Thanks Robbbert. The schematic is correct, thanks for you patience.

I will replace all caps and be very carefull when I start.

I do have a question, is the 1954 supposed to be a light globe and not a valve, the schematics shows a resister in a circle and not a filament globe like the dial lights.

I have never seen this setup before.

John

I just checked radio museum and saw that the 1954 valve, looks like a globe, and is a 0.15 amp current regulator, so that helps abit.


John

A barretter is basically a light globe. As it gets hotter its resistance increases, so reducing its output voltage. In this way it functions as a crude current limiter. An ordinary light globe works in a similar way, but of course current limiting isn't what its designed for.

A barretter is easier to use than a massively giant hot resistor, or even the dreaded line cord.

If you don't know what a line cord is, it looks like a power cord and is used as one, but it's actually a long flexible resistor. If you didn't realise and replaced it with a normal power cord, you'd blow up your radio and wouldn't know why.

Now, to the maths. The valves and the 2 dial lamps all up (being in series) require around 136 volts at 150ma. This leaves 104 volts to be wasted away somehow. So you could use a 700 ohm resistor to dissipate 16.5 watts of heat - or use the barretter, where 16.5 watts is not a problem.

It's also worthy to note that if one of the dial lights blows, the valves won't light up. The replacement lamp must be 150ma type - a 300ma type will unbalance everything. Thus the pitfalls of series heater strings become obvious.

Some background on barretter: https://en.wikipedia.org/wiki/Iron%E2%80%93hydrogen_resistor

The 1954 barretter is rated at 100 to 160 volts at 150mA, so the mains plug (240V) looks correct, with 150mA through the filament/heater chain and the full mains voltage available for HT.
Harold

The barretter differs from a globe /lamp. The filament is of Platinum, Tungsten, & Iron. in an atmosphere of Hydrogen.

USA made a lot of these "hot chassis" radios as did Europe. They were not popular with radio service people here because of the real risks.

A lot of there servicing of them in USA in particular is via an isolation transformer.

Do be aware that coupling instruments to it can be perilous, as they cannot be grounded. Make sure Neutral ends up on the chassis. No isolation transformer, use a plug in or line cabled RCD if not fitted to circuits.

Meters & such used on these and tube radio's in general, need a voltage class / rating That can handle AC mains and high AC/ DC volts around a KV. Much modern instrumentation can't.

Thanks guys, that's all very good information, and extremely useful to me.... what a great forum this is.

I was reading about AC/DC radio circuits last night (Elements of radio servicing) by Levy) which has helped me better understand about berretta's and the information you all provided.

Although I haven't started checking the radio yet, I do think about the risks of hooking test equipment to this radio.

I do have a variac, isolation transformer and dim bulb setup, and am aware of the dangers.

Well, I just popped up the shed as I was curious about the barretta and found my barretta is a philips 40 watt bayonet mounted incandescent bulb.

The bulb looks quite old, so maybe it run OK like this.....yeh I know, maybe it didn't.


John

I'd be looking for a small, 50VA, 110V output transformer to make that radio safe as well as cooler running.

A mains transformer out of a beyond-restoration AWA P1 TV would do the job and is quite compact.

I like the transformer idea Ian

John

I was very fortunate in respect of an isolation transformer. (Do note the Variac (tm) / Slide regulator is not an isolation transformer.)

Someone in the radio club got onto some 3A commercial ATCO ones used in lighting. To this I have added a circuit breaker on the secondary to better protect it. And a kill switch, plus my usual filters in respect of what gets onto the mains here.

Do note that the fuse box RCD will not see an earth leakage fault on the secondary of a non iso transformer & I have seen some spectacular damage in a Thorn stereogram and a couple of stick welders, when their secondary's went feral.

Below 130VAC (output side) of a Variac, a 30mA RCD will not trip.

From my understanding of the circuit, the radio is designed for a 240V mains supply.
The barretter limits the filament/heater current to 150mA with the drop across the barretter disposing of the excess voltage.
Refer to my post #20.
Harold

You could replace the 1954 ballast (the American term for a barretter) with a 2,4μF 220VAC capacitor . This cap will cause reactive current of 150ma through the heaters, at 240V 50Hz. A 2μF cap in parallel with a 0.39μF should get you there (measure the current through the heaters and adjust this cap accordingly. A 2μF cap on American 240V 60Hz gave me the 150mA current. An advantage is less waste heat, and for you a sub for a likely unobtanium barretter tube.

The 2μF cap might be sourced from a multispeed ceiling fan. Or other fan. You might find it at a fan repair shop.

Be sure the plug's neutral pin goes to this hot chassis radio's chassis. And the hot plug pin goes to the radio's power switch. This should make this radio safer to work on. But check that the chassis does in fact measure zero volts AC to ground before working on it.

Be sure to replace all the wax paper caps and electrolytic filter caps too.

Don't know if they have a minimum order but Wiltronic's has them for around $2.15 new. Others probably but make sure its a run capacitor.

Re caps for multispeed fans I have had to have these replaced in three identical fans which got slower over twenty odd years. Not a bad life I suppose but may be a factor to consider.

A warning from an electrical rewinder here to me was that many of the newer rectangular shaped ones were crap. I have replaced a few in pumps & compressors. I did have one in a 3HP go to ground a couple of years ago. It tested good so I just replaced its jacket: It had to close to 50 years old. Some of the eight foot 2 by 85Watt flouro's here had caps that were pre 60's.

It all comes down to quality & fitness for purpose. Just imagine the stresses on the Electric Fence ones (pulse grade) that have run virtually non stop since the eighties.

The cheap fans I have seen tend to do more bearings than caps. Plain oil bearings used in these things do not have much life. Yet the old Marelli "Breezo" ceiling fan in one shed has been there since the 80's. It has roller bearings that have been repacked and runs slowly 24/7.

I have never been able to find it's true age: Its got three aluminium blades with an open cage cast motor and its heavy.