Power tube 42 is supposed to be 6.3 Volt filament,
but transformer only puts out 5 Volts.

http://www.radiomuseum.org/tubes/tube_42.html.

I could try a second transformer which could be hidden.

I was testing right across the secondary for that reading, and although the CT is earthed, the radio was not earthed at the time.

Earthing the chassis will not alter voltage readings, or at least it shouldn't. The fixed neutral and earth wiring are bonded at your main switchboard along with your water pipes and in the case of newer dwellings, your roof guttering may under certain conditions also be bonded to the neutral.

If the earth wire inside your radio (or the chassis if you have earthed it) comes into contact with either the active or neutral wire or the transformer's primary shorts to the chassis your RCD (if one is fitted in your switchboard) will trip.

That is the extent of the influence of a mains earth wire to your radio. If the transformer's secondary spits out 795 volts it will do it whether the chassis is earthed or not.

As far as I know, valve shields only exclude RF interference. Removing them will not stop audio from working.

Measuring across the transformer can kill modern voltmeters, many are flat out at 400V.

In my opinion, most digital multimeters are just junk. As a licenced electrician I have always placed my trust in Fluke. They are more expensive than other brands but the bottom line is that you are buying a superior product in every way.

On most Fluke models you can measure up to 600VAC however the test probes are rated at 1000 volts to protect the user from being zapped through the insulation.

The Fluke model 27 can measure to 1000 volts, AC or DC.

On top of this you can get a 3kV adaptor from Fluke which solves the problem of limited voltage range. The other good thing about Fluke meters is the auto ranging. I've never seen a need to buy a meter with dozens of selectable ranges when you can just choose "AC Volts" and get a correct reading every time.

False alarm with the power transformer voltage.. it does have both 5 and 6.3 volt outputs, One is hidden where it enters the chassis.

The amplifier works fine if I connect my cheap battery radio to the phono input.
Just a bit of squeal as I turn the volume then.

Image Link

Might see how I go replacing the radio valves.

If you have an 80 or 5Y3G as the rectifier then it would have to be that way as the filaments double as the cathode rather than heating a separate one. You wouldn't want any HT feedback hitting the dial lamp sockets which are powered by the 6.3v winding that powers the heaters for the remaining valves.

Yes it's an 80 rectifier.
When I only saw one filament secondary, I just assumed there was only one.
The schematic of course, omits the 6.3 Volt circuit.

Funny this.. If I switch from phono back to radio,
absolute silence, even if the amp is running loud.

Usually, there is no line diagram showing wiring to the valve heaters. 'Tails' usually come out of the valve symbol and end in arrows.

Fixing these you work backwards. Once the power supply and audio are working, then you sort out the front end.

Check the wiring again in the audio albeit what you did can affect stability. There are three things that destabilise. things

Moving parts about especially around the detector causing feedback.

6D6 (6U7; 78; with a different bases) would be one of the worst valves made for going tropo, if you leave the shields off.

Lead dress: Viz sky wiring where the wires are not as close to the chassis as they should be and they induct, or radiate. I have seen two sets doing that lately, one of which was a factory oversight. I ended up rewiring that lot and adding a shielded cable. If its a lead dress issue, prodding wires & parts with a chopstick will change the frequency of the whistle at the point of induction.

Marc

It's 2x 6D6 and a 6A7 in the radio,
The whistling is an odd one, it only seems to occur only while changing the volume
while using external input, and with the radio switched in, it's the same, but no radio sound,
so then the whistling is the only thing I can hear.

Problems with changing parts makes sense.
I only kept two micas, and a variable thing in wax,
and higher wattage resistors.
Most resistors were out of the 20% of their value.

Wouldn't you think you'd still get some radio sound?
It can't be 6D6 etc, anyway, because the radio is switched out when the phono input is being used.

I should be able to prove that by pulling the radio valves out and using phono input.

I am wondering if there is a wiring mistake 85 is a double diode, Triode so there may be AGC./AVC. Is there a circuit for this?

The set was made by Airzone 1931, possibly built by EMMCO it has to be 1934, or after as 6A7 & 6D6 were released in 1934 and 85 in 1932.

The whistle may be related to an AGC fault? Has this thing got a pickup plug for a turntable? If it has the turntable normally had some sort of low impedance input and would be in the cathode circuit of the 85.

When the turntable was not connected there was a pin bridging the two input holes. That grounded the cathode of the 85. If the pin has gone AWOL ..85 will not operate & will have a voltage on the cathode (pin5) of near B+

Marc

I'll get back, I think you're onto something there.
There is a radio phono switch, and yes there are two input holes,
one is connected direct to chassis inside, but otherwise there's nothing connected between them.

Another problem could be that the valve cap of valve 85 is not connected.
When I pulled the cap off to remove the valve for the first time,
I also pulled the cap off the valve, and later soldered a piece of capacitor wire
to the remaining pigtail, fed it through the hole in the valve cap, and soldered it back on.
It could have desoldered at the pigtail end when I was soldering the cap on,
only this time it's superglued. I committed to either fixing it or breaking it.

Though I haven't been told in so many words, I figure I'm not supposed to post schematics
to prevent someone like me coming along and collecting them all
for the purpose of getting into some business (and rightfully so).
I will try to isolate something in the circuit and post that, because I find something interesting.

Brad, you might as well make a list of modern things that are not junk,
it would be a much shorter list ;)
but I did accidentally measure across the high voltage when trying
to measure the 5 Volt secondary, and mine survived.
I'll just have to test some known voltages before trusting it again.
They are made to measure more modern junk.
Before this, as a hobbyist, there was never any reason for me to measure high voltages.

There's several sites out there that post schematic diagrams. If you have one you want to share or use to explain a problem then it is okay to post it here in the same way as for images.

Ok, thanks Brad
Not everyone here will have my eyes, I will email it if required.

Image Link

If the phono input was bridged, it's switched out when the radio switch is in the radio position anyway.

But I would like to know what would happen if I pulled the cap off valve 85? ... if the amplifier would still work, but not the radio.

Marcc, you're right, it says on radiomuseum that Airzone became "Airzone 1931 Ltd." when they moved to Sydney.
So I've got a radio with an original label on it saying
"Manufactured by Airzone (1931)".
So I have to wait another 3 years for it to hit 100.

The radio won't work but sections of the tuned circuit will still have power applied to it. It'll be at least another nine years before the oldest radio receivers in Australia turn 100 years old.