps) at or above the "striking" voltage will cause a neon tester to glow. Easily mis-interprated by the unwary, and possibly leading you on a wonderful adventure to nowhere.
Useful if used correctly, but as Boater Sam says: they tell lies. Tony

[SteveCG]

What a glowing neon tester is telling you that there is a voltage present that you should check further with a proper meter.

As boxdoctor and Boater_Sam say: Neon testers are useful but they have their limitations.

[G8UWM-MildMartin]

It isn't an autotransformer!

[ms660]

There's no auto transformer in the schematic I'm looking at (Trader).....

EDIT: Post crossed

Lawrence.

[John10b]

As previously mentioned, the Bush AC 11 uses a Mains transformer. Do you have the circuit diagram ?
Cheers
John

[Boater Sam]

The Bush AC11 uses a common chassis with the DAC11 which is an AC/DC set. The valve line up is the same, the only difference is the mains supply arrangements.
The AC11 has a mains transformer isolating the chassis from the mains. The valve heaters and the HT are fed by this transformer and the chassis can be earthed.
The Trader service sheet 851 covers both models.
Sam.

[David G4EBT]

True, it has an isolating transformer and the circuit shows an earth direct to the chassis (or in the case of the DAC11, via an isolating capacitor). Some Bush AC only sets such as AC91 do use an autotransformer, so the HT is derived directly from the mains, though to the uninitiated, without studying the circuit, they may mistakenly believe that it's a full mains transformer with a primary and secondary.

Incidentally, Bush's own service data is invariably more comprehensive than Trader Sheets and runs to many more pages. For example, the Bush AC11 service data runs to 11 pages - the Trader Sheet is four. The Bush DAC90A service sheets run to eight pages - the Trader Sheet is just two. Always state which sheet is being used, because there are differences between manufacturers' data and Trader Sheets. For example, the audio coupling cap ('that cap') on the AC11 trader sheet is shown as C29, but on the Bush sheet is called C33. Hence, if someone working from the Bush data says "make sure you change C33", and you're using the Trader Sheet, 'C33' is the 32 uF section of the twin 32/16uF HT smoothing, and so on it goes.

[merlin1]

Good afternoon again, gentlemen. Thank you very much for your comments. I appreciate your help. Chris.