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Old 25th Jan 2014, 12:12 am   #1
John_BS
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Default Fed up of Purley.... (part one)

I don't often work on valve radios, but before Christmas I discovered that a friend's son would like a "retro" radio, so I thought I'd buy a set with SW and get it working.

Started work on a 1940's Cossor, which presented the usual requirement of replacing every single paper & electrolytic capacitor, plus many resistors way out of tolerance, and two duff valves.

On powering up, there was some evidence of life on MW, but after a few seconds there was arcing and pyrotechnics from beneath V1, which turned out to be the result of a failed valve-holder (the Paxolin has previously carbonised and tracked). With a sigh I snipped all 8 connections, dug it out, and managed to find an octal socket in the junk collection.

Re-powering I discovered that there was no oscillator on SW, so had a closer look at the components in this area: the SW Osc coil was shown on the circuit as dual-winding, but appeared to have a single spaced winding of c 20swg wound on a spiral-cut Bakelite former. Then I noticed some loose wire hanging off the former.....it turned out to be 44swg silk-covered: this was the second winding! The service data gave the resistance as c. 30 ohms, which I thought was a misprint, as it's only a few inches of wire. However, upon measuring a section I discovered it was actually resistance wire!

By some fluke, the only resistance wire I possess is 44swg double-silk covered, so I decided to rewind. However, the last turn disappears though a hole in the former and returns to the top of the coil on the inside. Cossor had thoughtfully secured the internal dust-iron tuning slug with a generous melting of dark wax, and the core was "tuned" by the operator lowering the core on a "sling" of fine thread, and then presumably pouring in the wax .....

When this job was completed, the set worked on SW but was mis-tracking on MW and LW. The IF was 18kHz out, so I thought I'd bring that right first: ....big mistake: as soon as the brass screws were adjusted, something inside the IF cans dropped off and the IF went way off tune. Gritted teeth; unsolder and remove IF can, to find that the dust-iron slugs are cantilevered off the end of the brass threaded adjusters, and presumably with time they bind on the surrounding coil and can't wait to break off.....

Set abandoned

John

In part two: the post Christmas Philips set !
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Old 25th Jan 2014, 12:27 am   #2
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Default Re: Fed up of Purley.... (part one)

John

It's a shame to have to give up after all that work.

It is often possible to repair IFT cores of that general type (see the Pye L75 B example below), where the brass adjuster has separated from the core.

I superglued the brass piece to the end of the core to get it in the right position, then reinforced the joint with a layer of Araldite Precision and finally gently filed off the surplus.

Ron
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Old 25th Jan 2014, 2:21 am   #3
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Default Re: Fed up of Purley.... (part one)

John, welcome to vintage radio, they are not Tektronix! , just grit yr teeth and man-up!
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Old 25th Jan 2014, 11:41 am   #4
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Default Re: Fed up of Purley.... (part one)

No, I agree with John! Cut your losses and treat it as an ornament or plug a mobile phone into it and listen to the phone's MP3 player and FM radio through its output stage.

At the moment, there are so many really nice sets very restorable condition that life's sometimes too short to deal with the really troublesome ones.

Your story reminds me of a Ferguson that I foolishly offered to bring back to life for a friend. Many hours later, the performance was still laughably bad, so I reluctantly had to admit defeat.

Nick.
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Old 25th Jan 2014, 12:20 pm   #5
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Default Re: Fed up of Purley.... (part one)

Quote:
Originally Posted by ronbryan View Post
It is often possible to repair IFT cores of that general type (see the Pye L75 B example below), where the brass adjuster has separated from the core.
I've done that, too - used a (stationary) lathe, between power chuck and tailstock chuck to keep the brass adjuster in line with the slug. Used a tiny drop of Araldite Precision, with a fan heater blowing for several hours, to tack it together - then removing from the lathe I used more Araldite, filled with glass microspheres (low-loss) to build it up, curing in an airing cupboard.

But yes, it is tedious and time consuming!

Also, yes, resistance wire is sometimes used for feedback windings, so as to not have any spurious high-Q resonances. Makes life very interesting, don't you think?
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Old 25th Jan 2014, 1:03 pm   #6
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Default Re: Fed up of Purley.... (part one)

It would probably work with copper wire and an added resistor in series.

David
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Old 25th Jan 2014, 2:50 pm   #7
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Default Re: Fed up of Purley.... (part one)

Ron/kalee20

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between power chuck and tailstock chuck
That's a very good idea. I still think the fundamental design is weak. The core is very brittle and crumbly, and only c. 5mm OD.
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Old 25th Jan 2014, 3:04 pm   #8
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Default Re: Fed up of Purley.... (part one)

I would be very interested to know which model of Cossor started this thread.
I have a pair of admittedly later Cossors to do. They are 500s and I hope I don't encounter the same slug problems!
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Old 25th Jan 2014, 4:04 pm   #9
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Default Re: Fed up of Purley.... (part one)

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It would probably work with copper wire and an added resistor in series.
Probably,on SW, yes.

What usually happens is that the winding itself has significant self-capacitance, and this causes spurious in-band resonances. In that case, adding an external resistor iun series would not help. On SW the turns are generally so widely spaced that this isn't an issue, and most of the capacitance is external to the coil. But on MW and LW, it can often have significant effect.

Asto the IFT slugs - yes they are often fragile. The good news is that if you snap them in half, they can be re-glued as long as the bond line is thin (and they are glued straight!).
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Old 25th Jan 2014, 5:26 pm   #10
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Default Re: Fed up of Purley.... (part one)

Quote:
Originally Posted by Boater Sam View Post
I would be very interested to know which model of Cossor started this thread.
Boater Sam
I would guess something like a 464 or one of the 470 series. I think these chassis are pretty similar and use resistance wire for the SW reaction winding.

I must have been lucky to date not have encountered either the crumbling cores or open circuit windings on the coils but I have had trouble with coils on the later 494 and 501's

Cheers

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Old 25th Jan 2014, 5:35 pm   #11
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Default Re: Fed up of Purley.... (part one)

It's a Cossor 464. Or rather, was.

You can see the SW coil in the attached photo: only the 20SWg winding is visible as the interleaved winding is so fine!

John
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Old 25th Jan 2014, 7:55 pm   #12
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Default Re: Fed up of Purley.... (part one)

I too have glued together slugs and glued them back onto their brass screwed rods. The important thing is to keep things concentric. A lathe helps, but it is possible to set things carefully by hand.

I prefer these type of slugs, compared iron dust or ferrite ones screwed directly into the cores, any day.
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Old 25th Jan 2014, 9:38 pm   #13
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Default Re: Fed up of Purley.... (part one)

I must have been lucky, I had to replace the 100pf caps in the same type of Cossor IFT. I didn't break the cores, but I snapped the fine wires coming out of one of the coils. Some careful repairing later and a continuity check and all was well. Until I went to re assemble it and broke the wires off the other coil! >:C

The radio is now working fine. Perhaps you will come back to this set at a later time and sort it? My Cossor was sat for over 2 years before I came back to it and found the fault, it had been wired wrong in the factory!
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Old 26th Jan 2014, 11:07 pm   #14
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Default Re: Fed up of Purley.... (part one)

Quote:
I had to replace the 100pf caps
Were they the same type as above, and what was wrong with them?

John
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Old 27th Jan 2014, 11:22 pm   #15
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Default Re: Fed up of Purley.... (part one)

Basically the same caps, but dark brown in colour. I'm sure I couldn't get the IF to align properly. The new caps sorted it. My memory isn't good, but the radio (Cossor 501AC) works really well and is frequently listened to.
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Old 28th Jan 2014, 9:37 am   #16
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Default Re: Fed up of Purley.... (part one)

AH!!!! Don't give up on it! You are on a forum full of resourceful people, spares and knowledge.
Possibly, put a wanted request here on the forum.


As suggested above, put the radio aside for now.

Try a repair when the sadness you feel has subsided!

SEAN
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Old 28th Jan 2014, 3:50 pm   #17
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Default Re: Fed up of Purley.... (part one)

Sean:

Wanted: glass tuning dial!

John
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