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Old 22nd Jan 2020, 6:03 pm   #1
Gabe001
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Default Using the PU jack

I am posting my experience connecting modern peripherals to 1930s radio sets via the PU jack, in case this helps anyone here. Most things were learnt through trial and error, asking questions here and googling. Below please find the things I wish I knew starting out.

1. On both my sets (GEC superhet 5 and Pilot u650) the PU jack requires an input via a 6.35mm mono plug. This automatically mutes the radio to allow sound via this 'line in' only. The problem is that not all 6.35mm plugs work. The overall length of the plug seems to vary by about 5mm. The shorter ones are a waste of money. I have found (after 3 attempts) that the kenable 6.35mm mono male to 3.5mm mono male audio cable (Amazon/eBay) works well.

2. There is a very well concealed mono mode on the later versions of android. This is accessible from
Settings->smart assistance->accessibility->mono
If you set this to mono and plug the 3.5mm mono plug in your phone's stereo headphone jack, you get great sound

3. If you can only get stereo sound from your device, you need a resistive circuit to convert stereo to mono. There is a thread here that explains how to make one. Alternatively, you can buy one from ebay, just search for
'smadt1 stereo to mono'. Its priced 7.99 but the seller accepted 6.50. Don't be tempted to use anything that shorts L+R - the sound will be terrible.

4. If you want to avoid cabling, there is a 3.6v 3 channel AM transmitter available on eBay or AliExpress (10-15) which i can verify works well provided it's very close to the radio.

Hope this helps someone
Gabriel
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Old 22nd Jan 2020, 8:47 pm   #2
Wellington
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Thumbs up Re: Using the PU jack

Hi Gabriel

Thanks for posting about your experiences connecting modern devices to your old radios. I was particularly interested in the 'mono' option you discovered on Android - well-concealed, as you say!

I was puzzled by your reference to a PU jack, as I'd not seen a radio with a jack socket fitted for Pick-up (or 'Gram') use. However, I looked up the thread on your Pilot U650 and lo, there it was. As David describes in that thread, insertion of a plug into the socket automatically mutes the RF/IF stages.

It looks a nice set. Glad to see that you're enjoying using it.

Last edited by Wellington; 22nd Jan 2020 at 8:48 pm. Reason: Name correction!
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Old 22nd Jan 2020, 8:54 pm   #3
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Default Re: Using the PU jack

Also worthy of note: many now-obsolete-and-worthless mobile phones have a FM-radio function; these use the 'headphone' lead that plugs into the 3.5mm jack as the antenna.

If you fit a suitably-long lead from this to a plug that fits into your vintage-radio's "PU" input, you have quickly and cheaply converted your old radio to receive FM broadcasts. The phone usually doesn't need an "active" SIM, just a SIM of some sort so the thing will boot-up.
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Old 22nd Jan 2020, 10:08 pm   #4
Gabe001
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Default Re: Using the PU jack

Wellington the GEC Superhet 5 BC3440K has one too. I don't think they're that uncommon.

The FM trick with an old mobile is great!
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Old 23rd Jan 2020, 1:09 am   #5
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Default Re: Using the PU jack

My very basic Pixi4 smart phone (now surplus as my children bought me a better one for my birthday) works Ok without a SIM,and could probably be used to access internat radio if I cold be bothered.

As a schoolboy in the early 1960's i used to play my EL3585 tape recorder through the gram input of an old radio. I only ever saw UK-made radios with 1/4" wander plug connections for the Gram input, although did find 4mm connectors used on some Philips models. The radios always had a Gram setting on the waveband select switch.
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Old 23rd Jan 2020, 8:06 am   #6
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Default Re: Using the PU jack

Talking about connections, has anyone of you used an AM transmitter for a true cable-less connection. I've got the one in the picture below (built a wooden enclosure for it) and you can hear the GEC receiving the MW signal (playing nirvana) from it in the video in the zip file. The USB cord is a modification I've done the recharge the battery. Range is only about 1m or so.
Attached Thumbnails
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Name:	-2079743332840561938.jpg
Views:	86
Size:	40.5 KB
ID:	197632   Click image for larger version

Name:	IMG_20200108_195018.jpg
Views:	76
Size:	58.7 KB
ID:	197633  
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File Type: zip amtrans.zip (3.60 MB, 12 views)
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Old 23rd Jan 2020, 12:44 pm   #7
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Default Re: Using the PU jack

Quote:
Originally Posted by emeritus View Post

......

I only ever saw UK-made radios with 1/4" wander plug connections for the Gram input, although did find 4mm connectors used on some Philips models.

.......
That'll be 1/8" for wander plugs. 1/4" for jack plugs, 4mm for banana plugs.
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Old 23rd Jan 2020, 12:57 pm   #8
emeritus
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Default Re: Using the PU jack

Indeed, sorry for the typo!
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Old 23rd Jan 2020, 1:46 pm   #9
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Default Re: Using the PU jack

Just a thought, but I think my 30's Murphy has a 1/4" jack. If you use it I think you have to be careful to load it correctly as this can affect the bias on the next stage. Will check latter.
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Old 23rd Jan 2020, 1:59 pm   #10
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Default Re: Using the PU jack

Ive seen those little "transmitters" on ebay, they work OK as you have seen, but not very well. They are just 3 crystal oscillators and a transformer, 3 for the different frequencies. Its about as simple as you can make yourself but they wont produce great audio like a better device would.

In terms of value, you would struggle to beat this device:

https://www.nfor.nl/radioforumservic...PLL_AM_Osc.pdf

Its IC based and uses a 4053 analog switch IC as the final. Very stable and running at 15v could well cover your whole house properly with the right antenna and other variables. It gets better range than my SSTran AMT3000 did

A kit for building this circuit can be bought here for 15-18

https://www.amateurradioshop.nl/

You need to use google translate though as its in dutch, I've found them very helpful by email as well. There are many different transmitters available, if you don't fancy building one the spitfire is ok, but I suspect the one I posted above is better.
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Old 23rd Jan 2020, 2:00 pm   #11
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Default Re: Using the PU jack

Just had a look. Using the jack will open circuit the control grid and possibly cause damage on this particular set. "Murphy A4"
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Old 23rd Jan 2020, 7:24 pm   #12
Gabe001
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Default Re: Using the PU jack

Quote:
Originally Posted by vintage_8bit View Post
Just had a look. Using the jack will open circuit the control grid and possibly cause damage on this particular set. "Murphy A4"
Can you explain why this is the case, in rather simple terms?
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Old 23rd Jan 2020, 11:23 pm   #13
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Default Re: Using the PU jack

Well a floating grid isn't good news but on a low level audio triode with significant anode load resistance it's unlikely to damage anything. However the signal ground on the PU input isn't chassis ground, it's a bias voltage decoupled to chassis by capacitor. A PU of the day would simply have been a floating coil with no ground and low enough resistance to couple the bias to the valve OK. Any input with one side grounded would short out the bias if the chassis has an RF ground connected and reduce bias voltages on other valves including the output which might be unwelcome.
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Old 24th Jan 2020, 10:22 pm   #14
Gabe001
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Default Re: Using the PU jack

Quote:
Originally Posted by Herald1360 View Post
Well a floating grid isn't good news but on a low level audio triode with significant anode load resistance it's unlikely to damage anything. However the signal ground on the PU input isn't chassis ground, it's a bias voltage decoupled to chassis by capacitor. A PU of the day would simply have been a floating coil with no ground and low enough resistance to couple the bias to the valve OK. Any input with one side grounded would short out the bias if the chassis has an RF ground connected and reduce bias voltages on other valves including the output which might be unwelcome.
I had to read this a few times. So if the radio antenna is not grounded, then there shouldn't be a problem?
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Old 25th Jan 2020, 12:05 am   #15
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Default Re: Using the PU jack

Not in terms of damage to the radio but grounding a point part way up the bias chain could be a recipe for hum. Best to use a floating (battery operated maybe) source or else feed the signal via a small audio isolating transformer. No particular high voltage isolation transformer is needed, it's not a safety issue.
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Old 25th Jan 2020, 12:25 am   #16
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Default Re: Using the PU jack

Does the attached help of the Murphy A4.
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Old 25th Jan 2020, 7:45 am   #17
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Default Re: Using the PU jack

Thank you both.
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