I was reading Vintage Radio in the Silicon Chip Magazine January 2017 on the Pye 1951 Model APL radio and Professor Graham Parslow highlighted the fact that this radio had a “Hot” output transformer. The body of the transformer is insulated from the chassis and is connected to the B+!

As the Pye APJ is similar to the Astor GN I checked the AORSM and found the same problem.

I know we are aware that there are high voltages on a valve radio chassis but this is defiantly a trap for the unwary!

A quick check of the AORSM Vol. 10 and 11 shows Astor models GQR, GQS, QQ, QS, NM & NK and Pye EMS and APS have the same issue. Also some Astor and Pye battery portables have 90 Volts on the transformer.

Pye APJ Circuit Diagram

Yep, such "hot" output transformers have been mentioned here before from time to time, and like all such traps for the unwary it needs to be on the repairer's beware list.

I wonder why they went to the trouble to do this. Only thing that comes to mind is to reduce the potential difference between the output transformer primary winding and body.

It didn't help though, those output transformers are just as likely to blow as those that are earthed.

Document uploaded.

I believe the Astor GN is the big radio known as the 'Big Bow'. The issue of the live transformer was discussed here some seven years ago - boy does that make me feel old.

https://vintage-radio.com.au/default.asp?f=1&th=177

With the live part carrying the B+ I would assume that touching the chassis would also be required to complete the circuit though the bottom line is, many (maybe most) of these radios are missing their Masonite backs, making them a very dangerous model to leave lying around, especially when kids are around.

Ha ha, yes my big Astor has an output transformer insulated from the chassis!

Luckily, my paranoid little habit of suspending the chassis upside down with the negative of my multimeter alligator clipped to the chassis, when first powering up, and indiscriminately probing EVERY single bit inside the chassis, reading voltages, I discovered to my surprise, some 350 odd volts on that transformer casing!
It's very handy knowing what will kill you and what won't.... Before anything else.

I just had a thought. If an refurbishment to these radios includes earthing the chassis then this will bypass the isolation properties of the mains transformer via the MEN link on your meter box and back through the neutral conductor. This is safe for the purpose of operating the radio but it will mean that just touching the body of the transformer will give you a shock in the same way that touching the mains active would.

A visual inspection which reveals a transformer sitting on a block of insulating material, should ring alarm bells: That lot also built PYE ones with that same."shocking" feature.

From memory that will have a resistor to 6V6 screen and not direct B+. 6V6 with 300V or so on it, has around 225V on the screen.

Be very careful as to the voltage of the filter caps. if it has a 5Y3, 600V is minimum. The only way to avoid that surge is to use a Sovtec 5Y3 or one with the same filament current and a cathode sleeve 5V4 has a lower voltage rating.

That sort of thing is why I warn against PA Line amps. Often tubes like 807 & 6CM5 are used and the TC is the plate. The Philips one I have has 300V on the plates of the 6CM5's and the transformer is capable of 300mA DC or more: That is in the killing Zone and DC "grabs".

QUOTE: I discovered to my surprise, some 350 odd volts on that transformer casing!

It'd never happen today, that's for sure. Then again, it does but to a lesser extent. In a switchmode power supply on a computer the voltage stepdown is managed at a very high frequency so a small transformer can be used. At the same time the voltage is quite high, typically more than 1,000 volts. Under normal use there's no problem. However if one was to stick a knitting needle in through the air holes then there'd be big problems, either for the power supply or the mug with the knitting needle.

These Astors were made much the same way. They had backs on the cabinets to stop prying fingers and the chassis were not earthed, so the mains transformer provided full isolation to the secondary circuits. Because the chassis is used as the return path for the two or three secondary circuits in most radios, this ends up connected to mains neutral via the MEN link in our meter boxes. This is little-known because most people don't know what an MEN link is or what its purpose is. I do know that this is a system used here as well as the US and NZ. I am not sure about Great Britain and other countries.

This issue with mains isolation is not relevant in modern appliances because secondary circuits don't generally use a chassis as a common rail. In addition, most appliances now are double insulated so they aren't provided with an earth connection.

I guess it is a classic case - we all trust ourselves, so most of us wouldn't have an issue operating one of these radios, even with the back missing. I don't think it is something to use when kids or other less-learned people are around though.

NSW only adopted the MEN system in the 1960s. My parents' second house, built in 1968, had MEN and an earthing rod below the meter box. Their first one, built in 1948, didn't have MEN and the main earth was a bare copper cable clamped to the water tap near the meter box.

NSW only adopted the MEN system in the 1960s.

Quite true and it closely followed the original (and optional) rollout of voltage operated earth leakage circuit breakers. Occasionally I still get to see an ancient meter box, which back then of course was just a sheet of ebonite on the brick wall with four insulators as standoffs. This would be fed with either single phase 6mm² mains or two or three phases with 4mm² mains on each phase if there was an electric stove instead of gas. No MEN link, the W/h meter vastly inferior in capacity to what is available now and only two or three porcelain fuse blocks plus the council fuse. Earth wire for the EP bond would have been 3/.036 bare copper all the way to the water pipe. Such bonds were always green with corrosion well before they ended up being replaced with 4mm² PVC building wire.

Depending on where the board was located, the weather would eventually erode the VIR insulation on the cables, leaving the likes of me with a birds nest of live wiring to work around when adding circuits to a board that the customer cannot afford to replace.

I checked the circuit on the Pye APS, AC/DC portable radio and found that when powered by AC it rectifies the incoming mains and also has the insulated output transformer! So you could have active 240V on the chassis and B+ on the transformer. Now that would give you a jolt!
I'll post the circuit.

Pye APS Circuit Diagram

If the GPO was wired backwards, yes it certainly would be there.