I have this interesting little set coming to me soon. I am enjoying finding the mistakes in the service folder. Two capacitors marked as 100MFD and 30MFD that are actually micas.

A hot chassis with a metal case ouch.

Me thinks you are reading that wrong: MMF is picofarads.

This is one of the latter "not quite as dangerous: Intrinsically unsafe type". These you service with an isolation transformer& pray the RCD works if you don't.

The metal chassis is floated above the mains by R3 which means everything is returned to a floating negative not the chassis. This means C6 should be a line cap. The American wiring code used to be fairly hap hazard. There was no really enforced code so Neutral (common) and Active (line) could be interchanged in the socket & with the socket. Switch mode PSU's and tube radio's are not a good mix and you need to be rather careful as to how you use test equipment.

N.B. Its a TRF not a superheterodyne.

It is desirable to have the chassis as Neutral.

http://www.nostalgiaair.org/PagesByModel/830/M0011830.pdf

Marc

Marcc in the paperwork I believe the capacitors are listed top to bottom electrolytic, non electrolytic and then mica. The two I was talking about just said the value as 100 and 30 but looking further over to the right it mentions where they are in the circuit which is pretty much common sense.

This one arrived today. Its a tiny little set. I will send photos to Brad.

Nomenclature: can get you into strife. It is very common to have Mica caps with nothing other than a number and 20 is liable to be 20pF they did that sort of thing here. Be wary, while American Mica's are prone to Silver Mica disease. and often short.

Beware of Mica caps on coils if they're not listed in dispatches and are on coils. If its got "Micamold" caps & resistors that is a trade mark: Not Mica. Most Micamold caps & resistors, will not have withstood the test of time.

Marc

Photos uploaded to Post 5.

Montgomery Ward's massive flagship department store was just west of the Wrigley bldg on the Chicago river. It was still going in the 1980's. I wish I had gone inside to experience the history and atmosphere.

Correction: the large complex was their mail order hq, their stores were elsewhere.

I would like to think there is a back for it? Using a DC power supply off of a transformer is likely possible with an LR8 and pass transistor like TIP 50 (has to handle the current) bearing in mind DC is more dangerous.

Definitely run it off of an isolation transformer. Beware some "Step-Down transformers" including "Slide Transformers", aka Variac's & Dimmerstat's (tm) are not "isolation transformers"; they are "Auto-Transformers" and offer no isolation: Therefore no protection.

Marc

Schematic shows chassis not actually hot. That's good of them.

Looked at it last night. Some twit cut one of the litz wires on the tuning coil. Fun . Also I will have to fit a dial string tensioner.
So what I will do is get the speaker, dial tensioner and Litz wire repaired. ( to judge if it's worthwhile) then order the valves.

Ahh! Famous last words.

If that circuit I posted is the right one? http://www.nostalgiaair.org/PagesByModel/830/M0011830.pdf

It has no transformer, therefore it is intrinsically unsafe. Its a fair bet that C6 is not an approved line cap and is leaking like a sieve.

The idea of R3 & C6 was to reduce the impact of the chassis being live to a dangerous level. That was the classic " Good intentions gone awry", as the twin core plug tops & the wiring code of the day, could easily see Active (line) on the chassis as the plug top and the wiring could get inverted. With the "floating system": Active could get onto a leaking C6.

To me it is desirable to ensure its run on an Isolation Transformer. You may live a lot longer.

There are isolation transformers here: One is 240 to 115V .0.87A "Ferguson" two pin US plug (out), I would say from its sheer weight, its continuous rated.

And another American Jefferson one that I think is similar but it has a Oz 3pin plug (out) {not a good idea; but that's how it came} but you can play "switcharoo" with its wiring to get 240 /480V or 240 /120V 0.1KVA

Knight Tube tester & Heathkit Grid dip Meter have dedicated step down isolation transformers.

Looking at the schematic, am I right in saying this is a TRF radio? I notice the schematic says it must NOT be earthed - I've noticed previously that the Americans take (or perhaps took) a lax approach to electrical safety. You want to be very careful while working on it.

The schematic uses the ancient MMF (micro-micro-farad) instead of pF.

Assuming it's the right schematic, the valves are:

12SG7 - RF amp (metal)
12SQ7 - detector and AF amp (metal)
50L6GT - AF output (glass)
35Z5GT - Half-wave rectifier (glass).

From the photo, it appears the 12SG7 is the only valve still there.

I assume the others would be fairly common in America.

America and Europe made a lot of radios without transformers and as I have noted some with Ballasts that look like transformers, just to add some treachery for the unwary.

One may find some 12V metals they were in some military stuff. The funny heater numbers add up to supply volts as they are in series.

There is no way, that you could earth the chassis unless it was totally isolated from the mains. The best you can do is ensure that Neutral which is at ground potential is on the Negative rail to keep the Active away from the chassis.

As our two wire plug tops are polarised they cannot be inverted at the socket, However, I have found inverted sockets here after work was done on the aerials.

The intrinsic safety is why it needs an isolation transformer.

I might even have one of those tubes in stuff from deceased estates?

Marc

Many old Oz TVs also had a crummy wax-paper cap from AC line to chassis, but since chassis was earthed, the cap would fail with a Bang not a Wimper