Hi guys - at the risk of displaying my ignorance.

I'm starting out my first restoration - an STC543.

Just doing a bit of background research to convince myself I sorta know what I'm doing (I definitely don't know what I'm doing) and came across this.

On the back of the chassis there is a two holed socket labelled "Pick-Up"

(See: https://myshed.mooo.com/Radio-STC543.html - the 4th and 6th photos)

Is this a connection for an external antenna?

Is "pick-up an old term for that - or am I completely on the wrong track.

I am currently interstate so I can't actually go and physically look at the radio - I suspect that would clear things up.

Thanks.

Polybus

Pick up is where you connect a record player or in these modern days a mp3 player will work. No not for antenna.

Is "pick-up an old term

Pick-up is indeed an old term for the transducer in the tone arm of a gramophone, often reduced to simply PU on radio chassis.

These are usually high impedance inputs and require some impedance matching (and possibly some DC decoupling) in order to effectively use the output of low impedance MP3 players.

GTC is correct . I wouldnt use a mp3 player but a CD player works quite well. I have one hooked into my Sky-Raider console and love it.

No problem using an MP3 player, just have to add a couple of passive components between it and the PU terminals.
JJ

Those "Pickup" terminals are usually just connected straight across the volume control with no attempt to silence the radio. They relied on the fact that the "Pickup" of the day was a low impedance device that would pretty much shunt the detector output when it was connected.

So directly connecting a low impedance source (MP3 or CD player) would do much the same thing. No need to add any parts.

That one puts the PU input across the volume control. You will need to reduce the signal to something like 30mV into the valve. If its stereo you will need some way to get it back to mono for the STC.

Perhaps advisable to get the set sorted first as I see issues, & would not power it. Once its going measure what voltage the detector is actually putting onto the top of the volume control. Circuit is on the same page as 510 in AORSM's.

Excellent - thanks for the info guys. I appreciate your thoughts.

Marcc - you said

"Perhaps advisable to get the set sorted first as I see issues"

I do intend to sort the set out first - to see what state its in but as this is my first restoration I don't have a lot of experience to rely on.

I note with interest the "I see issues........"

I assume you have looked at the photos and seen something of note - would you like to share your thoughts with a newbie??

Thanks

Ian, most MP3 players have earphone output only.
So as I said, it will need a couple of passive components, to make any sort of professional match to the PU terminals.
And I would also give some DC isolation to the network as well.
JJ

Johnny, I know this. I actually design this kind of gear for production.

A low impedance output (e.g. earphone) will happily drive a high impedance input with just a direct connection. We do it on a daily basis in hundreds of installations. The signal levels are directly compatible.

Exactly what passive components do you think you will need?

Maybe I’m old school, but would have suggested around 22 ohms to give the MP3 player something to look at as a load.
As many earphone outputs don’t like open circuits or high resistive loads.
Then I would experiment with perhaps 47K resistance in series with the PU input.
With maybe a .047 mfd blocking cap as well.
That way the volume on the MP3 player will not be so sensitive into the supposed relatively high impedance volume pot of the PU input..
An 8 ohm load straight into a 1 meg volume pot does not ring true to me.
A gross impedance missmatch, but as you say, works. Just not right.
JJ

It used to matter in the days of class B transformer coupled amps. You needed to consider the maximum power transfer theorem and what would happen if the load was open circuit (not good).

Now, there is a near to perfect AC voltage source with large amounts of negative feedback around it. You are generally looking into a small signal source impedance of near enough to zero ohms. Plus any internal series resistors that might be present.

You can always check this by measuring the AC voltage output, playing a tone. Add parallel resistance until the voltage drops to half. Measure the resistance. That's your AC source impedance. It will generally be vanishingly small compared with the input impedance. That's OK.

If you have LTSpice you can model it.

I can’t get my head around the fact that a solid state output stage that’s meant to look into a 8 ohm load, would be happy to operate without side effects into a 1 megohm load.
Appreciate your modelling programs, and have had some experience with antenna modeling software.
JJ

As a rough but ready analogy I look at it like this:

A low impedance output will not be loaded down when fed into a high Z device, therefore there will be a transfer of signal.

However, a high impedance output will see a low Z load as a dead short. Full stop. No signal transfer.

Think of a crystal earphone (remember those) "listening" in to a coil and capacitor. No problems--plenty of hiss, crackle, hum and perhaps one or two broadcast stations if a diode is thrown in.

Now, try doing that with a 32 Ohm headphone. Three chances of picking up anything---Fat, Buckleys and none!

So, rough as guts rule of thumb----Low Z to High Z, Signal Fed.

High Z to Low Z,Signal Dead.


Too many beers last night resulted in stumbled wording in the last sentence. Now re-edited to how it should be written.

Thanks Brad for saving the day, and my reputation---and double thanks for your tolerance.

Much appreciated,

G.