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Old 19th Apr 2003, 7:33 pm   #1
quantum
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Default Cossor 500AC SW oscillator coil failure

This is a fault that I have come across on a couple of occasions with Cossor 500AC 'Melody Maker' sets, although the same problem could occur with any of the Cossor series with identical RF stages with their associated coils, namely the 494AC/U, 501AC/U and the 520AC.

With these sets the primary of the short wave oscillator coil is permanently in circuit, regardless of which waveband the set is tuned to. The primary of the MW/LW osc coil is connected in series with the SW osc coil primary, being shorted out when the the set is switched to SW. Thus if the osc coil primary is open circuit the set would not function on any waveband.

I have found sets with o/c osc coil primaries on two occasions - bypassing this primary restores the sets to operation on MW and LW, although not SW of course.

Why the osc coil primary should go o/c is not certain - it does not carry DC so perhaps it is just age or a quality problem in the original coil - after all, the sets were not designed with the thought of how they would operate in 50 years time!
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Old 20th Apr 2003, 1:10 am   #2
Norm_Leal
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Default Re: Cossor 500AC SW oscillator coil failure

Hi

Here in the US we find coils open even though windings don't carry high current. Often with a green spot where wire has corroded through. Probably caused by contamination on wire along with moisture in the air. Very common on Philco radios.

Norm
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Old 20th Apr 2003, 6:56 pm   #3
quantum
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Default Re: Cossor 500AC SW oscillator coil failure

Norm,

Yes, coils occasionally can go o/c in any set - I have also come across this problem. The particular problem with the Cossor models is that an o/c in the SW osc coil primary will stop operation on all wavebands, because of the design of osc stage.

Ironically, the design of the SW osc coil appears more robust than the MW/LW osc coil, so theoretically should be less likely to fail due to humidity, contamination etc..
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Old 20th Apr 2003, 7:23 pm   #4
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Default Re: Cossor 500AC SW oscillator coil failure

I was once told it was due to the handling that the wire received when the coils were wound, something to do with the acids on the skin attacking the insulation and the copper. this would probably explain why some manufacturers sets are more prone than others (different procedures and machinery)

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Old 21st Apr 2003, 11:10 am   #5
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Default Re: Cossor 500AC SW oscillator coil failure

Jim,

I believe you are correct on this. At the risk of going off topic, I remember reading some time ago that a transformer company, Woden I think, carried out some research into this in the 1950s as a result of complaints about breakdowns in their transformers. During WWII and for many years afterwards more women were employed in electronics factories as the men were away on service, and apparently during 'certain times of the month' the moisture in their fingertips is very mildly acidic, which over time can eat through thin covered wires. When Woden discovered this they required all their female operatives to wear light cotton gloves when working on winding coils and transformers.
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