pye: P76F; connection to ipod via 3.5mm jack

Hi,

I'd like to connect an ipod to my P76f using the gram setting on the waveband selector

I've ordered a 3.5mm jack lead with bare wires.

What other connectors will I need and what plugs should I connect to. There are four plugs on the back left:. One for Aerial, one for earth, and 2 that are marked P.U.

Will I need an audio transfomer?

Also, the chassis have 60 years of dust - Which is the best way to clean this off.

Thank for your help.

Best wishes

David

• lead (5 KB)
• Plate from seller - just for interest (111 KB)

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Put a 33 Ohm resistors each in series in tip and ring of jack (White and Red). Connect the other ends together  to signal socket of Gram in (marked P.U.). The sleeve / earth of jack connects to the other socket (braid / screen / plain wire). I make plugs out of cut up pens and curled coffee tin as the 1/8th inch approx wander plugs are hard to find. The active gram input makes speaker buzz when you touch it, the earth input is usually the lower hole.

I hope the electronics have been restored, the paper diaelectric capacitors only lasted about 15 years. The leakage is excessive and can destroy audio transformer, mains transformer, output valve and rectifier.

I use  a long stiff dry artist's brush to remove excess dust.

An audio transformer is only needed for live chassis (which rarely have a gram socket, but some did, with capacitor isolation, which needs replaced by 1KV X Y types. I'd not use those type sets for external audio). Some older sets are for crystal cartridge, which is much higher level than mp3 player/phone. In such case the junction of the 33 Ohms resistors and jack sleeve can connect to 600 Ohm side of  step up transformer used for Microphones (600: to 5K 10K  or 20K). A 1:5 ratio is  600 Ohms to 15 K Ohms, which is fine.

Note such audio transformers don't usually provide isolation for live chassis sets (your set isn't live chassis). A 1:1 600 Ohm modem transformer does provide isolation, but isn't remotely HiFi.

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Hi Michael,

Thank you so much for your very helpful reply.

I have ordered some resistors and will throughly enjoy putting it all together.

I had no idea that my radio might need new paper dielectric capacitors. It's been in my possession for 25 years so it must need new ones by now. What is the best way to go about this?

Thank you again.

David

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DO NOT SWITCH ON AGAIN till you check or replace the faulty caps. They would have needed replacement in the 1970s!

Likely the EL41, EZ40, audio transformer and Mains transformer have been highly stressed any time it's been used since 1970s, though it's a progressive failure.

Use same or higher voltage rating plastic film types.

old 0.5uf = 0.47uF

old 0.4uF = 0.39 uF

old 0.2 uf = 0.22uF

etc i.e. 2 - > 2.2, 4 -> 3.9

You may need to extend legs of PCB types. Replace only one at a time.

Download schematic and put highlighter on all from 1nF (0.001uF) to 0.5uF values. Find them on chassis. If they are wax covered card, tar, black or brown crumbly plastic case Hunts brands, or metal with rubber bung at each end, (all cylindrical with a wire at each end) then they are likely paper diaelectric.

Only replace one at a time. Any that are connected to chassis at one end I leave in place, only cutting the other end. Between the EBC41 Anode and EL41 grid is the most destructive. If there is a paper  capacitor on EL41 anode or primary of Audio Transformer I usually use a 1KV ceramic type (it's often 220pF or 470pF and usually the lowest value paper type). Note I've not looked at schematic. But it's similar to the very many UK models I've restored.  

An ordinary DMM is no use to test. You need to measure leakage with at least 250V isolated DC via a 1M ohm safety resistor with one end disconnected from circuit, but very likely they are all bad on that modem.

Silver Mica (retangular), Ceramic (usually hollow) and polystyrene (clear plastic with visible foil and fine wires) types don't need replaced. Nor do the Philips/Mullard "mustard" coated  polyester types. But I think this model is too early for those.

There are electroylic types (Polarised). If it's working without hum (AFTER replacing the other caps!) then likely they are OK.

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Hi Michael,

Thank you again for all your help and advice. It is very much appreciated.

It looks like this is turning into a full scale project for me. Ideal for the winter months. Lots of research needed before getting started.

I had no idea the wireless could be so fragile. Up to about 20 years ago we used it every day.

My grandfather had a beautiful valve radio and we used to listen to the one o'clock news together, then one day it stopped working and Grandad got a transistor radio but it wasn't the same.

Anyway enough of the wittering.

I'll let you know what transpires.

Thank so much again.

David

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Hi Michael,

I had the problem to connect a MP3 players to an old tubes radio by the P.U. gram socket and thanks to your suggestions i was able to do that.

later, being curious, I have looked into Internet to go deeper on this subject  and i found another one, similar to the yours, but using resistors 10k ohm and with a capacitor of 100nF put after the joined resistors.

Then i tested different configurations even connecting directly the output of the jack without any resistors and capacitor,  without noticed any difference.  

Considering that i'm not an expert in radio technique, to not saying that i am very poor about this subject, I wonder why there are no differences in the different configurations and at the end, what are the need of the resistors and the capacitor and what are their functions from a theoretical point of view? I guess a hyphotesis, could be to protect the source device from any electrical voltage coming from the old radio?

 
Many thanks in advance for your attention.

Best Wishes

Roberto

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The modern players (MP3. phone, tablet) are stereo, L and R with low impedance. Thus you can't short the Left to the Right. The old radios etc are mono.

A pair of 33 Ohm is chosen as modern gear is for 32 Ohms rather than 8 Ohms earphones (earlier was 600 Ohms and before that 2,000 Ohms).

The two resistors have little effect when L = R as the radio input resistance is 10,000 to 470,000 Ohms or higher. If a signal is ONLY on the L or R, then it will be 1/2 volume.

A 2KV  isolation transformer is needed for "live chassis". Some of these have poor isolation gram / PU via 0.1uF (100nF) or 10nF 600V rated capacitors that usually need replaced due to leakage.

Some older sets are for crystal pickups on 78 rpm only players, which might be x10 voltage of later low tracking weight ceramic pickups on 33 or 45 rpm. Then a 1:4 or to 1:6 stepup  transformer may be needed as some players have limited output level (The French first limied playback level for headphones.).

A capacitor should not be needed.

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Dear Michael,
many thanks for your fast and detailed esplications, the radio i had connected is a "Sondyna 6032 Stereo" that i don't think is "live chassis". 
But by the way, when you talk about ceramic pickup in 33 rpm I thought that i have in my collection published in RDM, a radio "Nordmende Phono Super L" that when i touch also gently the cartridge or the arm of the turntable the noise i reproduced amplified, could it depends by some capacitors? 

Still many thanks for dedicating me your time and your experience 

Roberto

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For later sets with a stereo amp, no combining resistors are needed. Those are only for mono Gram/PU inputs. Some 1950s German sets with Mono VHF-FM do have true stereo mode mode for tape or gram/PU. Some Philips "bi-amp" models simply had two speakers on a mono amp, like the 1930s McMichael radios.

Ceramic cartridges are quite microphonic compared to the later HiFi magnetic moving iron or moving coil types. The Crystal ones are worse, but I doubt any still work as the crystal (Rochelle salt?) succumbs to damp. The  1970s HiFi magnetic types are quite different to the 1930s magnetic type, often with single play steel needles. The 1930s autochangers obviously had to use sapphire as the steel needles are really single play.

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Dear Michael,

many thanks for your answers, that means the problem is in the cartridge that works like a microphone due to the fact that is almost 60 years old and the cristals doesn't work well and the capacitors are not responsible of this problem.

Thanks a lot

Roberto
 

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