Portadyne CG.68 Radiogram.

[vintage_8bit]

Hi all.

I've had this a while and its coming up for restoration soon, has anyone had experience of these? I will have a go at repairing the crystal cartridge on this if its not working, I doubt if it is. I've not done that before, perhaps fit it with a piezo crystal element? I'm also struggling to find any circuits, Trader or otherwise. I have worked on radios of this age and without a circuit diagram before, but l would be more confident if I had one. I have noticed if you look at the back panel image, the full model No may be CG.68.S. Colin

[AC/HL]

It appears to be an AC only version of the U680 chassis (possibly AG80?): https://www.radiomuseum.org/r/dynapo...dyne_u680.html.
Portadyne (Dynaport) info is a bit thin on the ground.

[vintage_8bit]

Thanks Bill. The U680 chassis looks very similar as does the dial glass. I see the radio museum doesn't have a circuit diagram as well. Colin

[Heatercathodeshort]

I have checked all my Portadyne info, no luck I'm afraid. It's probably the standard 5 valve superhet ECH35 EF39 EBC33 EL33 and an AZ31 series rectifier. The pick up will probably be a magnetic type with a 2000ohm coil. You may be lucky and just find the damping rubbers U/S. The arm is of light construction and you should find a medium output crystal cartridge suitable but you may have to find some way to reduce the arm weight. Really only suitable for 78 records. John.

[vintage_8bit]

Thanks for your research John. I assumed the pick up would be crystal. I'm not going to start restoration right now, but I will look at the pick up now. Colin

[vintage_8bit]

Couldn't resist a little look.
The pick up measures 2.3K Ohm. Looking at its output on a scope at 5mV/div there is no change when needle rubbed. I've no experiance of these, but hoping now it is magnetic and its just jammed up? I can see some of the rubber surrounding the needle carrier, it looks cracked and hard.
The valve line up currently fitted is : 6K8GT, EF39, 6Q7GT, 6V6GT, 5Z4G.
I couldn't see a date on the electrolytics so I removed the Elac speaker in search of a date code, see images, not sure if the numbers mean anything. I did check speaker and o/p transformer whilst removed & thats all good. Colin

[PaulR]

Here is a thread containing information a similar pickup https://www.vintage-radio.net/forum/...d.php?t=121284

[Heatercathodeshort]

The needle armature will be magnetically stuck to one of the pole pieces due to rock hard rubber suspension. The centering rubber will no doubt be in the same condition.

Paul's link will explain it better but when you decide to have a go at it. Take great care handling the pick up coil. It is very delicate, wound with hair thin wire and thousands of turns!

The valves are the usual Brimar set up with equivalents in the Mullard red E range.
ECH35/6K8G-EF39/6K7G-EBC33/6Q7G/- 6V6G [not direct equivalent but it will work the Mullard EL33] and the rectifier a GZ30/5Z4G. The 5Z4 is indirectly heated. Hope you get a squeak out of it. It should rock when working correctly. 6V6 is around 4.5W RMS. John.

[vintage_8bit]

John / Paul, thanks for the advice. I have read through the link, very interesting. Its really got my interest up for this now, so may make this radiogram the next project. I will tackle the pickup first and let you know how I get on. Colin

[vintage_8bit]

No going back now. Once I had freed off the armature I connected it to a scope, see image, off load 30V! this was with armature being flicked from pole to pole with the top bridge & rubber centering piece removed. Now trying to find some silicon tube. Ive ordered some with an I.D. of 2mm to try. It all came apart quite easily. The hardest part was freeing the coil from the pole pieces as it was glued in. Thanks for giving me the confidence to proceed, with all your advice gents. Colin

[Techman]

Looks like you're doing a grand job!

A lot of folk don't seem to realise that if you don't rebuild the pickup suspension and try playing 78s with it as is, that the lack of compliance will completely ruin the records.

[Edward Huggins]

Originally Posted by vintage_8bit

I couldn't see a date on the electrolytics so I removed the Elac speaker in search of a date code, see images, not sure if the numbers mean anything. I did check speaker and o/p transformer whilst removed & thats all good. Colin

Most, if not all, ELAC speakers, will have their size printed on the chassis e.g. 95RM = 9" x 5" unit. The letter/s indicate the strength of the magnet.

[vintage_8bit]

I have now assembled the pick up. I was very pleased with the armature bushes. I used pink rubber Hellemann sleving with an O.D. of around 0.114" before strething over the armature. The top rubber on the armature was more difficult to get right. I tried a piece of rubber around 0.05" thickness this seemed to thick. Next I tried a piece around 0.020" this worked quite well but I felt not quite thick enough. Lots of output with this set up though. Rubbing the needle across my finger showed an unloaded output on scope of > +/- 500mV. Having never seen a good cartridge I will leave as is. When I get the chassis going I can see how well it works. It's quite easy to change the top rubber then if required. Colin

[PaulR]

I have had the most trouble with the top rubber. Too thick and the compliance is greatly reduced, too thin and the armature sticks to the magnet or you get odd high frequency distortion.

[vintage_8bit]

With the 0.20" rubber I can get it to stick but you have to push the needle harder than you would expect in normal use. I obtained the rubber samples from a bicycle inner tube and rim tape, which is the thinner sample. Colin

[PaulR]

It will be interesting to know what it sounds like when you try it

[vintage_8bit]

I have now tried it on the gram input on my Wyler 55 amp. Its difficult to judge how good it sounds as im not sure of the condition of my records, and I've never heard a good one before. I used the same needles that my wind up gramophone uses. (New one every record) I have only ever used soft needles but not sure what I should use in this pick up.

Edward, sorry missed your earlier post. I couldn't find any code on the speaker other than the images in post 6. I will look again though.

Colin

[Heatercathodeshort]

Make sure you can set the needle armature centrally by adjustment of the rubber and the locking screws.
Do a little work on the amp first. Change the coupling cap from the EBC33 anode pin 3 to the control grid of the 6V6 pin 5 [probably via the volume control.
If the smoothing caps look OK give it a bit of power with the lamp limiter. I bet it works! Good solid kit Portadyne. Simple and always sound good. John.

[vintage_8bit]

I will change the coupling cap to the output stage thanks. I usually power up very slowly with a variac and AVO monitering the current, & with just the rectifier pluged in first. Will report back when I know more. Colin

[PaulR]

The ones I have succeeded with sound ok. Better than acoustic gramophones in the HMV 100 range level but not as good as with a crystal or later cartridge. At least you have a volume control!