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Old 27th Jun 2007, 12:10 pm   #1
M0TAW Tony
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Default Cossor 464: No short wave

Can someone give me some pointers please:
Have been repairing a Cossor 464 and it now works fine on M & LW. Plenty of volume, good selectivity and sensitivity working off its frame aerial. However
I get no stations on SW. I get background hum when tuning cap vanes are fully unmeshed hum decreases to little at half meshed. Wire aerial is fitted as frame aerial is for M & LW only. Wire aerial makes some slight improvement to reception on these bands. Have cleaned switch, tested SW coils for O/C - OK and tested associated caps for o/c and values.
Could the frequenct changer valve OM10 (ECH35) be failing at the higher frequencies? It looks like an original - all in the line up are Cossor.
Tony
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Old 27th Jun 2007, 1:18 pm   #2
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Default Re: Cossor 464: No short wave

Tony. I think this is a fairly common problem now. You will probably find the short wave oscillator coil is O/C. I don't have the circuit here at work so can't quote circuit references. You may be lucky and find just the end of the coil has come adrift. If not it may be the dreaded 'green spot' and the coil is useless.

This happened to a 464 I repaired for someone years ago. In this case however, the owner was quite happy for shortwaves not to work. The radio is still working on MW and LW only.



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Old 27th Jun 2007, 7:19 pm   #3
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Default Re: Cossor 464: No short wave

try plugging an aerial in.

Sometimes the SW is not connected to the frame aerial on some sets.
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Old 27th Jun 2007, 9:11 pm   #4
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Default Re: Cossor 464: No short wave

Um...yes! good point!! Sometimes one overlooks the obvious as a starting point.


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Old 28th Jun 2007, 1:50 am   #5
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Default Re: Cossor 464: No short wave

High Tony.
Yes could be mixer valve low emission. They tend to loose "oomph" at higher frequencies. In extreme cases sets might work on LW and not on MW. I recently had a marconi 274 which went deaf over 400m on the MW band. A new mixer (FC4) cured the problem. Measure the voltage on the triode grid of the OM10,(-ve) probably there on MW& LW(oscillator running), but not(or reduced) on SW. Also check resistor and decouplers ascociated with the OM10.
I used to have a 474, which used to howl if you went near it. Microphonic valve somewhere!
I also have a Cossor 494 which drove me nuts(and still isn't completely fixed)-I think the SW osc coil is faulty on that one.

Finally Good luck. I hate RF faults.
(Have some NOS Z&I ECH35 if you need one. £4.00 inc P&P) or £2.00 used tested Mullard.
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Old 28th Jun 2007, 7:39 am   #6
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Default Re: Cossor 464: No short wave

First, Give the wavechange switch a good clean.

Second, check the wavechange coils. Switch to SW on the wavechange switch, and there should be a reading of several ohms betweeh the Anode of the Triode (Pin 6) of V1 and the not earth end of C14.

If there is, then there should also be a reading of Several Ohms across L6. If there is, then go across C31. If no reading here, look at C11.

Also, check C8 and R3.

If this lot comes out OK, then try the valve...

Cheers,

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Old 28th Jun 2007, 11:52 am   #7
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Default Re: Cossor 464: No short wave

If you have another SW radio of any type, you can use it to check to see if the local oscillator is working:

Tune the working set to a broadcast say, in the 6 MHz band (49 metres).

Put the faulty one next to it and tune from about 5 to 7 MHz.

If the LO is working, you'll hear it on the good set as a whistle as the LO radiates and is picked up as a carrier and mixes with the station tuned in on 6 MHz.

There's nothing special about 6 MHz, but it's going to be on any set with a shortwave band.

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Old 28th Jun 2007, 7:36 pm   #8
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Default Re: Cossor 464: No short wave

Wow - many thanks to you all for your advice. As I said in my first, I did connect a wire aerial.
I will follow through with the suggestions and report back.
Tim - thank you for offer of ECH35. If I get nowhere I will send you a PM.
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Old 6th Jul 2007, 11:53 am   #9
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Default Re: Cossor 464: No short wave

Thanks to all who offered advice and especially Tim for off-line suggestions & ECH35. I have followed-up on all suggestions and also tried the replacement valve. It is now apparent that the osc SW reaction coil is o/c (Trader L9). As I dont see a way forward with that I guess I will have to accept MW & LW only on this set.
Tony
Afterthought:I'll see if anyone is breaking a 464 chassis for spares in the relevant place on the forum
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Old 7th Jul 2007, 5:18 pm   #10
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Default Re: Cossor 464: No short wave

That's a common fault with 494, 500 & 501, as well. That winding is resistance wire ("approx 29.5 ohms" according to the service sheet.)
It is easy enough pick all the bits of the old winding off the former and rewind by hand with ordinary enamelled wire, and it's not critical on gauge or if you miscount a turn or two, with it being the feedback winding, hence untuned.
Good luck,
Martin.
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Old 7th Jul 2007, 11:01 pm   #11
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Default Re: Cossor 464: No short wave

I think my 494 has a similar problem with the MW/LW oscillator coil. It works fine on SW, is completely dead on LW and only recieves a few stations at the high frequency end of the MW.All caps etc changed.
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Old 10th Jul 2007, 10:33 am   #12
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Default Re: Cossor 464: No short wave

OK - I've removed the coil from the chassis. I sweated out the large amount of wax which was holding a ferrite bobbin in place. Sure enough the resistance wire was broken where it went through a small hole in the coil former on its way back up inside to the solder tag.
I've removed the resistance wire (seems about 40swg) which is wound in a shallow groove (8 turns) inside the tuning coil (this one is a sturdy 24 swg or so).
Question now is do I have to wind on 29R5 worth of enamelled copper wire or just replace with copper wire to the same length of the old resistance wire? Maplin does a reel of resistance wire albeit at 28 swg but not sure if it has an insulating coating - but it would cut down the number of turns considerably IF it has an insulated surface.
Grateful if Martin or anyone else could give me a steer.
Tony
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Old 10th Jul 2007, 6:31 pm   #13
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Default Re: Cossor 464: No short wave

Just the same number of turns and in the same direction.
You could add an appropriate resistor (27R) if necessary, but the two I did were fine without.

(I don't know whether the winding was resistive to stop squegging or to stop parasitics. Anyone who knows?)
Martin.
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Old 15th Jul 2007, 7:58 pm   #14
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Default Re: Cossor 464: No short wave

Thanks Martin
Today I rewound the reaction coil with a single strand from some multi-strand wire and added a 22R resistor in series. Re-inserted the ferrite slug and melted the wax back into the former. Re fixed the coil on the chassis and solered it back into cct.
Am now listening to SW across the dial!
Thanks to all for the help in getting the radio back to fully functioning.
Tony
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